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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)
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