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6 theoretical problems 3 practical problems

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INTERNATIONAL CHEMISTRY OLYMPIAD, 1995

THE TWENTY-SEVENTH
INTERNATIONAL CHEMISTRY OLYMPIAD
13–20 JULY 1995, BEIJING, CHINA

THEORETICAL PROBLEMS
PROBLEM 1
1.1 Excavated Chinese ancient bronze musical instrument, carillon, was covered entirely
by rust. Chemical analysis showed that the rust contains CuCl, Cu2Oand Cu2(OH)3Cl. Simulation experiments showed that CuCl was formed first under the action of both air and Cl containing aqueous solution and then Cu2(OH)3Cl produced through the following two different ways:
Cu2O (a) (c) Cu (BRONZE) CuCl Cu2(OH)3Cl (b)

Using the molar standard formation Gibbs free energies of the species concerned answer the following questions:

Species
∆fG (298 K) -1 ( kJmol )
0

Cu2O(s) CuO(s) CuCl(s) Cu2(OH)3CI(s) -146 -130 -120 -1338

Cl(aq) -131

OH(aq) -157

H2O(l) -237

i) ii)

Write balanced equations for reactions (a), (b) and (c). Calculate the molar standard Gibbs free energy ∆fG (298 K) for reactions (a),
0

(b) and (c). iii) Decide the spontaneous direction of reaction (a) in air through calculation, when T = 298K, c(HCI) = 1.0×10 mol dm.
-4 -3

THE COMPETITION PROBLEMS FROM THE INTERNATIONAL CHEMISTRY OLYMPIADS, Volume 2 Edited by Anton Sirota, ICHO International Information Centre, Bratislava, Slovakia

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1.2 Rate constants kc for reaction (c) were measured at various temperatures in a
simulation experiment in order to obtain its kinetic parameters. On thebasis of the data given below answer the following questions:

t° C kc / mol dm s
i)
-3 -1

25 1.29×10
-4

40 2.50×10
-4

Write the equation for calculating the activation energy of reaction (c) and fínd the value.

ii) iii)

Assign the overall reaction order of reaction (c). Knowing that the rate determining step of reaction (c) is the monolayer adsorption of O2 (g) on solid CuCl,write the overall rate equation of this heterogeneous reaction (c). Under what condition might the reaction order be the same as that you have given in ii)? Assume only O2 can be adsorbed.

1.3 A copper plate was divided into two parts, Cu(1) and Cu(2). Cu(1) was then
hammered so that Cu(1) and Cu(2) are different in certain thermodynamic properties. i) An galvanic cell with Cu(1) and Cu(2) wasdesigned as Cu(1)lCuSO4(aq)ICu(2) and the electromotive force E of the above cell was expressed as E = ΦR – ΦL, where ΦR and ΦL being the right and left electrode potentials (i. e. half-cell potentials), respectively. Please choose the correct E value from the following and give the thermodynamic reason for your choice. (A) E 0 (D) It cannot be decided

Write the net cell reaction for the cell.1.4 In a Cu-Zn alloy the molar fractions of Cu and Zn are 0.750 and 0.250, respectively.
The structure type of the alloy is the same as that of pure copper, except Zn atoms substitute some Cu atoms randomly and statistically, i. e. at every atomic position, the probability of the occupation of Cu and Zn is proportional to the composition of the alloy. In this sense the alloy can be consideredas composed of statistical atoms CuxZn1-x. X-ray analysis shows that the arrangement of atoms in the alloy is of the cubic face-centred close packing type. Density of the alloy d = 8.51 g cm . Calculate the radius of the statistical atoms in the alloy. Given: Ar (Cu) = 63.5, _______________
THE COMPETITION PROBLEMS FROM THE INTERNATIONAL CHEMISTRY OLYMPIADS, Volume 2 Edited by Anton Sirota, ICHOInternational Information Centre, Bratislava, Slovakia -3

Ar (Zn) = 65.4.

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SOLUTION
1.1 i)
a) b) c) ii) 2 CuCI(s) + H2O(l) → Cu2O(s) + 2 H (aq) + 2 Cl (aq)
+ –

Cu2O(s) + ½ O 2(g) + H2O(l) + H (aq) + Cl (aq) → Cu2(OH)3Cl(s)
+ –

2 CuCl(s) + ½ O 2 (g) + 2 H2O(l) → Cu2(OH)3Cl (s) + H (aq) + Cl (aq)
+ –

a) b) c)...