Igcse Chemistry Sample Chapter

7/28/2019 Igcse Chemistry Sample Chapter

1/20

Chemistryfor

IGCSE

Roger Norris Series Editor:Roger Standbridge Lawrie Ryan

Sample Chapter


2/20


3/20


4/20

Unit 6 Electricity and chemistry 70

6.1 Electrolysis (1) 706.2 Electrolysis (2) 72

6.3 Explaining electrolysis 746.4 Purifying copper 766.5 Electroplating 786.6 Extracting aluminium 806.7 Conductors and insulators 82

Summary / Exam-style questions 84

Unit 7 Chemical changes 86

7.1 Heat changes in reactions 867.2 Fuels and energy production 887.3 Energy from electrochemical cells 907.4 Fuel cells 92


Unit 8 Speed of reaction 96

8.1 Speed of reaction 968.2 Interpreting data 988.3 Surfaces and reaction speed 1008.4 Concentration and reaction speed 1028.5 Temperature and reaction speed 1048.6 How light affects reactions 106


Unit 9 Chemical changes 110

9.1 Reversible reactions 1109.2 Changing the equilibrium 1129.3 Redox reactions 1149.4 Oxidation numbers 116


Unit 10 Acids, bases and salts 120

10.1 How acidic? 12010.2 Properties of acids 12210.3 Bases 12410.4 More about acids and bases 12610.5 Oxides 128


Unit 11 Making and identifying salts 132

11.1 Making salts (1) 13211.2 Making salts (2): Titration 13411.3 Making salts (3) 13611.4 Whats that gas? 13811.5 Testing for cations 14011.6 Testing for anions 142


Introduction 1

Unit 1 Particles and purification 2

1.1 Solids, liquids and gases 21.2 Diffusion 41.3 Apparatus for measuring 61.4 Chromatography 81.5 Is that chemical pure? 101.6 Methods of purification (1) 121.7 Methods of purification (2) 14


Unit 2 Atoms, elements and compounds 18

2.1 Inside the atom 182.2 Isotopes 20

2.3 Atomic structure and Periodic Table 222.4 Elements, compounds and mixtures 242.5 Metals and non-metals 26


Unit 3 Structure and bonding 30

3.1 Ionic bonding 303.2 Covalent bonding (1): simple molecules 323.3 Covalent bonding (1): more complex

molecules 343.4 Ionic or covalent? 363.5 Giant covalent structures 38

3.6 Metallic bonding 40 Summary / Exam-style questions 42

Unit 4 Formuale and equations 44

4.1 Chemical formaulae 444.2 Working out the formula 464.3 Chemical equations 484.4 More about equations 50

4.5 Reacting masses 52 Summary / Exam-style questions 54

Unit 5 Chemical calculations 56

5.1 Chemical calculations 565.2 How much product? 585.3 Using gas volumes 605.4 Yield and purity 625.5 More chemical calculations 645.6 Titrations 66


1

Contents

3


5/20


6/2096

Some reactions, such as an old car rusting, are very slow. Otherreactions are very fast. When you mix solutions of silver nitrate andpotassium iodide, you get a yellow precipitate straight away.

The speed of a reaction usually called rate of reaction tells ushow fast the products are formed from the reactants. Knowing howfast a reaction goes helps chemical companies make their products asquickly and as cheaply as possible.

Following the progress of a reaction

To find the rate of reaction we can either:

measure how quickly the reactants are used up measure how quickly the products are formed.

Calculating rate of reaction depends on measuring something whichchanges with time, for example: volume of gas, mass of the reactionmixture or amount of light transmitted through a solution.

PRACTICAL

Following change in mass

You can use this method for reactions which give off a gas which

is allowed to escape. As the reaction takes place the mass of thereaction mixture decreases. Record the mass at intervals of time.The loss of mass is equal to the mass of gas given off. When thetotal mass is plotted against time you get a graph like the oneshown here.

Modern balances can be attached to a data logger and computerso that loss in mass can be recorded continuously.

LEARNING OBJECTIVES

1 To describe a practicalmethod for measuring speedof reaction where a gas isproduced

2 To understand that speed ofreaction involves quantity ofmaterial and time

3 To know how to investigatethe effect of a given variable

on the speed of a reaction

EXAMINERSAYS

Rememberthatrateofreactiondependsontwothings:(1)thechangeinamountorconcentrationofreactantorproductand(2)thetimetakenforthischange.

We run at different rates. The chemical

reactions in our bodies supply our muscles

with food at the rate needed.

8 Speed of reaction

8.1 Rate of reaction

Extension

Marble chips andhydrochloric acid

Cotton woolbung

Conical flask

Top-panbalance

Time

Massofreactingmixture


7/2097

PRACTICAL

Following change in the volume of gas given off

If a gas is given off in a reaction, you can collect it in a gassyringe. The volume of gas is recorded at intervals of time.

You can draw a graph of volume of gas against time.

DID YOU KNOW?

Some reactions can be followedby recording tiny changes inthe volume of a solution whichtakes during some chemical

reactions.

KEY POINTS

1 We can follow the progressof a chemical reaction bymeasuring how fast thereactants are used up or howfast the products are formed.

2 We can use change in volumeof gas, loss of mass ofreactant or the time taken fora precipitate to make a markdisappear to measure the

rate of reaction

SUMMARY QUESTIONS

1 Sketch a graph to show how the volume of gas changes withtime for the reaction

Mg + 2HCl MgCl2

+ H2

2 Put the following in order of increasing speed of reaction

cement setting firework exploding iron rusting

A variety of methods

There are many methods for measuring rate of reaction. You can useany property which changes during a reaction. You could use a pHmeter or electrical conductivity meter if the hydrogen ions are used

up in the reaction. You could also record changes in pressure forreactions involving only gases.

Extension

Time

Volumeofgasproduced

Marble chips andhydrochloric acid

Rubber tubing

Gassyringe

`ii

/i

v}>i`L

PRACTICAL

Following the progress of a precipitation reaction

If a precipitate is one of the products in a reaction, thesolution goes cloudy. We can follow the reaction by:

placing the paper with a letter A on it underneath theflask containing one of the reactants

starting the reaction by adding the other reactantand starting the clock

recording the time taken for the A to disappear.

We can also measure the amount of light passingthrough the solution. We do this by using a lightmeter attached to a data logger and computer.This gives a graph as shown.


8/2098

Figure 8.2.2

Constants and variables

Many factors such as temperature, concentration of reagent andparticle size can affect the rate of reaction. How can you comparethe results of a series of experiments in a fair way? You need tokeep some things the same (controlled variables) and only changethe one that you are interested in (the continuous variable). If youwant to find out how the concentration of hydrochloric acid affectsthe rate of reaction of the acid with calcium carbonate you haveto keep the temperature and particle size of the calcium carbonatethe same in each experiment. You only change the concentrationof the hydrochloric acid and record the time taken to get a certainvolume of gas. Since you decide to when to take the time, the timeis called the independent variable. And the volume of carbon dioxide

produced is called the dependent variable because it depends on thetime you selected.

Calculating rate of reaction

In a reaction such as

Mg + 2HCl MgCl2

+ H2

the time taken for the magnesium to disappear completely can beused as a measure of the rate of reaction. The reaction rate can beworked out by using the equation:

rate of reaction = amount of product formed or reactant used uptime

A more accurate definition of rate of reaction is

rate = change in concentration of reactant or product

time

However, we often need to know how the reaction rate changes asthe reaction proceeds. If we look at how the volume of gas given offin a reaction changes with time we see that the reaction is fastestnear the start but then gets slower and slower until it finally stops.The gradient (slope) of the graph gives us the reaction rate at any

particular time.

If we look at a graph of loss of mass of the reaction mixture againsttime we can see a similar thing happening.

From these graphs we can find out:

how long it takes for a reaction to produce a given volume of gas

the volume of gas produced in a given time.

8.2 Interpreting data

LEARNING OBJECTIVES

1 To be able to interpret graphsof volume of gas producedagainst time

2 To be able to interpret graphsof mass of reactant changingwith time

3 To be able to calculate speedof reaction

4 To be able to calculate whichreagent is limiting in a reaction

Reaction stopsafter60s

Volumeofgas/cm3

Time/s

40

30

20

10

00 20 40 60 80

35cm3 gas produced

After 37s, 30cm3

of gas produced

Extension

Figure 8.2.1

No furtherreaction

Reaction is slowerwhere gradient isless steep

Reaction is faster wheregradient is steeper

Volumeof gas

Time

xgives the ratey

x

y

Time

Massesofreactingmixture

Reaction fast

Reaction slow


9/2099

EXAMINERSAYS

Youshouldalsobeabletocalculatetherateofreactionformthesegraphs.

EXAMINERSAYS

Makesurethatyouknowhowtointerpretthedifferentpartsofagraphofvolumeofgasreleasedorlossinmassofthereactantsagainsttime.

DID YOU KNOW?

The time taken for one form ofaspartic acid to change into adifferent form of aspartic acid infossil bones is 1 00 000 years!

KEY POINTS

1 Rate of reaction is calculatedby dividing change in theamount of reactant orproduct by time.

2 As a reaction proceeds therate of reaction decreases asone or more of the reactantsgets used up

3 A reaction stops when thelimiting reactant is completelyused up.

SUMMARY QUESTIONS

1 Sketch a graph to show how the mass off the reaction mixturechanges with time for the reaction:

MgCO3

+ 2HCl MgCl2

+ CO2

+ H2O

On your graph show (i) where the reaction had just stopped(ii) where the reaction is fastest (iii) where the reaction isvery slow

2 a student adds some pieces of calcium carbonate to some acid.At the start of the reaction many bubbles are seen. After 10minutes some calcium carbonate is still present but the bubbleshave stopped. Which reactant is in excess? Explain your answer.

3 Copy and complete using words from the list:

fast limiting slows stops

When excess calcium carbonate reacts with hydrochloric acid,the reaction is very at first but then down untilit completely. The reaction stops because the

reactant has been completely used up.

Extensio

n

Extension

Limiting reactants

When carrying out an experiment we sometimes use an excess ofone of the reactants. The reactant that is not in excess is called thelimiting reactant. The reaction stops when the limiting reactant iscompletely used up. You can work out which reactant is limitingby calculating which reactant has the least number of moles for

reaction. You must also take into account the mole ratio of thereactants in the equation.

Example: In the reaction Mg + 2HCl MgCl2

+ H2

If 2.4 g of magnesium react with 50 cm3 of2 mol/dm3 hydrochloric acid

moles of Mg = 2.4/24 = 0.1 mol;moles of HCl = 2 x 50/1000 = 0.1 mol

The limiting reactant is hydrochloric acid because if we look at theequation: for every mole of magnesium used we need 2 moles of

hydrochloric acid. So there is still 0.1/2 = 0.05 mol of magnesiumleft when the hydrochloric acid has been used up.

The graph shows how thevolume of gas changeswhen hydrochloric acidis the limiting reactantat several differentconcentrations.

Figure 8.2.3

6iv}>

/i

x`>V`

`>V`

`>V`


10/20100

8.3 Surfaces and reactionspeed

Surface area and rate of reaction

If you want to make a fire quickly, you are more likely to succeed ifyou try to light small, thin pieces of wood rather than large pieces.We see a similar effect when solids react with solutions. A large lumpof marble reacts slowly with hydrochloric acid but powdered marblereacts very quickly. Why is this?

The rate of reaction depends on how often the particles of acid collidewith the particles on the surface of the marble. The greater the surfacearea of the marble, the more particles there are available to react. If wecut up the marble into smaller pieces, the surface area and the numberof particles of marble which can react are both increased.

LEARNING OUTCOMES

Describe the effect of particlesize and catalysts on thespeeds of reactions

Explain why fine powders cancause explosions e.g. in flourmills and in mines

EXAMINERSAYS

Itisacommonerrortothinkthatlargerparticleshavealargersurfaceareathansmallerones.Thinkofalargecubecutupbycutting,youareexposingmoresurfaces.

Figure 8.3.1

cut into eight smaller piecesSurface area 48cm3

Large piece of marbleSurface area 24cm3

2cmMore surface exposed

2cm

2cm

PRACTICAL

Investigating the effect of surface area

Marble is a form of calcium carbonate. You can investigate theeffect of increasing surface area by reacting different sized marblechips with hydrochloric acid. You can carry out the reaction eitherby measuring the loss in mass as carbon dioxide is released or byrecording the volume of carbon dioxide given off.

Figure 8.3.2

Marble chipsand hydrochloricacid

Cotton woolbung

Conical flask

Top-panbalanceTime

Massofreactingmixture


11/20101

Explosive reactions

Many industrial processes cause fine powders to get into the air.These powders are highly combustible. They burn very readily in airbecause of their very large surface area. A lit match or a spark froma machine can cause them to explode. Examples are flour fromflour mills, wood dust from sawmills and coal dust in coal mines. In

coal mines there is another hazard. The methane gas which is oftenpresent can form an explosive mixture with air.

PRACTICAL

Explosive milk!

If you try to set fire to a pile of milk powder, it will just go black onthe surface. If you sprinkle the milk powder onto a burning splintthe flame will suddenly flare up as the milk powder explodes.

Catalysis

A catalyst is a substance which speeds up a chemical reaction.The catalyst is not used up in the reaction it remains the samesubstance as it was at the start of the reaction.

A catalyst works by allowing the reactants to get close together on

its surface so that less energy is needed to get the reaction to occur.Catalysts are generally used in the form of pellets or wire gauzes. Thisgives them a large surface area for reaction.

We only need tiny amounts of catalyst to speed up the reaction butthey are often expensive. However they can be used over and overagain. They are important in speeding up the reactions in manyimportant industrial chemical processes. In this way chemicals canbe produced quicker and at a lower temperature compared with theuncatalysed reaction.

All living things contain particular types of catalysts called enzymes.These speed up all the chemical reactions in the body (see Unit 20.4).

DID YOU KNOW?

The earliest recorded explosionin a flour mill was in Italy in1785.

Catalytic converters are important in reducing

pollutant gases from cars.

SUMMARY QUESTIONS

1 Copy and complete using thewords below:

again increasesrate unchanged

A catalyst is a substancewhich theof a chemical reaction. Thecatalyst is at the endof the reaction, so it can beused .

2 Factories which cut metalsoften have special fans

to remove metal dust.Explain why metal dustin the atmosphere can bedangerous.

3 The inside of a catalyticconverter in a car hasthousands of tiny beadscoated with the catalyst.Explain why these beads areused rather than having largelumps of catalyst.

Figure 8.3.3

Pile of milkpowder

Fine particlesof milk powder

Wooden splint


12/20102

Extension

8.4 Concentration and rateof reaction

How concentration affects the rate of reaction

The Taj Mahal, a beautiful building in India, is being eroded away!Over recent years the concentration of acids in rainwater all overthe world has been steadily increasing. The acids in the air reactwith the marble and damage the surface of the building. The higherthe concentration of acid in the air, the quicker a building made ofmarble or limestone will react.

LEARNING OUTCOMES

Describe how concentrationaffects rate of reaction

Describe and explain howconcentration affects therate of reaction in terms ofcollisions between reactingparticles

Extension

DID YOU KNOW?

Cars with steel bodies arelikely to rust faster if you livenear the sea because there is ahigher concentration of salt inthe atmosphere and rainwater.

PRACTICAL

How changing the concentration of acid affects reactionrate

The rate of reaction of marble chips (calcium carbonate) iscarried out with hydrochloric acid of different concentrations.

CaCO3

+ 2HCl CaCl2

+ CO2

+ H2O

We can follow the reactionby measuring the increasein volume of carbon dioxidegiven off or by measuringthe decrease in mass ofthe reaction mixture. Thetemperature, mass and sizeof marble chips are kept

the same as well as thevolume of hydrochloric acidused. The concentration ofhydrochloric acid is variedbut it is always in excess.

For each concentration ofhydrochloric acid used,record the volume of gas attime intervals.

A graph of the results isshown.

You can see that as the concentration of acid increases, the rate ofreaction increases. The final volume of carbon dioxide released is thesame in each experiment because hydrochloric acid was in excess.

Using the collision theory

A concentrated solution has more particles of solute per unit volumethan a dilute solution. In the reaction between calcium carbonate andhydrochloric acid, the important solute particles are the hydrogen

ions in the hydrochloric acid (see Unit 10.4).

Gas syringe

Rubber bung

Hydrochloric acid

Marblechips

Deliverytube

6ivV>L``i

/i

>V`-}iVVi>

x`>V`-iVVi>

>V`

Limestone statues are damaged by acid rain.

This damage increases as the concentration

of the acids in rainwater increases.


13/20103

EXAMINERSAYS

Whenexplainingtheeffectofconcentrationonreactionratedontjustrefertomorecollisionsoftheparticles.Itisthemorefrequentcollisionoftheparticleswhichisimportant.

EXAMINERSAYS

Rememberthatincreasingtheconcentrationofareactanthasnoeffe

ctontheforcewithwhichtheparticleshiteachother.

A reaction occurs when particles with enough energy collide. Themore concentrated the hydrochloric acid, the more hydrogen ionsthere are in a given volume to collide and react with the carbonateparticles in the calcium carbonate. The rate of reaction depends onthe number of successful collisions per second. If there are morecollisions per second, the rate of reaction is faster.

KEY POINTS

1 Increasing the concentrationof reactants increases therate of reaction.

2 Increasing the concentrationof reactants increases thefrequency of collision of theparticles and so increasesreaction rate.

SUMMARY QUESTIONS

1 Copy and complete using words from the list below:

concentration increases rate

The of a chemical reaction when the ofone or more of the reactants increases.

2 a When barium carbonate reacts with hydrochloric acid,carbon dioxide gas is given off. Sketch a graph to showhow the mass of the reaction mixture changes using dilutehydrochloric acid.

b On the same graph axes sketch the line you would expect forthe same experiment but using hydrochloric acid of half theconcentration.


collide collision concentrated frequently unit

The effect of increasing reactant concentration on the rateof reaction can be explained by the theory. When thereactants are more there are more reactant particlespresent per volume so they more .

4 Use ideas from the collision theory to suggest why a reactionslows down as time goes on.

Figure 8.4.3

Calciumcarbonateparticles

Water molecules not shown

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

High concentration of acid

Many acid particles (H+ ions) per dm3.

More chance of particles colliding

and reacting.The frequency of collisions is high.

Low concentration of acid

Few acid particles (H+ ions) per dm3.

Less chance of particles colliding

and reacting.The frequency of collisions is low.

In a reaction involving gases, increasing the pressure has a similareffect to the increase in concentration. Increasing the pressurepushes the particles closer together so that they collide with a greaterfrequency and the rate of reaction increases.

Extension


14/20104

8.5 Temperature and rateof reaction

How temperature affects rate of reaction

We can change the rate of a chemical reaction by heating or coolingthe reaction mixture. Food goes mouldy quicker when left out of therefrigerator. This is because the reactions which make food rot arefaster at higher temperatures. Some surgical operations are carriedout below room temperature to slow down the chemical reactions inthe body.

LEARNING OUTCOMES

Describe how temperatureaffects the rate of reaction

Describe and explain howtemperature affects thespeed of reaction in terms ofcollisions between reactantparticles

Extension

DID YOU KNOW?

Food cooks quicker at a highertemperature because theparticles have more energy andthere are more collisions persecond between the moleculesin the food.

You can plot a graph of the time taken for the X to disappearagainst temperature. The shorter the time taken for the X todisappear, the faster the reaction is. This is because rate of reactionis proportional to amount of product formed (or reactant used)divided by time. If you look at the graph carefully, you can see thatfor every 10 C rise in temperature, the rate of reaction approximatelydoubles.

Moving faster means its more likely that

youll bump into someone else and the

bump will be harder too!

PRACTICAL

Investigating the effect of temperature on reaction rate

When we react sodium thiosulfate with hydrochloric acid aprecipitate of sulfur is formed.

You place the flask of sodium thiosulfate on top of the cross onthe paper. You add hydrochloric acid and record the time takenfor the letter X to disappear. Repeat the experiment at different

temperatures using the same amounts of acid and thiosulfateheated up separately before mixing them together.

Sodium thiosulfatesolution and dilutehydrochloric acid

Time how long it takes forthe A to disappear whenviewed from above

Figure 8.5.2

Temperature

Timetakenforsolution

togocloudy


15/20105

EXAMINERSAYS

Notethatasthetemperatureincreases,eachparticlecollideswithagreaterforce.Itisalsomoreaccuratetowritethattherearemorefrequentcollisionsthanjustmorecollisions.

EXAMINERSAYS

1Whenwritinganswerstoquestionsaboutratesofreaction

itisimportanttousewordslikefasterorslowernot

justfastorslow.

KEY POINTS

1 The higher the temperaturethe greater the rate of reaction.

2 The rate of reaction increaseswith increase in temperaturebecause particles have moreenergy so they move fasterand collide more frequently.The collisions that take placeare also more energetic soare more likely to result in areaction taking place.

SUMMARY QUESTIONS


longer rate slower time

The the taken for a reaction to be complete thethe of reaction.

2 A student followed the rate of reaction of calcium carbonatewith hydrochloric acid at different temperatures. At each

temperature she recorded the volume of gas produced 30seconds from the start of the reaction. Sketch a graph to showhow the volume of gas changes with temperature.


effective energy faster increases minimum more

When the temperature of a reaction mixture is increased theparticles move because the of the particlesincreases. At a higher temperature there are also particleswith the amount of energy to react when they collide. Sothe collisions are more and the rate of reaction .

Using the collision theory

There are two reasons we can use to explain why increasing thetemperature increases the rate of reaction. The second of these is themost important.

1. When we heat up a reaction mixture, the particles gain energy.

When particles gain energy they move faster and collide moreoften. The frequency of collisions is increased. This results in anincreased rate of reaction.

2. In order to react, particles must collide with a minimum amountof energy. This is called the activation energy. As the temperaturegets higher more and more particles have this minimum amountof energy to react when they collide. In other words, as thetemperature is increased there is more chance of a collision betweenthe reactant particles being successful. We say that the number ofeffective collisions increases as the temperature increases.

Extension

Figure 8.5.3

Lower temperature.Particles have less energy.They move more slowly and

collide less frequently.The collisions are not very effective.

Higher temperature.Particles have more energy.They move faster and collide

more frequently.The collisions are very effective.

Extension

Extension


16/20106

EXAMINERSAYS

Itisimportanttorealisethatlightonlyaffectsafewreactions.Theonlyonesyouhavetoknowaboutarephotosynthesis,theconversionofsilverbromidetosilverandthereactionofhalogenswithchlorine(seeUnit18.1)

A few chemical reactions are started by ultraviolet or visiblelight. These are called photochemical reactions. We rely on the

photochemical reactions in plants to provide the oxygen webreathe and the food we eat as well as removing carbon dioxidefrom the atmosphere. The photochemical reaction in plants iscalled photosynthesis. Not all photochemical reactions are useful.Compounds called CFCs are broken down by ultraviolet light andthe products are responsible for the depletion of the ozone layeraround the Earth.

Photosynthesis

Plants use the energy from sunlight to make glucose. This process iscalled photosynthesis.

6CO2

+ 6H2O C

6H

12O

6+ 6O

2

carbon dioxide water glucose oxygen

The glucose is turned into macromolecules called starch and cellulose(see Unit 20.3)

Photosynthesis is catalysed by the green pigments in plants calledchlorophylls.

PRACTICAL

How light affects the rate of photosynthesisWhen light shines on the pondweed, bubbles of oxygen slowlyform on the leaves and then rise into the syringe. You can recordthe volume of oxygen collected over several days. Then you canrepeat the experiment using stronger (more intense) light. Thegraph shows how the rate of photosynthesis changes with lightintensity. The graph levels off because carbon dioxide becomeslimiting.

LEARNING OBJECTIVES

1 To describe the effect of lighton the rate of some reactions

2 To describe the use of silversalts in photography

3 To describe the process ofphotosynthesis in simpleterms

8.6 Light sensitivereactions

The plant on the left looks unhealthy

because it has not had enough light

Photo:Two plants grown with and

without light

Volumeofoxygen

Light intensity

Weak light

Strong light

Small glasssyringe

Water

Funnel

Pondweed

Lightsource


17/20107

Extension

Photography

The surface of black and white photographic film contains tinycrystals of silver bromide mixed with gelatine. When light shines onthe film the silver bromide is activated. Some of the silver bromidedecomposes (breaks down) to form silver.

2AgBr 2Ag + Br2silver bromide silver

(very pale cream) (black)

In this reaction the silver ions in the silver bromide accept electronsfrom the bromide ions and become silver atoms. This is a redoxreaction (see Unit 9.3). The silver ions get reduced because theyaccept electrons and the bromide ions get oxidised because they loseelectrons (see unit 9.3)

2Ag+ + 2e 2Ag (a reduction reaction)

2Br Br2

+ 2e (an oxidation reaction)

The parts of the film exposed to stronger light appear black and theparts not exposed appear white. The greater the intensity (strength) ofthe light the faster the reaction. A positive print is made by shining lightthrough the negative onto a piece of photographic paper behind.

DID YOU KNOW?

Some spectacle lenses darkenin bright light and get moretransparent in dim light. Thisis because light reacts with thesilver chloride in the lenses.

SUMMARY QUESTIONS

1 What do you understand by the following terms:a Photosynthesis

b reductionc photochemical reaction?

2 Copy and complete using words from the list.

black bromide crystals light particles silver

Photographic film contains tiny of silver Whenshines on the film the bromide breaks down toof silver which are in colour.

3 How does the rate of decomposition of silver bromide varywith the strength (intensity) of light?

PRACTICAL

A light sensitive reaction

Place a plastic disc on a piece of lightsensitive photographic paper in the dark.The paper and disc are then left in the

light. After a short time the paper willturn black but if you remove the discyou can still see a circle of white paper.This shows that light is needed for thereaction to happen. i> vi>vii

}ii>i

*>V`Vi``>i}

7i

>V

KEY POINTS

1 The rate of some reactionsis increased by increasingthe intensity of light. Thesereactions are photochemicalreactions.

2 Photosynthesis is a process bywhich plants change carbondioxide and water to glucoseand oxygen using chlorophyllas a catalyst

3 In the photographic reactionsilver bromide is decomposedto silver and bromine.


18/20108

SUMMARY QUESTIONS

1 Match the method for measuring reactionrate on the right with the type of reaction onthe left:

a purple solutionchanges to a

colourless solution

measure thevolume of

gas

hydrogen is releasedduring a reaction

electricalconductivity

two solutions reactslowly to form aprecipitate

measure thelight transmittedthrough a solution

there are more ionsin the reactantsthan in theproducts

seeing how longit takes for a letterA under a flask todisappear

2 Write definitions of the following:

(a) rate of reaction

(b) catalyst

(c) gradient

3 Sketch a graph to show how the masschanges in the reaction:

CaCO3(s) CaO(s) + CO

2(g)

4 Complete the following phrases:

(a) increasing the surface area of a solid

the rate of reaction.(b) As a reaction proceeds the of

reaction .

5 What effect does each of the following haveon the rate of reaction:

(a) diluting the reaction mixture

(b) using large lumps of solid rather than small

6 Use your knowledge of the kinetic particletheory to describe and explain how increasingthe pressure affects the rate of the following

reaction:

2N2O(g) 2N

2(g) + O

2(g)

7 Write definitions of:

(a) limiting reactant

(b) photochemical reaction

(c) photosynthesis

8 Us the kinetic particle theory to suggestwhy food cooks more quickly when thetemperature is higher.

EXAM-STYLE QUESTIONS

1 Which one of the following statements aboutcatalysts is true?

A They are always non metals

B They do not take part in chemical reactions

C They have no effect on rate of reaction

D Their mass remains unchanged at the endof the reaction

(Paper 1) [1]

1 The table shows the volume of oxygen givenoff when hydrogen peroxide decomposes at40 C in the presence of a catalyst

Time / s Volume of oxygen/ cm3

0 0

5 22

10 34

20 48

30 56

40 59

50 60

60 60

a) Plot a graph of the results with time onthe x axis and volume of oxygen on they axis. [3]

b) On the same axes sketch a curve forthe same reaction but carried out at atemperature of 50 C. [2]

c) What results would you expect if thecatalyst was not present? Explain youranswer. [2]

(Paper 2)

2 A student compared how well differentcompounds catalysed the reaction betweenzinc and hydrochloric acid. The results areshown in the table.

Compound Time taken for all

the zinc to react / s

No catalyst 500

Copper(II) sulfate 150

Copper(II) chloride 175

Manganese(IV) oxide 390

Sodium chloride 500

Sodium sulfate 500

E

xtension

108


19/20109

a) What do you understand by the termcatalyst? [1]

b) Which is the best catalyst for thisreaction? [1]

c) What things must you keep constant in

these experiments if it is to be a fair test? [3]d) Which compounds are not catalysts? [1]

(Paper 2)

3 Magnesium ribbon reacts with hydrochloricacid. The equation is

Mg + 2HCl MgCl2

+ H2

a) Suggest two methods that you could useto follow the progress of this reaction [4]

b) For one of these methods describe how

you can calculate the rate of reaction [3]c) Explain using the kinetic particle theory how

and why the rate of reaction changes when

(i) the concentration of hydrochloric acidis increased [3]

(ii) the temperature of the reactionmixture is lowered [3]

(iii) powdered magnesium is used ratherthan magnesium ribbon [3]

(Paper 3)

4 A student investigated the reaction between40 g calcium carbonate and 30 cm3 of 2.0 mol/dm3 hydrochloric acid.

CaCO3

+ 2HCl CaCl2

+ CO2

+ H2O

a) Draw a sketch graph to show how the lossin mass of the reaction mixture changeswith time. [2]

b) By referring to the shape of your graphyou have drawn in part (a) explain how therate of reaction decreases with time. [2]

c) Use the idea about colliding particles toexplain why the rate of reaction decreaseswith time. [3]

d) (i) Calculate the number of moles ofcalcium carbonate and hydrochloricacid at the start of the reaction. [2]

(ii) Was the hydrochloric acid or thecalcium carbonate the limitingreactant? Explain your answer. [2]

e) In another experiment the studentinvestigated how the rate of this reaction

varies with temperature. The results areshown in the table.

Temperature / C 20 30 40 50

Time taken for a piece

of calcium carbonate

to dissolve / s

64 32 16 8

(i) Draw a suitable graph to displaythese results [3]

(ii) From your graph predict how manyseconds it would take the piece ofcalcium carbonate to dissolveat 60 C. [1]

(Paper 3)

5 Crystals of silver bromide were brushed onto asheet of damp filter paper. A cardboard letter

H was placed

on top ofthe paper.

The filter paper and cardboard H was left inthe light for time. After a few hours the filter

Paper around the H had turned black butunderneath the H it was still white.

a) Explain these results. [4]

b) How can you make the filter paper go

black quicker? [1]c) Write an ionic equation to show what is

happening to the silver ions when thelight strikes the paper silver bromideon the paper. [1]

(Paper 3)

6 Zinc powder reacts with excess aqueous iodineto form zinc iodide.

Zn(s) + I2(aq) ZnI

2(aq)

grey solid brown colourless

solution solution

a) Describe a suitable method for followingthis reaction. [3]

b) Which is the limiting reagent in thereaction? [1]

c) Use the kinetic particle theory to explainhow and why the rate of reactionchanges when large pieces of zinc areused in place of zinc powder. [3]

(Paper 3)

>`L>ìi

i>i

LiìLì


20/20

IGCSE Science is a unique series o titles rom Nelson Thornes,

that match CIE specifcations and the needs o students and teachers

o the CIE 0610, 0620 and 0625 syllabuses.

A team o CIE IGCSE Principal Examiners has been instrumental in the

planning, writing and reviewing o the series. IGCSE Science has been

created specifcally or the international school market and incorporates

extensive market research among teachers in various parts o the world.

Also in the IGCSE Science series:

For urther inormation please contact

our International Sales TeamT: +44 (0) 1242 267369

F: +44 (0) 1242 267311

E: [email protected]

www.nelsonthornes.com/international

Preview Copy: this sample chapter will allow you to ully review specimen content o Chemistry

Igcse Chemistry Sample Chapter

Documents

Transcript of Igcse Chemistry Sample Chapter