Edexcel IAL Chemistry 12.7 Entropy change of the system

Multiply every S° value by its balanced-equation coefficient, total products and reactants separately and calculate products minus reactants with sign.

Exam points

Calculate ΣS° for products and reactants with all stoichiometric coefficients applied.
Subtract reactant entropy from product entropy and report sign and J K⁻¹ mol⁻¹ units.

[Maximum number: 2]

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})

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.

Table for Question 12.7—The entropy change of the system for a reaction, ∆Ssystem, given the entropies of the reactants and products question 1 — Edexcel A-Level Chemistry A2

states or uses equation

- calculate $S^{\circ}$ products

(1)

(1)

$\begin{aligned} \Delta S_{\text {system }}^{\ominus} & =S_{\text {products }}^{\ominus}-S_{\text {reactants }}^{\ominus} -98.0 & =S_{\text {products }}^{\ominus}-((0.5 \times 192)+(1.5 \times 131)) S_{\text {products }}^{\ominus} & =292.5-98 S_{\text {products }}^{\ominus} & =(+) 194.5 / 195\left(\mathrm{~J} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}\right) \end{aligned}$

If units are given they must be correct

Allow TE on incorrect $S^{\ominus}$ reactants

Comment

Correct answer with no working scores (2)
$\begin{aligned} S_{\text {products }}^{\ominus} & =63.5 \text { scores max (1) } S_{\text {products }}^{\ominus} & =225 \text { scores max (1) } \end{aligned}$

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12.7—The entropy change of the system for a reaction, ∆Ssystem, given the entropies of the reactants and products question 1