This question is about enthalpy changes of reactions involving hydrocarbons.
A student determines the enthalpy change of combustion, ΔcH\Delta_{\text{c}}HΔcH, of heptane, C7H16\text{C}_7\text{H}_{16}C7H16, using the following method:
The temperature of the water increased by 18.2 ∘C18.2\text{ }^\circ\text{C}18.2 ∘C.
The spirit burner decreased in mass by 0.320 g0.320\text{ g}0.320 g.
Use the student's results to determine the enthalpy change of combustion of heptane, ΔcH(C7H16)\Delta_{\text{c}}H(\text{C}_7\text{H}_{16})ΔcH(C7H16), in kJ mol−1\text{kJ mol}^{-1}kJ mol−1. Assume the specific heat capacity of water is 4.18 J g−1 K−14.18\text{ J g}^{-1}\text{ K}^{-1}4.18 J g−1 K−1.
Nonane, C9H20\text{C}_9\text{H}_{20}C9H20, can be broken down by heat to form pentane, C5H12\text{C}_5\text{H}_{12}C5H12, and ethene, C2H4\text{C}_2\text{H}_4C2H4:
C9H20(g)→C5H12(g)+2C2H4(g)ΔH=+156 kJ mol−1Reaction 1 \text{C}_9\text{H}_{20}(\text{g}) \to \text{C}_5\text{H}_{12}(\text{g}) + 2\text{C}_2\text{H}_4(\text{g}) \quad \Delta H = +156\text{ kJ mol}^{-1} \quad \text{Reaction 1} C9H20(g)→C5H12(g)+2C2H4(g)ΔH=+156 kJ mol−1Reaction 1The enthalpy changes of combustion of C9H20(g)\text{C}_9\text{H}_{20}(\text{g})C9H20(g) and C2H4(g)\text{C}_2\text{H}_4(\text{g})C2H4(g) are shown in the table below:
| Hydrocarbon | ΔcH/kJ mol−1\Delta_{\text{c}}H / \text{kJ mol}^{-1}ΔcH/kJ mol−1 |
|---|---|
| C9H20(g)\text{C}_9\text{H}_{20}(\text{g})C9H20(g) | −6120-6120−6120 |
| C2H4(g)\text{C}_2\text{H}_4(\text{g})C2H4(g) | −1411-1411−1411 |
Use ΔH\Delta HΔH in Reaction 1 and the enthalpy changes of combustion in the table to determine the enthalpy change of combustion of C5H12(g)\text{C}_5\text{H}_{12}(\text{g})C5H12(g).
