Edexcel IAL Chemistry A2 Topic 12 Entropy and energetics

Pearson Edexcel IAL Chemistry syllabusChemistry YCH11First assessment 2019

Translate equations and physical changes into entropy direction, calculate each contribution with compatible units and use total entropy rather than system entropy alone for…

Exam points

Explain entropy changes through molecular dispersal, energy distribution, state and gas moles.
Calculate ΔSsystem and ΔSsurroundings from tabulated entropy, enthalpy and temperature data.
Combine contributions to obtain ΔStotal and judge feasibility at a stated temperature.

[Maximum number: 2]

A simplified Born-Haber cycle for the formation of lithium iodide is shown.
$\begin{aligned}+520 \mathrm{~kJ} \mathrm{~mol}^{-1} \uparrow & +\underset{\substack{\text { electron } \\ \text { affinity }}}{\mathrm{Li}^{+}(\mathrm{g})} \xrightarrow{\mathrm{Li}(\mathrm{g})}+\underset{-270 \mathrm{~kJ} \mathrm{~mol}^{-1}}{\mathrm{LiI}(\mathrm{s})} \\ +160 \mathrm{~kJ} \mathrm{~mol}^{-1} \uparrow & +107 \mathrm{~kJ} \mathrm{~mol}^{-1} \uparrow \\ \mathrm{Li}(\mathrm{s}) & +\end{aligned}$

(a)

The enthalpy change of atomisation of iodine $\left(+107 \mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ is given by the equation

$1 / 2 \mathrm{I}_{2}(\mathrm{~g}) \rightarrow \mathrm{I}(\mathrm{g})$

$1 / 2 \mathrm{I}_{2}(\mathrm{~s}) \rightarrow \mathrm{I}(\mathrm{g})$

$\mathrm{I}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{I}(\mathrm{g})$

$\mathrm{I}_{2}(\mathrm{~s}) \rightarrow 2 \mathrm{I}(\mathrm{g})$

[ 1 ]

(b)

Use the information in the cycle to calculate the electron affinity of iodine.

$-298 \mathrm{~kJ} \mathrm{~mol}^{-1}$

$-242 \mathrm{~kJ} \mathrm{~mol}^{-1}$

$+242 \mathrm{~kJ} \mathrm{~mol}^{-1}$

$+298 \mathrm{~kJ} \mathrm{~mol}^{-1}$

[ 1 ]

What is the standard entropy change of the system, in $\mathrm{JK}^{-1} \mathrm{~mol}^{-1}$ , for the reaction between nitrogen and hydrogen to form ammonia?

\mathrm{N}_{2}+3 \mathrm{H}_{2} \rightarrow 2 \mathrm{NH}_{3}

□ A -198.8
□ B -129.9
□ C +129.9
□ D +198.8

[Maximum number: 1]

The enthalpy change of solution of sodium sulfate, $\mathrm{Na}_{2} \mathrm{SO}_{4}$ , may be calculated using three pieces of data. Which of these pieces of data is not required?

lattice energy of $\mathrm{Na}_{2} \mathrm{SO}_{4}$

enthalpy change of hydration of $\mathrm{Na}^{+}$

enthalpy change of formation of $\mathrm{Na}_{2} \mathrm{SO}_{4}$

enthalpy change of hydration of $\mathrm{SO}_{4}^{2-}$

[Maximum number: 5]

The standard enthalpy change of solution for ammonium nitrate, $\mathrm{NH}_{4} \mathrm{NO}_{3}$ , is $+25.7 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

(a)

Calculate the value for the standard entropy change in the surroundings, $\Delta S_{\text {surroundings }}^{\ominus}$ , when ammonium nitrate dissolves in water at 298 K .
Include a sign and units with your answer.

[ 2 ]

(b)

Explain what can be deduced from your answer in (a) about the sign and the value of the standard entropy change in the system, $\Delta S_{\text {system }}^{\ominus}$ , when $\mathrm{NH}_{4} \mathrm{NO}_{3}$ dissolves.

A
"
$\Delta S_{\text {surroundings }}^{\ominus}$ , when ammonium nitrate dissolves in water at 298 K .
Include a sign and units with your answer.
(b) Explain what can be deduced from your answer in (a) about the sign and the value of the stana
$\mathrm{NH}_{4} \mathrm{NO}_{3}$ dissolves.

[ 3 ]

[Maximum number: 6]

*18 The table shows the theoretical and experimental (Born-Haber) lattice energy data for two metal halide compounds, sodium chloride and magnesium iodide.

Using the data, compare and contrast the type and strength of bonding in these compounds.
Give reasons for your answers.
(6)

[Maximum number: 7]

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

1 / 2 \mathrm{~N}_{2}(\mathrm{~g})+1 \frac{1}{2} \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons \mathrm{NH}_{3}(\mathrm{~g})

(a)

At 298 K the standard entropy change of the system, $\Delta S_{\text {system }}^{\ominus}=-98 \mathrm{JK}^{-1} \mathrm{~mol}^{-1}$ .

Calculate the standard entropy of one mole of ammonia.
Use the value of $\Delta S_{\text {system }}^{\ominus}$ and the data in the table.

[ 2 ]

(b)

The relationship between $\Delta S_{\text {total }}$ and 1 / T can be found by combining the two equations:

\Delta S_{\text {total }}=\Delta S_{\text {surroundings }}+\Delta S_{\text {system }}

and $\Delta S_{\text {surroundings }}=-\Delta H / T$
to give

\Delta S_{\text {total }}=-\Delta H / T+\Delta S_{\text {system }}

[ 2 ]

(i)

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

[ 1 ]

(ii)

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

[ 1 ]

(c)

The industrial synthesis of ammonia

1 / 2 \mathrm{~N}_{2}(\mathrm{~g})+11 / 2 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons \mathrm{NH}_{3}(\mathrm{~g})

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

[ 3 ]

(i)

Explain why $\Delta S_{\text {total }}$ decreases with an increase in temperature.

[ 3 ]

[Maximum number: 3]

The reversible reaction between hydrogen chloride and oxygen produces water vapour and chlorine.

4 \mathrm{HCl}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{~g})+2 \mathrm{Cl}_{2}(\mathrm{~g}) \quad \Delta H=-114 \mathrm{~kJ} \mathrm{~mol}^{-1}

Draw a sketch of entropy against temperature for water to illustrate the entropy changes as temperature increases, including when water changes state.

A scale is not required for the vertical axis

Exam points

Question 1

Question 1(a)

Question 1(b)

Question 2

Question 3

Question 15

Question 15(a)

Question 15(b)

Question 18

Question 22

Question 22(a)

Question 22(c)

Question 22(c)(ii)

Question 22(c)(iii)

Question 22(d)

Question 22(d)(iii)

Question 20(c)