Chemistry 9701 · AS & A Level · Chemical energetics

Chemical energetics — practice question

Entropy measures how disordered a system is.
(a)[2]

Assume that the entropy, $S$, of $\text{H}_2\text{O}$ is zero at $0\,\text{K}$. Sketch a graph on the axes to show how the entropy of $\text{H}_2\text{O}$ varies between $0\,\text{K}$ and $300\,\text{K}$.

(b)[1]

Put one tick (✓) in each row of the table to indicate the sign of the entropy changes, $\Delta S$. solid dissolving in water water boiling to steam

(c)[2]

The equation for a reaction that forms methanol is shown. $\text{CO}_2(\text{g}) + 3\text{H}_2(\text{g}) \rightarrow \text{CH}_3\text{OH}(\text{g}) + \text{H}_2\text{O}(\text{g})$ Use appropriate bond energies from the Data Booklet to calculate the enthalpy change, $\Delta H$, for this reaction in the gas phase.

(d(i))[2]

At $298\,\text{K}$, both products in this reaction are liquid. $\text{CO}_2(\text{g}) + 3\text{H}_2(\text{g}) \rightarrow \text{CH}_3\text{OH}(\ell) + \text{H}_2\text{O}(\ell)$, $\Delta H^\circ = -131\,\text{kJ mol}^{-1}$ Calculate the standard entropy change, $\Delta S^\circ$, for this reaction.

(d(ii))[2]

Calculate the standard Gibbs free energy change, $\Delta G^\circ$, for this reaction at $298\,\text{K}$.

(d(iii))[1]

Predict the effect of increasing the temperature on the feasibility of this reaction.

(e(i))[1]

Use the Data Booklet to write an equation for the overall cell reaction.

(e(ii))[1]

Use $E^\circ$ values to calculate the $E^\circ_{\text{cell}}$ for this reaction.

Worked solution & mark scheme

This 12-mark question has a full step-by-step worked solution and mark scheme. One marking point: Entropy rises continuously from $0$-$300\,\text{K}$ except at melting point

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