Combustion of ethanol is used in spirit burners to heat water in calorimetry experiments. When ethanol burns, the temperature of the water in the calorimeter rises. Explain what this temperature rise tells us about the combustion of ethanol, and describe how calorimetry could be used to determine the enthalpy change of this reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student uses a spirit burner containing ethanol to heat 200 g of water. The temperature of the water rises by 15 °C during the experiment.
Model answer (4 marks)
The rise in temperature shows that heat is released into the water, so the combustion of ethanol is exothermic.
In a calorimeter the heat released (or absorbed) is measured by the temperature change of the water:
Q = m·c·ΔT.
The mass of ethanol burnt is converted to moles (using its molar mass). The heat Q calculated above is divided by the number of moles of ethanol to give the molar enthalpy change, ΔH°.
Because the reaction releases heat, ΔH° is negative.
In a calorimeter the heat released (or absorbed) is measured by the temperature change of the water:
Q = m·c·ΔT.
The mass of ethanol burnt is converted to moles (using its molar mass). The heat Q calculated above is divided by the number of moles of ethanol to give the molar enthalpy change, ΔH°.
Because the reaction releases heat, ΔH° is negative.
Examiner tips
- State that the temperature rise indicates an exothermic reaction.
- Show the calorimetry equation Q = mcΔT and explain how Q is found.
- Convert the mass of ethanol to moles and divide Q by this to get ΔH°.
- Remember to give ΔH° a negative sign for an exothermic reaction.
Common mistakes
- Forgetting to convert the mass of ethanol to moles before calculating ΔH°.
- Using the wrong sign for ΔH° (positive instead of negative).
- Not including the calorimetry equation or the temperature change in the explanation.
Mark scheme (4 marks)
- The temperature of the water rises, so energy is transferred to the surroundings / water, meaning the combustion of ethanol is an exothermic reaction.
- Calorimetry measures the amount of heat energy released (or absorbed) during a chemical reaction / Q = mcΔT is used to calculate the heat energy change.
- The moles of ethanol burned are calculated (from the mass of ethanol used), and the heat energy Q is divided by the number of moles to give the molar enthalpy change.
- The molar enthalpy change value is given a negative sign because the reaction is exothermic.
Key terms in this question
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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