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Name TA Name Chemistry 130 (1,2) Dr. J. F. C. Turner Examination 3 2 hours Please answer all parts of all questions and show all your work You may use: Non-programmable calculators You may not use: Any written materials (cards or notes etc) or any other electronic device

ZnS 1. Zinc sulfide, s ¡ , exists in two different crystalline phases, sphalerite and wurtzite. Sphalerite and wurtzite

have different solubilities in water and are both sparingly soluble. The solubility constants are ¢

wurtzite ¥§¦ 2

¨ © K ZnS £ 3 10

sp s ¤

¢ ¦

sphalerite ¥ 4

¨ ©

K sp ZnS s ¤ 2 10

ZnS (a) Write down the balanced solution equilibrium for s ¡ in water.

4 points (b) Write down the expression for the solubility equilibrium for solid zinc sulfide in equilibrium with a saturated

2+ 2 -

Zn S £ ¤ solution of aq and aq .

4 points

sphalerite

sphalerite §¦ 4

£ ©

given by ¨ . ¤ K sp ZnS s 2 10

6 points

2+ 2 - £

Zn £ S ¤ (d) Calculate the concentration of aq ¤ and aq in a saturated solution in equilibrium with sphalerite.

6 points wurtzite

(e) Calculate the molar solubility of wurtzite given that the solubility constant of wurtzite, £ , is given ¤ K sp ZnS s

wurtzite §¦ 2

£ ©

by ¨ . ¤ K sp ZnS s 3 10

6 points

2+ 2 - £

Zn £ S ¤ (f) Calculate the concentration of aq ¤ and aq in a saturated solution in equilibrium with wurtzite.

6 points

(g) Write down the relationship between the standard Gibbs function and the equilibrium constant K eq

2 points ¡

(h) Calculate the change in the standard Gibbs function for the dissolution of sphalerite, Gsol, sphalerite . You should

1 1

£ £ assume that R ¢ 8.314 J mol K

6 points ¡

(i) Calculate the change in the standard Gibbs function for the dissolution of wurtzite, ¤ Gsol, wurtzite . You should

1 1

£ £ assume that R ¢ 8.314 J mol K

6 points (j) Write down a Hess cycle that represents the formation of aqueous zinc sulfide from both sphalerite and wurtzite as well as the conversion between sphalerite and wurtzite. You can either write the Hess cycle in equations or draw the

2+ 2 -

Zn S ¡ cycle. You should assume that the concentrations of aq ¡ and aq are identical for the dissolution of sphalerite and wurtzite.

The Hess cycle should contain the following equilibria: ¤

2+ ¤ 2- 2 + 2-

£ £ £ £ £ £ £

1 ZnS £ Zn S 2 ZnS Zn S 3 ZnS ZnS

£¢ ¤ ¤ ¤¥¢ ¤ ¤ ¤¦¢ ¤ wurtzite ¤ aq aq sphalerite aq aq sphalerite wurtzite

6 points

§ § §

(k) For each of the changes in standard Gibbs function for the three equilibria in (j), G1 , G2 and G3 , write down the numerical value where known.

6 points (l) Calculate an estimate for the change in standard Gibbs function for the conversion of sphalerite to wurtzite.

6 points (m) Calculate the equilibrium constant for the equilibrium between sphalerite and wurtzite.

6 points 70 points 2. The standard state for elemental carbon is graphite at 1 atm pressure and 298 K. Both diamond and graphite can be hydrogenated to give methane. The stoichiometric reactions and associated changes in the standard Gibbs

functions are given by

¦£¢

¡ 1

£ ©

C 2H £ CH G 74.6 kJ mol ¤

graphite 2 g ¤ 4 g 1

¦¤¢

¡ 1

£ ©

C 2H £ CH G 76.5 kJ mol ¤ diamond 2 g ¤ 4 g 2 (a) Calculate the standard Gibbs function of formation of diamond from graphite.

6 points ¥ (b) Calculate the equilibrium constant for the reaction Cgraphite Cdiamond

6 points (c) Why is the interconversion of graphite and diamond rarely if ever observed at 298 K, even though the thermodynamic description of the system implies that at equilibrium both should be present in substantial amounts?

4 points (d) From a thermodynamic perspective, is the title of Ian Fleming's fourth novel, 'Diamonds are forever' justified? 4 points (e) Does the title 'Graphite is forever' have the same ring? 4 points (f) Write down the change in the standard Gibbs function in terms of the change in standard enthalpy and the change in standard entropy at a temperature T.

4 points (g) Which part of the change in the standard Gibbs function gives the change in the entropy of the surroundings?

Underline your answer.

¡ ¡ ¡

S G H 2 points

¡ ¡£¢ 1

(h) The standard enthalpy of formation of methane is H f ,methane 50.5 kJ mol © . Calculate the standard ¡ entropy of formation of methane, S f ,methane .

6 points

(i) The reaction for the change in the standard Gibbs function for the formation of methane is given by

¡ ¡

¦£¢ ¦ ¢

¡ 1 1

£ £

¤ ¤ ©

1 1

£ £ Calculate the equilibrium constant for this reaction at 298 K, given that R ¢ 8.314 J mol K

6 points (j) Write down the expression that gives the change in equilibrium constant with temperature.

4 points

¡ £¢

¦ 1 ¤ (k) The heat of formation of methane is given by H f ,methane 50.5 kJ mol © . Calculate the equilibrium constant for the formation of methane at 2000 K.

6 points ¤

K CH £ ¦¥ (l) Is the value of eq 4 g ¤ at 2000 K in accordance with Le Chatelier's principle? Justify your answer.

4 points 56 points (3)

(a) Write down the relationship between either K A , K B and K W or pK A , pK B and pK W

4 points ¦ - pK 9.46 K CN (b) Hydrogen cyanide, HCN is a weak acid with A . Calculate B for cyanide ion, aq ¡ .

6 points (c) Calculate p H and p OH for an aqueous solution of hydrogen cyanide that has a molarity of 0.8 mol l-1.

8 points (d) Is hydrogen cyanide in aqueous solution a strong or weak electrolyte? 2 points

KCN £ (e) A 5 M solution of potassium cyanide, aq¤ is prepared. Calculate p H and p OH for this solution.

8 points 28 points

End of Examination