An investigator burned four liquid alkanes to compare the quantity of heat energy they released, with the aim of determining which is the most effective fuel. The heat energy released was transferred to raise the temperature of 150 g150\text{ g}150 g of water. For each alkane, the investigator recorded the mass of fuel burned and the temperature rise of the water. The results are presented in the table below:
| Fuel | Relative formula mass | Mass of fuel burned in g\text{g}g | Amount of fuel burned in mol\text{mol}mol | Increase in temperature in ∘C^{\circ}\text{C}∘C |
|---|---|---|---|---|
| pentane | 72 | 1.2 | 0.017 | 15 |
| hexane | 86 | 2.1 | 0.024 | 24 |
| heptane | 100 | 3.2 | 0.032 | 34 |
| octane | 114 | 4.5 | 0.039 | 45 |
One student suggests that pentane is the best fuel. Explain why, using the information in the table.
Another student suggests that octane is the best fuel. Explain why, using the information in the table.
In another trial, the student calculated the experimental enthalpy of combustion of pentane to be −2150 kJ/mol-2150\text{ kJ/mol}−2150 kJ/mol. The accepted literature value is −3509 kJ/mol-3509\text{ kJ/mol}−3509 kJ/mol. Suggest two reasons why the experimental value is less exothermic than the literature value.
35 exam-style questions on Edexcel IGCSE Chemistry Energetics. Each one has a worked solution and a mark scheme showing where the marks go.