NZIC

SCHOLARSHIP CHEMISTRY93102

2012

Time allowed: 3 hours

Marks: 48

Answer all questions

A periodic table is included at the back of this booklet

© New Zealand Institute of Chemistry 2013

QUESTION ONEPart AA complex ammine, Cu(NH3)nX2, was crystallised from an ammoniacal solution of a copper(II) halide. Given the following analytical data, determine the value of n. Include all relevant equations and show all working.

(a) 0.845 g of the solid was dissolved in 50 mL of water. Excess potassium iodide was added and the resultant solution was titrated with 0.1070 mol L–1 sodium thiosulfate. The average titre was 27.0 mL.

The equation for the reaction between copper ions and iodide ions is:

2Cu2+ + 4I– 2CuI + I2

(b) 0.1457 g of the solid was dissolved in aqueous potassium hydroxide contained in a distillation flask. This flask was heated and the ammonia was distilled into a flask containing 50.0 mL of 0.0740 mol L–1 hydrochloric acid. Back titration of the excess acid required 19.2 mL of 0.0885 mol L–1 sodium hydroxide.

Part B

Data: E(Cu2+/Cu) = +0.34 VE(Ag+/Ag) = +0.80 V

E(I2/I–) = +0.62 V

Ks(AgI) = 8 10–17

(a) Using the data above discuss whether the two reactions below are feasible or not.

(i) 2Ag+(aq) + Cu(s) 2Ag(s) + Cu2+(aq)

(ii) 2Ag+(aq) + 2I–(aq) 2Ag(s) + I2(aq)

(b) When 0.1 mol L–1 solutions of silver nitrate and potassium iodide are mixed no brown iodine is observed. Use the data above to explain this observation.

(c) Describe an experiment which would allow reaction (a) (ii) to proceed. A labelled diagram would be acceptable.

QUESTION TWO(a) On Titan, the largest moon of Saturn, nitrogen from the atmosphere and methane are the raw

materials in the moon’s chemical manufacturing plant. Among the substances formed are organic molecules HC4H and HC3N.

(i) How many valence electrons does each compound have?

(ii) Complete plausible Lewis diagrams (electron dot diagrams) for both molecules and comment on their shape. The atom arrangements are:

HC4H H C C C C H HC3N H C C C N

(b) Ethyl ethanoate can be prepared from ethyne as the only organic starting material. The first step involves partial hydrogenation to ethene. Devise a scheme that will successfully complete this synthesis. Write equations for all the reactions involved. Include non-organic reagents and reaction conditions.

(c) (i) Caprolactam has the structure shown and is used to make nylon. This is achieved by strongly heating caprolactam with water. Draw structural formulae for the initial product of hydrolysis and a section of the nylon polymer with at least three monomer units.

(ii) Nylon garments are more easily damaged by accidental spillage of acid than are Orlon garments. (Orlon is made by the polymerisation of acrylonitrile, CH2=CH–CN) Explain this difference in the behaviour of the two polymers.

(d) A colourless liquid contains 1-chlorobutane (CH3CH2CH2CH2Cl) and 1-aminobutane (CH3CH2CH2CH2NH2). Describe carefully a chemical method (not distillation) by which the two could be separated.

QUESTION THREEPart A

(a) Listed below are a number of physical or chemical processes with the relevant standard enthalpy changes. Complete the table by filling in the missing equations. Assume that all processes occur under standard conditions.

process equation Enthalpy valueformation of magnesium bromide

fHo = –524 kJ mol–1

sublimation of magnesium

subHo = +148 kJ mol–1

double ionisation of Mg Mg(g) → Mg2+(g) + 2e– Ho = +2187 kJ mol–1

vaporisation of Br2(l) vapHo = +31 kJ mol–1

dissociation of Br2(g) into isolated atoms

atHo = +193 kJ mol–1

lattice energy of magnesium bromide

MgBr2(s) → Mg2+(g) + 2Br–(g) latHo = +2421 kJ mol–1

Given the above information, calculate Ho for electron gain by bromine:Br(g) + e– → Br–(g)

(b) Would the value of Ho for electron gain by chlorine be more or less exothermic than the value for bromine? Justify your answer by discussing the factors involved.

Part B

Ultra-high-molecular-weight polyethylene (UHMWPE), and Kevlar are both polymers which can be spun into fibres which are much stronger than the same weight of steel. Kevlar is used in bullet proof vests and UHMWPE is used in rock-climbing equipment.

Both Kevlar and UHMWPE consist of very long molecules oriented parallel to each other. Molar masses are approximately as follows: Kevlar: 1 105 g mol–1; UHMWPE: 4 × 106 g mol–1.

Part of Kevlar’s repeating structure is shown below

Describe the nature of the interactions between the polymer chains in each material, and explain why they are unusually strong in these two materials

QUESTION FOUR

Part A

(a) A dilute aqueous solution contains Cl– ions and CrO42– ions in equal concentration. A small

volume of silver nitrate is added. What precipitate will form. Use Ks values to justify your answer. Ks(AgCl) = 2 10–10, Ks(Ag2CrO4) = 4 10–12

(b) Sodium chlorate is produced industrially by the electrolysis of a hot concentrated aqueous solution of sodium chloride. The anode and cathode are only 3 - 5 mm apart.

The key reactions are shown below. They all take place in aqueous solution and are written as they appear in the original scientific paper, e.g. H+ instead H3O+.

At the cathode: 2H2O + 2e– → O2 + 2OH–

At the anode the sequence of reactions is:

2Cl– → Cl2 + 2e–

Cl2 + H2O HClO + H+ + Cl–

HClO ClO– +H+ (Ka = 2.95 10–8)

2HClO + ClO– → ClO3– + 2Cl– + 2H+

(i) Determine the amount of chlorate ion formed in this reaction sequence per mole of electrons passing through the external circuit. Show your working.

(ii) In light of the reactions represented above, suggest a reason why the electrodes have to be so close together.

Part B

The following information may be used in answering both (a) and (b) sections of Part B.

reduction Eo / V conditions MnO4

– │ Mn2+ 1.49 in acidic solutionVO3

– │ VO2+ 1.00 VO2+ │ V3+ 0.34

SO42– │ SO3

2– 0.17V3+ │ V2+ –0.26

MnO4– │ MnO4

2– 0.56 in alkaline solutionO2 │ OH– 0.40

(a) When solid potassium hydroxide is added to potassium permanganate solution, it dissolves and the solution slowly changes colour from purple to green. Explain this colour change in terms of the electrode potentials and write equations for the reaction taking place and state the changes in oxidation states.

(b) VO3–(aq) is yellow; VO2+(aq) is blue; V3+(aq) is green; V2+(aq) is violet

In acid conditions, 22.1 mL of 0.0500 mol L–1 potassium permanganate solution is required to convert 20.0 mL of violet V2+(aq) to a yellow solution containing vanadium only as VO3

–(aq).

Write ion-electron half-equations and a balanced net ionic equation for the reaction taking place.

What volume of 0.0500 mol L–1 sulfite solution would now be required to convert the yellow solution entirely to blue VO2+(aq)?

If excess sodium sulfite solution was added, what would the final colour be? Justify your answer.

QUESTION FIVEThe letters V, W, X, Y and Z represent five elements. These letters bear no relationship to the symbols of the elements.V has the lowest atomic number of the set.The atomic number of Z is twice that of V.Z and Y are not in the same period.V, W and X are all in the same periodX and Y are in the same group of the Periodic Table.The elements form compounds with the formula AX3 where A can be V, W, Y or Z

(a) Identify the elements and the compounds represented by the four formulae, AX3 and explain your decisions.

The elements V, W, Y and Z form a second compound with X each with a different formula.

(b) (i) VX3 forms a dimer under certain conditions. In terms of electron arrangements explain why this should happen.

(ii) For the two compounds of W and X describe the differences in molecular structure, shape and polarity.

(iii) For the two compounds of Z and X give the electron configuration of Z.

(iv) The second compound of X and Y resembles, in appearance, an element in the same group. Identify the element and explain why the similarities exist.

QUESTION SIX10.00 mL of solutions of acids HX and HY are titrated with the same solution of NaOH and the pH measured at intervals resulting in the graphs below.

(a) Determine the concentration of HX and HY

(b) Calculate the pH of the resulting solution of the salt NaX and compare your answer with data from the graph.

(c) It has been suggested that the Ka of a weak acid can be accurately determined using a solution of the weak monoprotic acid and a solution of sodium hydroxide without knowing the concentration of either but having standard laboratory equipment and indicators available. Is this possible? Justify your answer.

END OF EXAM

1 PERIODIC TABLE OF THE ELEMENTS 18Hydrogen

1

H1.008 2 13 14 15 16 17

Helium2

He4.003

Lithium3

Li6.943

Beryllium4

Be9.012

Boron5

B10.81

Carbon6

C12.01

Nitrogen7

N14.01

Oxygen8

O16.00

Fluorine9

F19.00

Neon10

Ne20.18

Sodium11

Na22.99

Magnesium12

Mg24.31 3 4 5 6 7 8 9 10 11 12

Aluminium13

Al26.98

Silicon14

Si28.09

Phosphorus15

P30.97

Sulfur16

S32.07

Chlorine17

Cl35.45

Argon18

Ar39.95

Potassium19

K39.10

Calcium20

Ca40.08

Scandium21

Sc44.96

Titanium22

Ti47.88

Vanadium23

V50.94

Chromium24

Cr52.00

Manganese25

Mn54.94

Iron26

Fe55.85

Cobalt27

Co58.93

Nickel28

Ni58.69

Copper29

Cu63.55

Zinc30

Zn65.39

Gallium31

Ga69.72

Germanium32

Ge72.59

Arsenic33

As74.92

Selenium34

Se78.96

Bromine35

Br79.90

Krypton36

Kr83.80

Rubidium37

Rb85.47

Strontium38

Sr87.62

Yttrium39

Y88.98

Zirconium40

Zr91.22

Niobium41

Nb92.91

Molybdenum42

Mo95.94

Technetium43

Tc(98)

Ruthenium44

Ru101.1

Rhodium45

Rh102.9

Palladium46

Pd106.4

Silver47

Ag107.9

Cadmium48

Cd112.4

Indium49

In114.8

Tin50

Sn118.7

Antimony51

Sb121.8

Tellurium52

Te127.6

Iodine53

I126.9

Xenon54

Xe131.3

Caesium55

Cs132.9

Barium56

Ba137.3

Lanthanum57

La†

138.9

Hafnium72

Hf178.5

Tantalum73

Ta180.9

Tungsten74

W183.9

Rhenium75

Re186.2

Osmium76

Os190.2

Iridium77

Ir192.2

Platinum78

Pt195.1

Gold79

Au198.0

Mercury80

Hg200.6

Thallium81

Tl204.4

Lead82

Pb207.2

Bismuth83

Bi209.0

Polonium84

Po(209)

Astatine85

At(210)

Radon86

Rn(222)

Francium87

Fr(223)

Radium88

Ra226.0

Actinium89

Ac‡

227.0

Rutherfordium104

Rf(261)

Dubnium105

Db(262)

Seaborgium106

Sg(263)

Bohrium107

Bh(264)

Hassium108

Hs(265)

Meitnerium109

Mt(266)

Darmstadtium110

Ds(271)

Roentgenium111

Rg(272)

Copernicium112

Cn(277)

114

(289)

116

(289)

† Cerium58

Ce140.1

Praseodymium59

Pr140.9

Neodymium60

Nd144.2

Promethium61

Pm(145)

Samarium62

Sm150.4

Europium63

Eu152.0

Gadolinium64

Gd157.3

Terbium65

Tb158.9

Dysprosium66

Dy162.5

Holmium67

Ho164.9

Erbium68

Er167.3

Thulium69

Tm168.9

Ytterbium70

Yb173.0

Lutetium71

Lu175.0

‡ Thorium90

Th232.0

Protactinium91

Pa231.0

Uranium92

U238.0

Neptunium93

Np237.0

Plutonium94

Pu(244)

Americium95

Am(243)

Curium96

Cm(247)

Berkelium97

Bk(247)

Californium98

Cf(251)

Einsteinium99

Es(252)

Fermium100

Fm(257)

Mendelevium101

Md(258)

Nobelium102

No(259)

Lawrencium103

Lr(260)