2018.2 L.33-34 ms -11 ha-final

of 52

L.33/34

Pre-Leaving Certificate Examination, 2018

Chemistry

Marking Scheme

Ordinary Pg. 4

Higher Pg. 31

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Chemistry

Ordinary & Higher Levels

Table of Contents

Ordinary Level Higher Level Section A Section A

Q.1 .................................................. 4 Q.1 .................................................. 31

Q.2 .................................................. 6 Q.2 .................................................. 34

Q.3 .................................................. 8 Q.3 .................................................. 36

Section B Section B Q.4 .................................................. 10 Q.4 .................................................. 39

Q.5 .................................................. 13 Q.5 .................................................. 42

Q.6 .................................................. 15 Q.6 .................................................. 44

Q.7 .................................................. 18 Q.7 .................................................. 46

Q.8 .................................................. 20 Q.8 .................................................. 48

Q.9 .................................................. 22 Q.9 .................................................. 50

Q.10 .................................................. 24 Q.10 .................................................. 52

Q.11 .................................................. 27 Q.11 .................................................. 55

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Chemistry

Ordinary & Higher Levels

Explanation

Conventions Used

1. A dash – before an answer indicates that the answer is a separate answer, which may be considered as independent of any other suggested answers to the question.

2. A single forward slash / before an answer indicates that the answer is synonymous with that which preceded it. Answers separated by a forward slash cannot therefore be taken as different answers.

3. A double forward slash // is used to indicate where multiple answers are given but not all are required.

4. Round brackets ( ) indicate material which is not considered to be essential in order to gain full marks.

5. ‘etc.’ is used in this marking scheme to indicate that other answers may be acceptable. In all other cases, only the answer given or ‘words to that effect’ may be awarded marks.

6. In calculations, 3 marks are deducted for a mathematical error but no further penalty is incurred if the problem, otherwise correct, is completed.

7. Answers which are given in this marking scheme should not be considered as the only possible answers that may be accepted. Answers which are synonymous with or equivalent to those in this marking scheme are also acceptable.

Current Marking Scheme

Assumptions about these marking schemes on the basis of past SEC marking schemes should be avoided. While the underlying assessment principles remain the same, the exact details of the marking of a particular type of question may vary from a similar question asked by the SEC in previous years in accordance with the contribution of that question to the overall examination in the current year. In setting these marking schemes, we have strived to determine how best to ensure the fair and accurate assessment of students’ work and to ensure consistency in the standard of assessment from year to year. Therefore, aspects of the structure, detail and application of the marking schemes for these examinations are subject to change from past SEC marking schemes and from one year to the next without notice.

Copyright

All rights reserved. This marking scheme and corresponding papers(s) are protected by Irish (EU) copyright law. Reproduction and distribution of these materials or any portion thereof without the written permission of the publisher is prohibited except for the immediate use within a classroom.

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Chemistry

Ordinary Level Marking Scheme (400 marks)

Answer eight questions in all. These must include at least two questions from Section A.

All questions carry equal marks (50).

SECTION A

Answer at least two questions from this section.

Section A Question 1 (50 marks)

1. Ethyne (C2H2) gas is the first member of a homologous series of unsaturated hydrocarbons. The diagram below shows an apparatus used to prepare and collect ethyne gas in the school laboratory.

Liquid X

A

Solid Y

(a) Explain the term homologous series. (11)

Any 2: (2 × 3m) – a family of compounds with the same general formula // – a family of compounds whose successive members differ by CH2 // – a family of compounds with the same functional group // – a family of compounds which have similar chemical properties // – a family of compounds which have gradation in physical properties // – a family of compounds with a similar method of preparation // etc.

** Accept other appropriate answers.

Name the homologous series to which ethyne belongs. (5m)

– alkynes

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Section A Question 1 (cont’d.)

(b) Identify the liquid X and the solid Y. (2 × 4m) (12)

liquid X – water / H2O solid Y – calcium carbide / calcium dicarbide / CaC2

Describe the appearance of the solid Y. (4m)

– grey-black / grey-brown / sandy coloured solid / brownish / dirty / stones, etc.

(c) Name the piece of equipment labelled A from which liquid X is added. (5)

Any 1: (5m) – dropping funnel // – addition funnel // – separating funnel

(d) Why are the first few test-tubes of gas collected discarded? (6m) (6)

– they contain displaced air / low yield of ethyne // etc.

** Accept ‘impure’ for 3m.

(e) When ethyne gas is mixed with another gas in suitable proportions, the mixture burns with a very hot flame. (10)

Identify the gas mixed with ethyne. (6m)

– oxygen

What use is made of this hot gas?

Any 1: (4m) – welding / oxyacetylene flame (torch) // – cutting torches // – cutting metals // – welding metals // etc.

(f) A sample of the ethyne gas collected is bubbled through bromine water. What change would you expect to observe in the appearance of the

bromine water? (6)

Any 1: (2 × 3m) – red/brown to – colourless

or

– yellow to – colourless

2018 LC Chemistry [OL]

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2. An experiment was carried out to find the concentration of a sodium hydroxide (NaOH) solution. The sodium hydroxide solution was measured in 25.0 cm3 portions and titrated with a 0.1 M standard solution of hydrochloric acid (HCl).

The equation for the titration reaction is:

HCl + NaOH NaCl + H2O

(a) (i) Name the piece of equipment used to measure out the 25.0 cm3 portions of sodium hydroxide solution into the conical flask. (3m) (9)

– pipette

(ii) Describe the procedure for rinsing this piece of equipment before using it to measure out the 25.0 cm3 portions of sodium hydroxide solution. (2 × 3m)

– rinse with deionised (distilled, pure) water – rinse with solution (sodium hydroxide, NaOH)

(b) (i) Name the piece of equipment used to measure out the hydrochloric acid solution for use in the titration. (3m) (9)

– burette

(ii) State two precautions, other than rinsing, that should be taken to ensure an accurate measurement when using this piece of equipment in the titration.

Any 2: (2 × 3m) – clamp vertically // – fill part of the burette below the tap // – remove any droplets adhering to the tap // – remove funnel before zeroing // etc.


(c) Name a suitable indicator for this titration. (9)

State the colour change of the indicator at the end point.

** Colours must match indicator named. ** Award 3m if colours correctly named but given in reverse order.

Indicator Colour change

Any 1: (3m) Any 1: (2 × 3m) – methyl orange // – orange (yellow) to – red (accept peach or pink)

– methyl red // – yellow to – red

– phenolphthalein – pink (purple, violet, red) to – colourless


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(d) It was found that exactly 25.0 cm3 of the sodium hydroxide solution required, on average, 22.5 cm3 of the 0.1 M hydrochloric acid solution for neutralisation.

Calculate the concentration of the sodium hydroxide solution: (12) (i) in moles per litre,

– 25.0 × M = 22.5 × 0.1 (6m) – M = 0.09 M (3m)

(ii) in grams per litre.

– 0.09 × (23 + 16 + 1) = 3.6 g/l (3m)

(e) A further titration was carried out using the same solutions to prepare and collect a common salt, sodium chloride.

(i) State one clear difference in procedure from the previous titration. (5m) (5)

– no indicator is added

(ii) How could a dry sample of salt be produced after this titration? (6m) (6)

– (by) evaporation


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3. A student carried out a number of investigations on a sample of sea water in the school laboratory.

(a) To measure the amount of suspended solids, the student filtered 250 cm3 of the sea water through a weighed clean dry filter paper. The filter paper was washed through with a little distilled water, dried and then reweighed. The filter paper had increased in mass by 0.23 g. The student found that a 50 cm3 sample of the filtered water contained 0.17 g of dissolved solids.

(i) Why did the student wash the filter paper with distilled water after filtering the sea water? (6)

Any 1: (6m) – to remove dissolved solids // – to wash through the dissolved solids

(ii) Express the concentration of suspended solids in parts per million (p.p.m. or mg per litre). (2 × 4m) (8)

– 250

)23.01000( = 0.92 g/l

– 0.92 × 1000 = 920 p.p.m. / mg/l

(iii) Describe how the student could have measured the concentration of dissolved solids in the sea water. (12)

Any 4: (4 × 3m) – weigh a beaker // – evaporate water // – filtrate // – reweigh the beaker // – increase in mass is the mass of dissolved solids

(iv) Express the concentration of dissolved solids in parts per million (p.p.m. or mg per litre). (2 × 4m) (8)

– 50

)17.01000( = 3.4 g/l

– 3.4 × 1000 = 3400 p.p.m. / mg/l

(b) Describe how the student could have tested the sample of sea water to show that chloride ions were present. (2 × 3m) (6)

– add silver nitrate / AgNO3 – white (pale) precipitate results

** Award (3m) for ‘add ammonia drop by drop and precipitate dissolves’.


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(c) A flame test was carried out to test for the presence of a sodium salt (NaNO3) in the dissolved solids collected.

What colour flame would indicate the presence of sodium ions? (4m) (4)

– amber / yellow

(d) Name the stage at which suspended solids are removed in the treatment of drinking water. (6m) (6)

– filtration


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SECTION B

Section B Question 4 (50 marks)

4. Answer eight of the following (a), (b), (c), etc. Award 1 bonus mark to each of the first two fully correct answers.

(a) Give one industrial source of hydrogen gas. (6)

Any 1: (6m) – steam reforming of methane / natural gas // – electrolysis of water

(b) What is meant by the term chemical compound? (2 × 3m) (6)

– two or more different elements – combined / bonded / joined chemically

(c) Identify one natural product that is extracted from plant material by steam distillation. (6)

Any 1: (6m) – clove oil (eugenol) // – rose oil // – oil of lavender // – citrus / orange / lemon oil // etc.


(d) Give the name or formula of an acid which is the cause of nettle or ant stings. (6)

Any 1: (6m) – formic acid // – methanoic acid // – HCOOH

(e) Name the English scientist, pictured on the right, who identified electrons as negatively charged subatomic particles in the 1890s. (6m) (6)

– J.J. Thomson

(f) Name the catalysts that are produced by living cells. (6m) (6)

– enzymes


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Section B Question 4 (cont’d.)

(g) Write the equilibrium constant (Kc) expression for the following reaction: (6m) (6)

H2 + I2 2HI

– Kc =

22

2

HI

HI

** Award 3m for top or bottom correct.

(h) State one use of the radioisotope cobalt-60 (60Co). (6)

Any 1: (6m) – cancer treatment // – food irradiation // etc.

** Award 3m for ‘medicine’. ** Accept other appropriate answers.

(i) The ancient Greeks, particularly the philosopher Empedocles, suggested there were four “elements”. One of these “elements” was earth.

Name two of the other three “elements” according to the ancient Greeks. (6)

Any 2: (2 × 3m) – wind // – fire // – water

(j) How many (i) protons and (ii) neutrons has Cl3717 ? (6)

(i) protons (3m)

– 17

(ii) neutrons (3m)

– 20


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(k) Answer part A or part B.

A Give two examples of how chemistry has contributed to society. (6)

Any 2: (2 × 3m) – healthcare products // – disease treatments // – crop pest control // – food processing / food production // – water treatment // etc.


or

B Sorting is an important stage in the recycling of plastics. Name two other stages in the recycling of plastics. (6)

Any 2: (2 × 3m) – shredding // – washing // – drying // – melting // – re-extrusion / melting and remoulding

** Accept any appropriate material.


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5. (a) Explain the term polar bond. (4m) (9)

– a bond where electrons are shared unequally between two atoms

Explain why the bond in water (H2O) is polar. (5m)

– difference in electronegativity is greater than 0.9 and less than 1.7

(b) Outline how you would use a burette to demonstrate polarity of water. (2 × 3m) (6)

– a thin stream of water is flowing from a burette – the steam of water is deflected towards a positively charged rod

(c) What are energy levels? (8)

Any 1: (4m) – fixed energy value that an electron in an atom may have // – a region of definite energy in an atom that an electron may occupy

Name the scientist, pictured on the right, who first proposed the idea of energy levels in his atomic theory. (4m)

– (Niels) Bohr

(d) What is the maximum number of electrons that can occupy the second energy level (shell)? (3m) (6)

– 8

How many electrons are in the third energy level (shell) of an atom of sulfur? (3m)

– 6


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(e) What change in atomic radius occurs as you go down a group in the periodic table? (6m) (9)

– increase

Give a reason for this change.

Any 1: (3m) – number of energy levels / shells increases // – increased screening effect

(f) The diagram below shows the bonding in a fluorine (F2) molecule. The electrons are represented by dots () and crosses (×).

F F

(i) What type of bonding is present in fluorine? (4m) (4)

– pure covalent

(ii) Name another type of chemical bond formed by fluorine and give an example of a compound in which fluorine forms this type of bond. (8)

Other chemical bond (4m)

– ionic

Compound with which fluorine forms ionic bond (4m)

– sodium fluoride / NaF

** Accept any correctly named ionic fluoride. ** Award full marks for correct name or structure given.


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6. (a) Alkanes are saturated and undergo substitution reactions.

Explain each of the underlined terms. (10)

Saturated (5m)

– (a compound that) contains only single carbon to carbon bonds

Substitution reactions (5m)

– (reactions in which) an atom or group of atoms in a molecule is replaced by another atom or group of atoms

(b) What are structural isomers? (2 × 3m) (12)

– compounds with the same molecular formulae – but with different structural formulae

Name and draw one structural isomer of butane (C4H10).

** Name (3m), Draw (3m). Any 1: (3m + 3m) – butane – //

– methylpropane / 2-methylpropane –

(c) Name the hydrocarbon produced by the decomposition of animal and vegetable waste in dumps. (3m) (6)

– methane

What environmental hazard is associated with this gas? (3m)

– global warming / the greenhouse effect


H

H

C

H H

H

C H

H

C

H

H

H

C

H

H

C

H

HC

CH3

H

H

H

C

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(d) A fractionating column, similar to the one shown, is used in an oil refinery to separate crude oil into different hydrocarbon fractions. (15)

(i) Name one of the hydrocarbon fractions obtained by this process.

(ii) From which part of the fractionating column (high up, middle or low down) is this fraction collected?

(iii) State one major use of this fraction.

** Fraction named (6m) ** Position collected from fractionating column (6m). ** Use (3m).

Any 1: (6m + 6m + 3m)

Fraction Where on column Major use

– refinery gas – top / high – heating // – cooking // etc.

– light gasoline (petroleum) – high – petrol

– naphtha – high / mid Any 1: – petrol // – plastics // – detergents // – solvents // etc.

– kerosene / paraffin – mid Any 1: – aviation fuel // – heating // etc.

– gas oil / diesel oil – mid Any 1: – truck fuel // – locomotive fuel // – heating // etc.

– lubricating oil – low Any 1: – engine oil // – lubricant // etc.

– fuel oil – low Any 1: – marine fuel // – heating plants // – power stations // etc.

– bitumen – bottom Any 1: – roofing // – roads // – waterproofing // etc.



Crude oil in

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(e) Benzene (C6H6) is sometimes added to petrol to improve its octane number. (7)

(i) To which group of hydrocarbons does benzene belong? (3m)

– aromatic

(ii) Draw the structural formula of benzene. (4m)

–


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7. (a) (i) Explain how and why the fluoridation of water is carried out. (6)

How (3m)

– addition of fluoride ion (F–) / sodium fluoride (NaF) / sodium hexafluorosilicate (Na2SiF6) / antimony hexafluoride (SbF6)

Why (3m)

– to prevent tooth decay

(ii) Why is chlorine added to drinking water? (3)

Any 1: (3m) – to kill germs / bacteria // – to prevent disease // – to sterilise it // etc.

(iii) Name a substance that is added to drinking water to increase its pH. (3m) (3)

– lime / Ca(OH)2 / CaO / base / alkali / named alkali

(iv) A test-tube containing limescale (taken from the inside of a water pipe) and dilute hydrochloric acid (HCl) is shown. As the gas X produced by the reaction is bubbled through a solution Y, the solution becomes milky (cloudy).

Identify the gas X and the solution Y. (2 × 3m) (6)

gas X – carbon dioxide liquid Y – limewater / Ca(OH)2 / calcium hydroxide

(v) What is meant by the term hard water? (5m) (5)

– (water that) does not easily form a lather with soap / wastes soap / produces a scum

(vi) State one advantage and one disadvantage of a hard water supply. (6)

Advantage

Any 1: (3m) – tastes nicer // – a good source of calcium // – good for brewing and tanning // etc.


X

Y

Limescale+ HCl

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(a) (vi) (cont’d.)

Disadvantage

Any 1: (3m) – wastes soap // – produces scum // – blocks pipes // – damages appliances // – decreases heat efficiency // – wastes detergent // etc.

(vii) How can temporary hardness be removed from water? (3)

Any 1: (3m) – boiling // – adding soap // – adding washing soda crystals

(b) The treatment of sewage involves three stages: primary, secondary and tertiary.

Name the stage which involves: (8) (i) the biological breakdown of the material present in the sewage, (4m)

– secondary (stage)

(ii) the removal of nutrients such as nitrates and phosphates. (4m)

– tertiary (stage)

(iii) Suggest two ways in which nitrates could get into a water source. (6)

Any 2: (2 × 3m) – sewage // – fertiliser run-off // – silage effluent // etc.

** Accept ‘run-off’ only for either fertilisers or silage but not for both; accept flooding or heavy rain.

(iv) What problem is associated with the release of nitrates and phosphates into rivers and lakes? (4)

Any 1: (4m) – eutrophication // – algal bloom

** Award 1 mark for ‘enrichment’.


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8. (a) The following are the molecular formulae of five organic compounds.

CH2CH2 CH3CH2Cl CH3CH3 C2H5OH CH3COOH

In your answer book, match each of the compounds above with their corresponding descriptions (A to E) in the table below. (5 × 3m) (15)

A Undergoes substitution reactions

B Contains only planar carbons

C Gives the sharp taste to vinegar

D Is produced in the brewing industry

E Requires UV light to form

A – CH3CH3 B – CH2CH2 C – CH3COOH D – C2H5OH E – CH3CH2Cl

(b) Consider the three compounds A, B and C.

X Y C2H5OH

A

CH2CH2

B

CH2BrCH2Br

C

(i) Give the IUPAC names for A, B and C. (3 × 4m) (12)

A – ethanol B – ethene C – 1,2-dibromoethane


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(b) (cont’d.)

(ii) Which of the compounds A, B or C can be polymerised? (3m) (9)

– B / ethene

State the name and give a use of the polymer formed.

Name (3m) Use

– polyethene / polythene Any 1: (3m) – plastic bags // – plastic tunnels // – plastic crates // – mineral bottles // – plastic sheeting // etc.

(iii) Categorise as an addition, elimination or substitution reaction (a) conversion X, (b) conversion Y. (8)

(a) conversion X (4m)

– elimination

(b) conversion Y (4m)

– addition

(iv) Give one reason why compound A is very soluble in water. (6m) (6)

– polar / polar OH group / can form hydrogen bonds with water


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9. Mass spectrometry (MS), thin-layer chromatography (TLC), gas chromatography (GC) and high-performance liquid chromatography (HPLC) are analytical techniques used in chemistry.

(a) Give one application of mass spectrometry. (5)

Any 1: (5m) – measurement of atomic radius // – measurement of molecular mass // – separation of isotopes // – determining structures // – identification of elements // – identification of compounds // – water analysis // – analysis of waste gases // – separation of ions // – measurement of abundance of isotopes // – measurement of mass of isotopes // etc.


(b) (i) Describe an experiment to separate a mixture of dyes or indicators using thin-layer chromatography. (18)

Any 3: (3 × 6m) – spot sample on plate (drawing line(s)) // – place in chromatography tank / beaker // – spot (line) on plate should be just above level of eluent (solvent) in tank // – elute (let solvent run up plate) // – remove plate from tank when eluted (solvent front nears top of plate) // – allow plate to dry (solvent to evaporate) and examine // etc.

** Accept answers using a diagram with correct description for full marks.

(ii) State one use that is made of this technique in forensic science. (6)

Any 1: (6m) – to examine (match) dyes and paints from crime scenes // – to test the purity of a drug // – to match blood samples // etc.


(c) Give one application for (12) (i) gas chromatography,

Any 1: (6m) – separating compounds for analysis by mass spectrometry // – drug testing on athletes // – testing meat for growth promoters (hormones / anabolic steroids) // – testing for alcohol in blood (urine) // etc.



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(c) (cont’d.)

(ii) high-performance liquid chromatography.

Any 1: (6m) – separation of compounds // – separation of growth promoters // – separation of vitamins in food // etc.


(d) What is the principle on which all of the three chromatographic techniques are based? (9)

Any 2: (6m + 3m) – the different compounds have a different attraction // – to the stationary // – and mobile phases used and so can be separated


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Section B Question 10 (2 × 25 marks)

10. Answer any two of the parts (a), (b) and (c).

(a) (i) Define a base according to the theory of Arrhenius. (5m) (5)

– (a substance that) produces OH– ions in water

(ii) Define pH. (4m) (4)

– –log10[H+] / minus log to the base ten of the hydrogen ion concentration

(iii) Gastric acid in the human stomach contains a solution of 0.1 M hydrochloric acid. Calculate the pH of this solution. (9)

– –log10[0.1] (6m) = –(–1) = 1 (3m)

(iv) What type of compound is usually present in medicines used to treat acid indigestion? (7)

Any 1: (4m) – antacid // – alkali // – base

Name or give the formula of one such compound.

Any 1: (3m) Name Formula

– sodium hydrogen carbonate / – NaHCO3 // sodium bicarbonate // – magnesium hydroxide // – Mg(OH)2 // etc. – Milk of Magnesia // – Rennie // – baking powder // – bread soda // etc.

** Accept other appropriate name or formula.


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(b) Hydrochloric acid (HCl) reacts with marble chips (CaCO3), producing carbon dioxide gas as one of the products. Flasks A and B were used in an experiment to examine the effect of concentration on the rate of a chemical reaction. Both flasks contained 2 g of marble chips. The marble chips in the two flasks were approximately the same size. A 100 cm3 portion of 1 M hydrochloric acid was added to flask A and a 100 cm3 portion of 2 M hydrochloric acid was added to flask B.

Flask A Flask B

100 cm3 of2 M HCl

100 cm3 of1 M HCl

2 g of marble chips

(i) Explain the underlined term. (4m + 3m) (7)

Rate of a chemical reaction

– rate of change of the concentration – of a product (or reactant) of a chemical reaction

or

– change of concentration of reactant (product) [accept ‘substance’] – in unit time (over time)

(ii) In which flask was the reaction faster (more vigorous)? (6m) (12)

– (flask) B

Why was this reaction faster? (6m)

– the acid (HCl) was more concentrated / because reaction rate depends on concentration

** Accept ‘more acid’ for 3 marks.

(iii) What would be the effect on the rate of reaction if this experiment was carried out at a higher temperature? (6)

Any 1: (6m) – faster rate of reaction // – more particles reach activation energy // – more particles have the minimum (required) energy


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(c) Define relative atomic mass. (4m + 3m) (7)

– mass of an atom relative to one-twelfth of the mass – of the carbon isotope C-12 (carbon-12)

Calculate the relative molecular mass of nitric acid (HNO3) from the relative atomic masses of its elements. (6)

– [1 + 14 + 3(16)] (3m) = 63 g (3m)

What is the percentage by mass of nitrogen in nitric acid? (6)

– 63

14 × 100 (3m) = 22.22% (3m)

How many moles of nitric acid are contained in 3.15 g of the acid? (6)

– 63

15.3(3m) = 0.05 moles (3m)


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11. Answer any two of the parts (a), (b) and (c).

(a) The diagrams below show the arrangement of particles in the three states of matter (gas, liquid and solid).

(i) Briefly outline the differences between the three states of matter in terms of the movement of their particles. (9)

Gas (3m)

– particles can move freely

Liquid (3m)

– some movement of particles allowed

Solid (3m)

– particles are not free to move

(ii) Explain the term diffusion. (2 × 3m) (6)

– movement / spontaneous spreading out – from high to low concentration / due to natural movement of particles / to fill a space /

to make concentration uniform

(iii) Describe a simple experiment to demonstrate diffusion. (4m + 3m + 3m) (10)

– potassium permanganate crystal – at bottom of beaker of water – colour movement is evidence for diffusion


Gas Solid Liquid

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(b) (i) State Charles’ law. (4m + 3m) (7)

– at constant pressure the volume of a fixed mass of gas – is directly proportional to its temperature (measured on the Kelvin scale)

(ii) A sample of carbon dioxide gas (CO2) has a volume of 12638 cm3 at a temperature of 308 K and a pressure of 2 × 105 Pa.

Use the combined gas law

2

22

1

11

T

VP

T

VP

to calculate the volume occupied by the gas at a temperature of 273 K and a pressure of 1 × 105 Pa. (3 × 3m) (9)

– 273

))(101(

308

)12638)(102( 255 V

– V2 = 22403.7 cm3

** Left-hand side correct (3m). ** Right-hand side correct (3m). ** Correct calculation of V2 (3m).

(iii) Taking the values 273 K and 1 × 105 Pa in (ii) as standard temperature and pressure, respectively, how many moles of gas are in the sample? (6m) (6)

– 1 mole

(iv) How many molecules are in this sample? (3m) (3)

– 1 × (6 × 1023) = 6 × 1023


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(c) Answer part A or part B.

A

Explain the difference between a batch process and a continuous process in the chemical industry. (8)

Batch process (4m)

– produced in discrete lots / quantity at a time

Continuous process (4m)

– produced continuously with addition of more raw materials and removal of product / ongoing

Explain the terms (i) feedstock and (ii) co-products as used in industrial chemistry. (8)

(i) feedstock (4m)

– reactants in an industrial process / prepared raw materials required

(ii) co-products (4m)

– products produced along with the desired product

Name three components of the feedstock in the case of any one of the following chemical industries: ammonia manufacture; nitric acid manufacture; magnesium oxide manufacture. (9)

Any 1: (3 × 3m)

Ammonia manufacture

– methane – steam (water) – air (nitrogen)

Nitric acid manufacture

– ammonia – air (oxygen) – water

Magnesium oxide manufacture

– limestone – freshwater – seawater (magnesium chloride)

** Accept other correct name or formula.

or


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(c) (cont’d.)

B

(i) Name the father and son team who were awarded a Nobel Prize for their work in developing the experimental technique for determining crystal structure. (4m) (7)

– (William and Lawrence) Bragg

Name the experimental technique they used to determine crystal structure. (3m)

– X-ray crystallography

(ii) Give an example of an ionic crystal. (7)

Any 1: (3m) – sodium chloride / NaCl // – potassium iodide / KI // – magnesium oxide / MgO


Name the binding force in this crystal. (4m)

– electrostatic attraction

(iii) Give an example of a molecular crystal. (11)

Any 1: (3m) – solid carbon dioxide // – iodine // – naphthalene // – sulfur // – solid hydrogen chloride / HCl // – ice // etc.


State two physical properties of molecular crystals.

Any 2: (2 × 4m) – low melting point // – usually quite soft // – do not conduct electricity // – solubility depends on nature of molecules at lattice points // etc.


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Chemistry

Higher Level Marking Scheme (400 marks)

Answer eight questions in all. These must include at least two questions from Section A.

All questions carry equal marks (50).

SECTION A

Answer at least two questions from this section.

Section A – Question 1 (50 marks)

1. A group of students determined the amount of dissolved oxygen in a sample of well water.

A sample bottle was washed before a sample was taken from below the surface of the water in the well and it was analysed immediately using the following procedure:

Winkler’s reagent was added to the water in the bottle and a white precipitate was seen to form. The stopper was carefully replaced and the bottle was shaken to ensure mixing of the reagent with the water. The white precipitate was seen to form a brown precipitate. On addition of concentrated sulfuric acid (H2SO4) the precipitate dissolved and a golden brown solution was formed. The concentration of iodine (I2) in this solution was found by titrating 100 cm3 portions against a 0.01 M sodium thiosulfate (Na2S2O3) solution.

The average titration figure was 15 cm3.

(a) Why is it necessary to analyse the water sample immediately? (3m + 2m) (5)

– so that the oxygen content does not increase due to photosynthesis by plants present in the water

– so that the oxygen content does not decrease due to respiration by micro-organisms present in the water

(b) How did the students add the Winkler’s reagent to the bottle? (4m) (4)

– using a dropper below the surface of the water

(c) What should the students have concluded if, after shaking, the white precipitate did not turn brown? (3m) (3)

– (that there was) no dissolved oxygen present.

examsDEB

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(d) Explain why this method is unsuitable for water which has been chlorinated. (3m + 2m) (5)

– chlorine is an oxidising agent – it (chlorine) would react with the iodide ions in potassium iodide to produce iodine

(e) Describe the procedure for rinsing and filling the burette with sodium thiosulfate solution. (12)

Any 4: (4 × 3m) – clamp burette vertically // – rinse with deionised water and then with the liquid it will contain // – use a funnel when filling the burette // – fill to the top, open the tap and allow the liquid to fill the part of the burette below the tap // – remove funnel after filling the burette // – adjust level of liquid until the bottom of the meniscus is on the zero mark // – read at eye level // – remove any drops of liquid from the tip of the burette by touching it against the side

of a glass beaker // etc.

** Deduct 3m if ‘read at eye level’ is not a given answer.

(f) Name the indicator used in this titration. (3m) (6)

– starch

State when it is added to the titration flask. (3m)

– when the solution turns straw yellow

(g) The balanced equation for the titration reaction is:

232O2S + 2I 2

64OS + 2I–

Calculate the concentration of the iodine solution in moles per litre. (2 × 3m) (6)

– 1

)100(

2

)1501.0( 2 M

0.075 = M2 × 100

– M2 = 100

075.0 = 0.00075 M

2018 LC Chemistry [HL]

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(h) For every one mole of dissolved oxygen (O2) in the water sample, two moles of iodine (I2) are liberated in this experiment.

Calculate the concentration of dissolved oxygen in the well water sample (9)

(i) in moles per litre, (3m)

– 2

00075.0 = 0.000375 M

(ii) in grams per litre, (3m)

– 0.000375 × 32 = 0.012 g/l

(iii) in ppm. (3m)

– 0.012 × 1000 = 12 ppm


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2. A chemistry student was required to perform a number of tasks as part of her practical work in a school laboratory. She had access to all the necessary reagents and glassware and also the required safety equipment.

(a) How could the student have carried out a test to prove that a solution contained hydrogencarbonate

( 3HCO ) anions and not carbonate ( 2

3CO ) anions? (6)

– add magnesium sulfate (or chloride) solution (3m)

Any 1: (3m) – a white precipitate does not form / solution remains clear if

hydrogencarbonate anions are present // – a white precipitate forms in presence of carbonate anions

(b) One of the tasks in the practical examination was to measure the heat of reaction between hydrochloric acid (HCl) and sodium hydroxide. (9)

List three precautions that should be taken by the student to obtain an accurate value for the highest temperature reached during the reaction.

Any 3: (3 × 3m) – use a thermometer accurate to 0.2 °C or better (temperature sensor, ‘sensitive’, ‘accurate’) // – add reactants quickly // – add reactants without splashing // – replace cover quickly (immediately) // – stir briefly // – after addition plot temperature at intervals and get highest temperature by extrapolating back

to time of mixing // – prevent heat loss (use suitable insulation) // etc.

** Do not accept ‘digital thermometer’.

(c) The student was supplied with a sample of ethene gas in a stoppered test-tube. (9)

Describe how she could have shown that the gas was unsaturated.

Any 1: (3 × 3m)

Test 1

– add / shake / bubble – bromine solution – goes from yellow to colourless

or

Test 2

– add / shake / bubble – acidified potassium permanganate solution – goes from purple to colourless


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(d) The student was then required to extract an organic substance from a plant material using steam distillation in the school laboratory. (6)

Name a substance she extracted by steam distillation. (3m)

– clove oil

Describe the appearance of the distillate collected. (3m)

– cloudy liquid / milky / white (creamy) liquid / emulsion

(e) Briefly describe a technique that the student could have used to isolate a pure sample of the organic substance extracted by steam distillation in (d) above from water. (12)

(3 × 3m) – organic layer is placed in a conical flask and magnesium sulfate is added as a drying agent – add magnesium sulfate until it has absorbed all the water – filter contents through fluted filter paper into a clean dry conical flask

Any 1: (3m) – add cyclohexane to dissolve the oil in the emulsion, but not mix with the water // – two layers are formed - aqueous layer (emulsion) and organic layer (cyclohexane) // – cyclohexane floats on the water as its less dense // – separate the layers using a separating funnel // – aqueous layer is allowed to run into a beaker // – the separating funnel is stoppered, then shaken back and forth // – the tap of the funnel is then opened to release pressure of vaporised liquid in the funnel // – aqueous layer is separated again // – procedure is repeated twice more // etc.

(f) The student carried out an experiment to oxidise phenylmethanol (benzyl alcohol) to benzoic acid with potassium manganate (VII) solution under alkaline conditions. (8)

Describe the appearance of the phenylmethanol at room temperature. (4m)

– colourless / pale yellow (clear) / oily / liquid

** Do not accept ‘clarified’ in place of colourless. ** Do not accept ‘solution’ or ‘solid’ in place of liquid.

What did the student add to the reaction flask to ensure alkaline conditions? (4m)

– sodium carbonate


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3. In an investigation into the effect of temperature on reaction rate, a student measured 100 cm3 of a 0.1 M solution of sodium thiosulfate (Na2S2O3) into a conical flask and added 10 cm3 of 1.0 M hydrochloric acid (HCl) at a number of different temperatures. The time (t) taken for the cross drawn on a sheet of paper to become obscured was measured.

Cross

Whitepaper

Na2S2O3 and HCl

The reciprocal of the time (1/t) was used as a measure of the initial rate of reaction in each case. The results are recorded in the table below.

Temperature (°C)

Time (t) (s)

Rate = 1/time (s–1)

10 100 0.010

20 59 0.017

30 36 0.028

40 21 0.048

50 12 0.083

60 7 0.143

(a) Identify the pale yellow precipitate that formed in the flask. (5m) (5)

– sulfur

(b) Plot a graph of reaction rate (1/time) versus temperature. (12)

Axes correctly labelled and scaled (2 × 3m) Points plotted correctly (3m) Line drawn (3m)


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(b) (cont’d.)

Temperature (°C)

Rat

e, 1

/tim

e (s

–1)

0.08

0.02

0.00

0.04

0.06

0.10

0.14

0.12

0.16

0 10 20 30 40 50 60

Rate at 35°C

(c) What conclusion can be drawn from the graph about the relationship between the rate of reaction and temperature? (2 × 3m) (6)

– the rate is not directly proportional to the temperature – as the temperature increases the rate increases at a greater rate

(d) Use your graph to find the time taken for the cross to become obscured if the reaction was carried out at 35 °C. (6)

Rate at 35 °C (read from graph)

– 0.036 s–1 (3m)

Time = Rate

1

Time = 0.036

1

– time = 27.8 s (3m)

** Accept any reasonable reading of rate from the graph.


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(e) Explain why increasing the temperature has a significant effect on the rate of reaction. (2 × 3m) (6)

– increasing the temperature increases the kinetic energy – so a larger proportion of molecules will have activation energy, so rate increases

(f) State and explain three other factors that have an effect on the rate of a chemical reaction. (15)

** Factor named (3m), Factor explained (2m). ** Explanation must be relevant to the factor named.

Any 3: (3 × 5m)

Particle size //

Any 1: – the more finely divided the solid, the more rapidly the reaction proceeds // – a greater surface area available for the reaction to occur

Concentration //

– the greater the concentration, the greater the number of collisions per second; the greater the rate of reaction

Nature of reactants //

Any 1: – ionic reactions are faster than covalent reactions - coming together of ions // – in covalent reactions, bonds have to be broken and then formed again so more time is needed

Presence of a catalyst

– alters the rate of a chemical reaction without being consumed in the reaction

** Accept other appropriate material.


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SECTION B


4. Answer eight of the following (a), (b), (c), etc. Award 1 bonus mark to each of the first two fully correct answers.

(a) How are heavy metals ions, e.g. mercury, removed from industrial waste before it is discharged into rivers, lakes and seas? (6m) (6)

– precipitation / tertiary treatment

** Accept ‘coagulation’ or ‘flocculation’ for full marks. ** Accept a suitable precipitating agents for 3 marks.

(b) Give an example of a useful radioisotope, and indicate one of its uses. (6)

Radioisotope named (3m), Use indicated (3m). ** Use should correspond to the radioisotope named.

Name Use

Any 1: (3m) Any 1: (3m) – americium-241 // – smoke detectors //

– carbon-14 // – carbon dating //

– cobalt-60 // Any 1: – cancer treatment // – food irradiation

(c) Why are mercaptans added to natural gas? (6)

Any 1: (6m) – for safety // – to detect gas leaks // – to give natural gas a smell

(d) Calculate the volume, in cm3, occupied by 3.0 × 1020 molecules of nitrogen at s.t.p. (2 × 3m) (6)

– 23

20

106

103

= 5 × 10–4 moles

– (5 × 10–4) × 22,400 = 11.2 cm3

(e) List the following types of radiation in order of decreasing penetrating power: (6m) (6)

alpha- (α-) beta- (β-) gamma- (γ-)

– gamma → beta → alpha


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(f) What mass of KMnO4 would be needed to make 150 cm3 of 2% (w/v) solution? (2 × 3m) (6)

– 2% (w/v) = 2 g in 100 cm3 – 3 g in 150 cm3

(g) Name the class of organic compounds that is responsible for the scent associated with fruits such as cherries, pears, oranges, bananas and strawberries. (6m) (6)

– esters

** Accept ‘terpenes’ for full marks.

(h) At what stage of sewage treatment does biological oxidation take place? (6m) (6)

– secondary treatment

(i) What instrument is used to measure heats of combustion accurately? (6m) (6)

– bomb calorimeter

(j) What is an ideal gas? (2 × 3m) (6)

– (a gas that) obeys the gas laws (Boyle’s law, kinetic theory, PV = nRT) – at all values of temperature and pressure / in all conditions / perfectly

** Accept ‘A gas that obeys the assumptions of the kinetic theory’ for 6m.


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(k) Answer part A or part B.

A What is meant by the term nitrogen fixation? (6m) (6)

– the conversion of atmospheric nitrogen to compounds that can be used by plants

or

B Give an example of (i) an ionic crystal, (ii) a covalent macromolecular crystal. (6)

(i) ionic crystal

Any 1: (3m) – sodium chloride / NaCl // – potassium iodide / KI // – magnesium oxide / MgO // etc.

** Accept other appropriate examples.

(ii) covalent macromolecular crystal

Any 1: (3m) – diamond // – quartz // etc.

** Accept other appropriate examples.


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5. (a) (i) State two characteristic properties of transition elements. (6)

Any 2: (2 × 3m) – have variable valency // – usually form coloured compounds // – can be used as catalysts

(ii) Define first ionisation energy. (2 × 4m) (8)

– the minimum energy that is needed to completely remove the first most loosely bound electron

– from a neutral gaseous atom in its ground state

(iii) Why is the first ionisation energy of neon higher than that of fluorine? (6)

Neon

Any 1: (3m) – full 2p sublevel // – full 2p6 sublevel

Fluorine

Any 1: (3m) – 2p sublevel is neither filled nor half-filled // – 2p5 sublevel // – unstable configuration

(iv) Why is the second ionisation energy of neon higher than that of fluorine? (6)

Neon

Any 1: (3m) – bigger effective nuclear charge // – electrons pulled more strongly towards the nucleus // – smaller atomic radius // – no change in screening effect

Fluorine

Any 1: (3m) – smaller effective nuclear charge // – less of a pull on the electrons towards the nucleus // – larger atomic radius // – no change in screening effect


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(b) (i) Using data in the Formulae and Tables booklet determine the type of bonding expected between atoms of hydrogen and sulfur. (3m) (6)

– 2.58 – 2.20 = 0.38 slightly polar covalent

Explain your answer. (3m)

– electronegativity difference is less than 1.7 (covalent) / small difference in attraction for shared electrons

(ii) Write the molecular formula for the compound formed when hydrogen and sulfur react. (3m) (6)

– H2S

Predict the shape of the molecule formed. (3m)

– V-shaped / distorted tetrahedron

(c) Describe the contribution of (i) Döbereiner and (ii) Newlands to the development of the periodic table. (12)

(i) Döbereiner (2 × 3m)

– discovered triads / groups of three elements with similar chemical properties – the atomic weight of the middle element is approximately equal to the average of the other two

(ii) Newlands (2 × 3m)

– elements arranged in order of increasing atomic weight – the first and the eighth element of each group have similar properties


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6. (a) One of the products produced in an oil refinery is refinery gas, from which liquid petroleum gas (LPG) is produced.

(i) Name the two components that are liquefied as LPG. (2 × 3m) (6)

– propane – butane

(ii) Name the homologous series to which the components named in (i) belong. (3m) (3)

– alkanes

(iii) Describe with the aid of a labelled diagram how crude oil is fractionated into useful products in an oil refinery. (9)

Diagram with one correct label (3m) ** Diagrams should show layers or outlets. ** Outlets may be shown by tubes (pipes), holes, gaps, lines, arrows (→).

–

Description (2 × 3m)

– heat (boil) / pass vapour up tower (column) / temperature gradient shown / decreasing temperature ascending – fractions separated based on boiling points and relative molecular masses

(iv) Name a fraction of crude oil which is used to form petrol. (4)

Any 1: (4m) – light gasoline – naphtha


Crude oil in

Tem

pera

ture

dec

reas

ing

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(b) Define heat of reaction. (5m) (5)

– the heat change when the number of moles of reactants indicated in the balanced equation for the reaction react completely

(c) Write a balanced equation for the combustion of methanol. (6m) (15)

– CH3OH + 1½O2 CO2 + 2H2O

** Equation correct (3m). ** Equation balanced (3m).

Calculate the heat of combustion of methanol, given that the heats of formation of methanol, carbon dioxide and water are –239, –394 and –286 kJ mol–1, respectively.

Hc = Hf(CO2 + 2H2O) – Hf(CH3OH + 1½O2) – Hc = [(–394) + (2(–286))] – [(–239) + (1½(0)] (3m) – Hc = –394 – 572 + 239 (3m) – Hc = –727 kJ mol–1 (3m)

(d) Dehydrocyclisation and isomerisation are used to improve the octane rating of a fuel. Explain the underlined terms. (8)

Dehydrocyclisation (4m)

– conversion of straight-chained hydrocarbons to ring compounds / ring compounds to aromatic compounds. Hydrogen is also lost

Isomerisation (4m)

– changing straight-chained hydrocarbons into branched-chained isomers


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(a) Infra-red spectrometry and ultra-violet spectrometry are widely used instrumental techniques in chemistry.

(i) Give one application of each of these techniques. (6)

Infra-red spectrometry

Any 1: (3m) – identification of plastics // – identification of drugs // – identification of functional groups // – to back up evidence for prosecutions arising from breathalyser tests // etc.


Ultra-violet spectrometry

Any 1: (3m) – quantitative determination of drug metabolites // – quantitative determination of transition metals // – quantitative determination of plant pigments // etc.


(ii) What are the main principles on which each of these techniques is based? (12)

Infra-red spectrometry (2 × 3m)

– molecules of a substance absorb infra-red light of different frequencies – the combination of frequencies absorbed is unique to the molecules of that substance

Ultra-violet spectrometry (2 × 3m)

– absorption of UV radiation by molecules results in the promotion of electrons from their ground state energy levels to higher energy states

– absorbance is directly proportional to concentration

** Accept other appropriate material.

(iii) Outline the processes that occur in infra-red spectrometry. (3 × 3m) (9)

– infra-red radiation passes through the sample – sample absorbs infra-red radiation at specific wavelengths, which are detected – an absorption spectrum is obtained


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(b) (i) What is the underlying principle of mass spectrometry? (5m) (5)

– positively charged ions are separated on the basis of their relative masses when moving in a magnetic field

(ii) Calculate, to two decimal places, the relative atomic mass of a sample of magnesium shown by mass spectrometry to be composed of 80% magnesium–24, 9% magnesium–25 and 11% magnesium–26. (3 × 3m) (9)

– 100 atoms = (80 × 24) + (9 × 25) + (11 × 26) – = 1920 + 225 + 286 = 2431 – 1 atom = 24.31

(c) State the type of chromatography most suited to: (9)

(i) the determination of blood alcohol levels, (3m)

– gas chromatography / GC

(ii) the detection of the presence of growth promoters in meat, (3m)

– high-performance liquid chromatography / HPLC

(iii) the separation of the components of food dyes.

Any 1: (3m) – paper chromatography // – thin-layer chromatography / TLC // – column chromatography // etc.


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8. (a) Benzene and methylbenzene are described as aromatic compounds. What is an aromatic compound? (3m) (3)

– (a compound that) contains a benzene ring

(b) Methylbenzene is often used in the laboratory in preference to benzene. Give a reason for this. (3)

Any 1: (3m) – benzene is toxic / a carcinogen // – methylbenzene is a good industrial solvent

(c) Draw diagrams to represent the molecular structure of (i) benzene and (ii) methylbenzene. (6)

(i) benzene (3m)

–

(ii) methylbenzene (3m)

–

(d) Is benzene soluble in water? (2m) (5)

– no

Give a reason for your answer. (3m)

– it is non-polar and cannot form hydrogen bonds with water


CH3

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(e) Give the IUPAC names for compounds A and B. (2 × 3m) (6)

OH

H

H

C

H

H

CH

H

H H

H

C C

A B

compound A – ethene compound B – ethanol

(f) (i) Name the reagent and catalyst needed for the reduction of propanal. (2 × 3m) (9)

reagent – hydrogen catalyst – nickel

(ii) Identify the product of this reaction. (3m)

– propan-1-ol

(g) (i) Describe the mechanism for the monochlorination of methane. (12)

Initiation (3m)

– Cl2 → Cl● + Cl●

Propagation (2 × 3m)

– CH4 + Cl● → HCl + CH3●

– CH3● + Cl2 → CH3Cl + Cl●

Termination (3m)

– Cl● + Cl● → Cl2 – CH3

● + CH3● → C3H6

– CH3● + Cl● → CH3Cl

(ii) Give two pieces of experimental evidence to support this mechanism. (6)

Any 2: (2 × 3m) – promoted by UV light at room temperature // – for every photon absorbed, many chloromethane molecules are formed // – ethane is formed // – adding a source of free radicals (tetramethyl lead) speeds up the reaction // – adding inhibitors (e.g. oxygen) slows down the reaction // – no H2 produced // – HCl produced // etc.


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9. (a) Define (i) an acid and (ii) a weak base according to the Brønsted-Lowry theory. (6)

(i) an acid (3m)

– a proton donor

(ii) a weak base (3m)

– a poor proton acceptor

(b) Identify the conjugate pairs in the following system: (6)

HClO4 + CH3COOH 23COOHCH +

4ClO

** Acid or base identified (3m), Conjugate base or acid identified (3m). Any 1: (2 × 3m)

– HClO4 4ClO

(acid) (conjugate base) //

– CH3COOH 23COOHCH

(base) (conjugate acid)

(c) A bottle of vinegar is labelled 4.8% (w/v) acetic acid (ethanoic acid, CH3COOH). The dissociation constant (Ka), for ethanoic acid is 1.8 × 10–5.

(i) Calculate the pH of the vinegar. (4 × 3m) (12)

– 4.8% (w/v) = 4.8 g in 100 cm3 = 48 g l–1

– 60

48 = 0.8 mol l–1

– [H+]2 = ]H[

8.0108.1 5

=

]0038.0[

8.0108.1 5

– pH = –log10 [H+] = –log10 8.0108.1 5 = 2.42

or

– 4.8% (w/v) = 4.8 g in 100 cm3 = 48 g l–1

– 60

48 = 0.8 mol l–1

– [H+] = MK a = 8.0108.1 5 = 0.0038

– pH = –log10 [H+] = –log10(0.0038) = 2.42

(ii) Find the concentration of a hydrochloric acid (HCl) solution that has the same pH as the vinegar. (3m) (3)

same pH = same [H+] HCl → H+ (monobasic and strong) – [HCl] = 3.8 × 10–3 mol/l


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(d) The free chlorine present in swimming pool water can be measured using a colorimeter or a comparator.

(i) What is free chlorine? (5m) (5)

– chlorine that exists in water as hypochlorous acid (HOCl) and hypochlorite ions (ClO–)

(ii) What is the general principle of all colorimetric and comparator techniques? (2 × 3m) (6)

– intensity (depth) of colour / absorbance / transmittance – is proportional to (varies directly with, directly related to, ) concentration

(iii) Describe briefly how you would estimate the concentration of free chlorine in a water sample using either a comparator or a colorimeter. (12)

Any 1: (4 × 3m) Colorimeter

– prepare standard solutions – place in the colorimeter and note the readings (absorbance/transmittance) – plot readings (absorbance/transmittance) vs. concentration – get concentration from the graph or curve

or

Comparator

– add reagent (DPD-1) to the sample of water – colour develops as DPD-1 reacts with the free chlorine in the water to give a pink-

coloured solution – compare with chart - which has been previously calibrated using solutions containing

known quantities of chlorine – best match gives concentration in p.p.m.


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10. Answer any two of the parts (a), (b) and (c). (2 × 25)

(a) (i) What is a catalyst? (4m) (4)

– (a catalyst is) a substance that alters the rate of a chemical reaction but is not consumed in the reaction

(ii) A catalytic converter is an engine component in every modern car. Name one element that acts as a catalyst in a catalytic converter. (3)

Any 1: (3m) – palladium // – platinum // – rhodium

(iii) What type of catalysis occurs in a catalytic converter? (6m) (6)

– heterogenous (catalysis)

(iv) Identify one reaction which is catalysed in the catalytic converter in a car. (12)

State one of the benefits to the environment of this reaction.

** Benefit should correspond to the reaction given. Any 1: (4m + 4m + 4m) Reactant Product Benefit to environment

– carbon monoxide – carbon dioxide – removes toxic CO gas

– hydrocarbons – carbon dioxide Any 1: and water – prevents smog // – reduces greenhouse effect

– nitrogen monoxide – nitrogen Any 1: – reduces acid rain // – removes toxic NO gas

** Accept equations (unbalanced) to illustrate product and reactants. ** Environmental benefit must be a primary effect or a specific and valid secondary

effect, e.g. respiratory problems (hydrocarbons), killing of fish (NOx). ** Do not accept general statements such as ‘prevents pollution’ or ‘harmful gases’. ** Award 4m for ‘prevents air pollution’ in all three cases.


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(b) In industry, ammonia may be obtained from the reaction of nitrogen and hydrogen.

N2(g) + 3H2(g) 2NH3(g)

The forward reaction is exothermic.

(i) Use Le Châtelier’s principle to predict the effect of (i) increasing the pressure, (ii) increasing the temperature on the equilibrium reaction. (6)

(i) increasing the pressure,

Any 1: (3m) – equilibrium shifts to the right / favours the forward reaction / exothermic reaction // – favours the side of the reaction with fewer moles / molecules // – equilibrium shifts to the right side - more ammonia / NH3 produced

(ii) increasing the temperature

Any 1: (3m) – favours the backward reaction / endothermic reaction // – favours the side of the reaction that absorbs the excess heat added // – equilibrium shifts to the left side - more nitrogen / N2 and hydrogen / H2 produced

In an experiment, 56 g of nitrogen and 12 g of hydrogen were reacted in a 10 litre container and reached equilibrium with ammonia at a certain temperature according to the balanced equation above. At equilibrium, 34 g of ammonia was present.

(ii) Write the equilibrium constant (Kc) expression for the above reaction. (6m) (6)

– 3

22

23

c]][H[N

][NHK

** Top correct (3m). ** Bottom correct (3m).

(iii) Calculate the value of Kc for this reaction under these conditions. (13)

Equation N2 3H2 2NH3

Start (2m) 28

56 = 2 moles

2

12 = 6 moles 0 moles

Change (3m) –1 –3 +2

At equilibrium (2m) 1 mole 3 moles 17

34 = 2 moles

Equilibrium concentration (3m) 10

1 = 0.1

10

3 = 0.3

10

2 = 0.2

– Kc = 3

2

)3.0)(1.0(

)2.0( =

0027.0

04.0 = 14.8 M–2 (3m)


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(c) (i) Explain oxidation in terms of electron transfer. (4m) (4)

– (oxidation is) the loss of electrons

A solution of potassium iodide, to which phenolphthalein indicator had been added, was electrolysed using inert electrodes.

(ii) Suggest a suitable material for the electrodes. (3)

Any 1: (3m) – graphite // – platinum

(iii) Write a balanced equation to explain the reaction that takes place at the cathode. (6m) (6)

– 2H2O + 2e– → H2 + 2OH–


(iv) What colour change was observed at the cathode? (2 × 3m) (6)

– (phenolphthalein turns from) colourless – to pink

(v) Write a balanced equation to explain the reaction which occurs at the anode. (6m) (6)

– 2I– → I2 + 2e–



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11. Answer any two of the parts (a), (b) and (c). (2 × 25)

(a) Describe a test that could be carried out to confirm the presence of nitrate ions in aqueous solution. (3 × 3m) (9)

– mix nitrate solution with freshly prepared iron(II) sulfate / FeSO4 solution – slowly pour concentrated sulfuric acid / H2SO4 down the inside of the test-tube – a brown ring forms at the junction of the two liquids

Define energy level and explain how the line emission spectrum of hydrogen provides evidence for the existence of energy levels. (7)

Energy level (4m)

– a region of definite energy within an atom that electrons can occupy

Evidence for existence of energy levels provided by line emission spectrum of hydrogen (3m)

– energy emitted (hf, h) corresponds to (=) difference between the two energy levels (E2 – E1), thus giving rise to a line on the spectrum / E2 – E1 = hf (h)

Describe a test that could be carried out to distinguish between the nitrate salts of barium (Ba(NO3)2) and potassium (KNO3). (9)

Any 1: (3 × 3m)

Method 1

– clean a platinum (nichrome) wire (rod, probe) in concentrated hydrochloric acid / HCl – dip wire in salt, then hold it in the hot (blue) part of a Bunsen flame – note the colour of the flame produced: barium – yellow / green potassium – lilac

Method 2

– use damp (wet) wood (splint, stick) – dip splint /stick in salt, then hold it in the hot (blue) part of a Bunsen flame – note the colour of the flame produced: barium – yellow / green potassium – lilac

Method 3

– prepare a solution of the given salt in water and ethanol (propanol) – spray solution onto (into) the hot (blue) part of a Bunsen flame – note the colour of the flame produced: barium – yellow / green potassium – lilac


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(b) A sample of benzoic acid was purified by dissolving it in the minimum amount of hot water and filtering. The resulting filtrate was allowed to cool.

(i) Why was it important to use the minimum amount of hot water? (2 × 3m) (6)

– to minimise the amount of benzoic acid dissolved at crystallisation – to maximise the yield of benzoic acid

(ii) What was the purpose of hot filtration? (3m) (3)

– to remove insoluble impurities

(iii) Give one precaution that should be taken to reduce recrystallisation in the filter paper. (3)

Any 1: (3m) – filter quickly under suction (Büchner funnel) // – (use a dropper) to add hot water // – heat filter funnel // – use a ‘stemless’ filter funnel // – flute the filter paper

(iv) What was observed as the filtrate cooled? (3m) (3)

– crystals formed

(v) How would you determine that this sample is a pure sample of benzoic acid? (2 × 3m) (6)

– measure the melting point (of the sample) – (the) melting point (of the sample) should be sharp

(vi) State one use of benzoic acid or its salts. (4)

Any 1: (4m) – food preservative // – disinfectant // – fungicide // etc.



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(c) Answer part A or part B.

A

What is meant by atmospheric pollution? (4m) (4)

– the addition of any damaging substance to the environment / that causes health risks

The pH of acid rain can range between 2 and 5. Name the two main gases that give rise to acid rain. (2 × 3m) (6)

– sulfur dioxide – nitrogen dioxide

Describe, using balanced equations, how one of these gases reacts to form acid rain. (9)

Any 1: (3 × 3m)

Sulfur dioxide

– S + O2 → SO2 – 2SO2 + O2 → 2SO3 – SO3 + H2O → H2SO4

Nitrogen dioxide

– N2 + O2 → 2NO – 2NO + O2 → 2NO2 – 2NO2 + H2O → HNO2 + HNO3

Outline two negative effects of acid rain on the environment. (6)

Any 2: (2 × 3m) – corrodes metal and stonework in buildings // – reduces crop yields // – prevents pollination // – damages / kills trees // – leaches metals from soils // – leaches metals into watercourses // – leads to fish kills in rivers and lakes // etc.



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11. (c) (cont’d.)

B

(i) Name a common ore of iron. (4)

Any 1: (4m) – hematite // – magnetite

(ii) Write the balanced equation for the reduction of ore to iron. (6m) (6)

– Fe2O3 + 3C → 2Fe + 3CO


(iii) Write a balanced equation for the formation of calcium silicate (a component of slag) from calcium oxide in steelmaking. (6m) (6)

– CaO + SiO2 → CaSiO3


(iv) What term is used to describe the iron leaving the blast furnace? (5)

Any 1: (5m) – pig iron // – cast iron

(v) In the making of steel, iron is purified using pure oxygen and then alloyed with a non-metal. Name this non-metal. (4m) (4)

– carbon


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2018.2 L.33-34 ms -11 ha-final

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