Entropy | Edexcel International A Level Chemistry Exam Questions 2017

2 marks

The equation for the formation of ammonia in the Haber Process is shown

1⁄2N₂(g) +1 1⁄2H₂(g) ⇌ NH₃(g)

At 298 K the standard entropy change of the system, $∆ S_{system}^{⦵}$ = –98 JK^–1mol^–1.

Calculate the standard entropy of one mole of ammonia.
Use the value of $∆ S_{system}^{⦵}$ and the data in the table.

Substance	Standard molar entropy, $S^{⦵}$ /JK^–1mol^–1
N₂	192
H₂	131

How did you do?

1b2 marks

The value of the total entropy change, ΔS_total , varies with temperature.

Data for the value of ΔS_total at different temperatures but at standard pressure of 100 kPa are given for this reaction.

Temperature / K	1/T / K^–1	ΔS_total / kJ K^–1mol^–1
250	4.00×10^–3	8.27×10^–2
375	2.67×10^–3	2.25×10^–2
500	2.00×10^–3	–0.764×10^–2
625	1.60×10^–3	–2.57×10^–2
750	1.33×10^–3	–3.77×10^–2

Plot a graph of ΔS_total against 1/T on the grid.
Include a line of best fit.

How did you do?

3 marks

The relationship between ΔStotal and 1/T can be found by combining the two equations:

ΔS_total = ΔS_surroundings + ΔS_system

and ΔS_surroundings = –ΔH/T
to give

ΔS_total = –ΔH/T + ΔS_system

Determine the gradient of the line plotted in (b), including units in your answer.

(1)

ii)

Identify the thermodynamic quantity that can be obtained from this gradient.

(1)

iii)

Determine the temperature at which the reaction ceases to be thermodynamically feasible at a pressure of 100kPa.

(1)

How did you do?

7 marks

The industrial synthesis of ammonia

1⁄2N₂(g) +1 1⁄2H₂(g) ⇌ NH₃(g)

is carried out at pressures of about 20000kPa and temperatures between 700 K and 750 K. These temperatures are higher than the answer to (c)(iii).

State the relationship between the total entropy, ΔS_total , and the equilibrium constant, K.

(1)

ii)

Calculate the value of the equilibrium constant K at 750 K.
[ΔS_total at 750K = –37.7JK^–1mol^–1]

(2)

iii)

Explain why ΔS_total decreases with an increase in temperature.

(3)

iv)

State how the Haber Process is made economically feasible at 750 K even though the total entropy change is negative.

(1)

How did you do?

1e6 marks

Ammonia from the Haber Process reacts with acids.
With phosphoric acid, H₃PO₄, a number of products are formed in solution.
One of these is the fertiliser diammonium hydrogenphosphate.

Write an equation for the production of this fertiliser.
State symbols are not required.

(2)

ii)

Write an ionic equation to show that ammonium ions are acidic in aqueous solution.
State symbols are not required.

(1)

iii)

A solution containing both ammonia and ammonium ions acts as a buffer.
Explain, using a relevant ionic equation, the effect of adding a small amount
of acid to this buffer.

(3)

How did you do?

Did this page help you?

Entropy (Edexcel International A Level Chemistry)

Exam Questions