A level chemistry Thermodynamics exam questions and mark scheme

THERMODYNAMICS

A level chemistry exam questions and mark scheme

,Question 1. The energy level diagram (Born-Haber cycle) for caesium chloride is shown below.

–
C s+ (g ) + C l(g ) + e

–1
H 4 = + 1 2 1 k J m o l
–1
H 5 = – 3 6 4 k J m o l
–
C s +(g ) + 12 C l 2(g ) + e

+ –
H 3 = + 3 7 6 k J m o l
–1
C s (g ) + C l (g )

C s(g ) + 12 C l 2(g )


H 2 = + 7 9 k J m o l – 1
H 6
1
C s(s) + 2 C l 2 (g )


–1
H 1 = – 4 3 3 kJ m o l

C sC l(s)


(a) Give the names of the enthalpy changes represented by H1, H2 and H5. (3)
(b) Calculate the value of the lattice energy H6. (2)
(c) Explain why the enthalpy change represented by H3 has a lower magnitude for caesium than for sodium.
(3) (Total 8 marks)

Question 2. Below is the energy level diagram (Born-Haber cycle) for the formation of rubidium iodide from its
elements.




–1 H 1
H I(R b ) = + 4 02 k J m o l

–
R b + (g ) + I (g )
R b (g ) + I(g )


–1
H at ( 12 I 2 ) = + 10 7 k J m ol


R b (g ) + 1
2 I 2 (s)


–1
H at (R b ) = + 8 5.8 k J m o l

–1
H 2 = – 6 0 9 k J m o l

–1
H f(R b I) = – 3 28 k J m ol


R b I(s)



(i) Complete the diagram giving the identities of the missing species (2)
(ii) Give the names of the enthalpy changes represented by H1 and H2. (2)
(iii) Calculate the value of the enthalpy change represented by H1. (2)
(Total 6 marks)

, Question 3. The following data relate to lithium chloride.
Standard molar enthalpy change of solution is –37.0 kJ mol–1.
Lattice enthalpy is –846 kJ mol–1.
+ –
L i (g ) + C l (g )




L i(s) + ½ C l 2 (g )
B
E n th alp y C
A

L iC l(s)

+ –
L i (aq ) + C l (aq )

(i) Give the name of each of the changes A and B. (2)

(ii) Calculate the value of the enthalpy change represented by C and suggest the name(s) of the
enthalpy change(s). (3)
(Total 5 marks)



Question 4. The following reaction occurs in the high-temperature preparation of titanium(IV) chloride.

TiO2(s) + C(s) + 2Cl2(g)  TiCl4(l) + CO2(g)

(i) Use the data given below to calculate the standard enthalpy change and the standard entropy change for this
reaction.
Substance TiO2(s) C(s) Cl2(g) TiCl4(l) CO2(g)
Hf / kJ mol –1 –940 0 0 –804 –394
S / J K–1mol–1 49.9 5.7 223 252 214

(ii) Calculate the temperature at which this reaction ceases to be feasible. (9)
(Total 9 marks)

Question 5. Sulphuric acid is manufactured by the Contact Process. One stage of the process is the oxidation of
sulphur dioxide according to the equation:
–1
2SO2(g) + O2(g) 2SO3(g) H = –196 kJ mol

Typical operating conditions are a temperature of about 450 °C, close to atmospheric pressure and the presence
of a catalyst. The use of a converter containing four catalyst beds results in a 99.5% conversion.
–1 –1
The value of the entropy change, S, for the reaction between sulphur dioxide and oxygen –188 J mol K .


(i) Explain why there is a decrease in entropy of the system. (2)

(ii) Use the equation G = H – TS to calculate the value of G for the reaction between sulphur dioxide and
oxygen at 25 °C. Explain the significance of the sign of the value obtained. (4)