Chemistry 9701 · AS & A Level · Hydroxy compounds

Hydroxy compounds — practice question

For many compounds, the enthalpy change of formation cannot be worked out directly. An indirect approach that uses enthalpy changes of combustion can be applied. A calorimetry experiment can be used to determine the enthalpy change of combustion, by using the heat released during combustion to warm a known mass of water and then measuring the temperature change.
(a(i))[3]

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

(a(ii))[1]

Write the equation that represents 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 for the combustion of $1$ mole of ethanol. Include a sign in your response.

(b(iii))[2]

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

(c(i))[3]

Construct a labelled energy cycle to show how these values may 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 $3\text{C}(s) + 4\text{H}_2(g) + \tfrac{1}{2}\text{O}_2(g) \rightarrow \text{C}_3\text{H}_7\text{OH}(l)$.

(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: “States enthalpy/energy/heat change when one mole of a substance reacts” …

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