Edexcel A-Level Chemistry A2 12.8 The Entropy Change in the Surroundings and Hence Stotal Questions

Use ΔSsurroundings = −ΔH/T with T in kelvin and matching joule units, retain the enthalpy sign and then add the system contribution.

Exam points

Calculate ΔSsurroundings = −ΔH/T after converting kJ to J and temperature to K.
Combine system and surroundings values to obtain total entropy with sign and units.
Rearrange the surroundings equation to calculate enthalpy from entropy and temperature.

[Maximum number: 1]

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})

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 }}

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

An answer that makes reference to the following points:

- enthalpy change of reaction / $\Delta_{\mathrm{r}} H$ (of the Haber process)

Allow $-_{\mathrm{r}} H$

Allow enthalpy change / $\Delta H /-\Delta H$

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Edexcel A-Level Chemistry A2 12.8 The Entropy Change in the Surroundings and Hence Stotal Questions question 1