This question is about enthalpy changes of reactions involving hydrocarbons.
A student determines the enthalpy change of combustion, ΔcH\Delta_{\text{c}}HΔcH, of octane, C8H18\text{C}_8\text{H}_{18}C8H18, using the following method:
The temperature of the water increased by 14.5 ∘C14.5\text{ }^\circ\text{C}14.5 ∘C.
The spirit burner decreased in mass by 0.250 g.
Use the student's results to determine the enthalpy change of combustion of octane, ΔcH(C8H18)\Delta_{\text{c}}H(\text{C}_8\text{H}_{18})ΔcH(C8H18), 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-1.
Decane, C10H22\text{C}_{10}\text{H}_{22}C10H22, can be broken down by heat to form hexane, C6H14\text{C}_6\text{H}_{14}C6H14, and ethene, C2H4\text{C}_2\text{H}_4C2H4:
C10H22(g)→C6H14(g)+2C2H4(g)ΔH=+194 kJ mol−1Reaction 1 \text{C}_{10}\text{H}_{22}(\text{g}) \to \text{C}_6\text{H}_{14}(\text{g}) + 2\text{C}_2\text{H}_4(\text{g}) \quad \Delta H = +194\text{ kJ mol}^{-1} \quad \text{Reaction 1} C10H22(g)→C6H14(g)+2C2H4(g)ΔH=+194 kJ mol−1Reaction 1The enthalpy changes of combustion of C10H22(g)\text{C}_{10}\text{H}_{22}(\text{g})C10H22(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 |
|---|---|
| C10H22(g)\text{C}_{10}\text{H}_{22}(\text{g})C10H22(g) | -6778 |
| C2H4(g)\text{C}_2\text{H}_4(\text{g})C2H4(g) | -1411 |
Use ΔH \Delta H\,ΔH in Reaction 1 and the enthalpy changes of combustion in the table to determine the enthalpy change of combustion of C6H14(g)\text{C}_6\text{H}_{14}(\text{g})C6H14(g)..
