Chemistry 9701 · AS & A Level · Hydroxy compounds

Hydroxy compounds — practice question

For a large number of compounds, the enthalpy change of formation cannot be determined directly. Instead, an indirect route using enthalpy changes of combustion is used. The enthalpy change of combustion may be measured by a calorimetry experiment in which the heat energy released during combustion warms a known mass of water and the temperature change is measured.
(a(i))[3]

Explain what is meant by the term standard enthalpy change of combustion.

(a(ii))[1]

Write the balanced equation for the complete combustion of ethanol, $\text{C}_2\text{H}_5\text{OH}$.

(b(i))[1]

Calculate the heat energy change, $q$, for the combustion of $0.23\,\text{g}$ of ethanol.

(b(ii))[1]

Calculate the enthalpy change when $1$ mole of ethanol burns. Include a sign in your answer.

(b(iii))[2]

Suggest two reasons why the enthalpy change of combustion of ethanol found using a simple laboratory calorimetry experiment is likely to be lower than the true value.

(c(i))[3]

Construct a labelled energy cycle showing how these values could be used to calculate the enthalpy change of formation of $\text{C}_3\text{H}_7\text{OH(l)}$, $\Delta H_f$, for the reaction: $\text{3C(s)} + \text{4H}_2\text{(g)} + \frac{1}{2}\text{O}_2\text{(g)} \rightarrow \text{C}_3\text{H}_7\text{OH(l)}$

(c(ii))[2]

Calculate the enthalpy change of formation, $\Delta H_f$, for $\text{C}_3\text{H}_7\text{OH(l)}$.

Worked solution & mark scheme

This 13-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Energy / heat / enthalpy change when one mole of a substance reacts” …

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