A student uses a spirit burner containing propanol to heat 150 g of water in a copper can.
The student's results are:
• mass of propanol burned = 0.90 g
• temperature rise of the water = 22.0 °C
Write the balanced equation for the complete combustion of propanol, C3H7OH\mathrm{C_{3}H_{7}OH}C3H7OH.
Calculate the energy released per mole of propanol burned, in kJ mol−1\mathrm{kJ\,mol^{-1}}kJmol−1. Use the equation: energy transferred to the water (J)=mass of water (g)×4.2×temperature rise (∘C)\text{energy transferred to the water (J)} = \text{mass of water (g)} \times 4.2 \times \text{temperature rise (}^{\circ}\text{C)}energy transferred to the water (J)=mass of water (g)×4.2×temperature rise (∘C). Relative formula mass of propanol = 60
The table shows data-book values for the energy released when one mole of some alcohols is burned completely.
| Alcohol | Energy released / kJ mol−1\mathrm{kJ\,mol^{-1}}kJmol−1 |
|---|---|
| methanol | 726 |
| ethanol | 1367 |
| propanol | 2021 |
| butanol | 2676 |
| pentanol | ? |
Predict the energy released per mole for pentanol. Justify your answer.
The student's value for propanol is much lower than the data-book value. Explain two reasons for this.
Another student compares the alcohols by using only the temperature rise of the water, without weighing the burners. Explain why this is not a valid way to compare the energy released by the alcohols.
43 exam-style questions on Edexcel GCSE Chemistry 10.4 Alcohols and carboxylic acids, covering 10.4.1 Alcohols and the -OH functional group, 10.4.2 Carboxylic acids and the -COOH functional group, 10.4.3 Oxidation of alcohols and homologous series, and 10.4.4 Fermentation and fractional distillation of ethanol. Each one has a worked solution and a mark scheme showing where the marks go.