7.8 Core practical: combustion of alcohols
5 exam questions worth 8 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Why is a draught shield used in this experiment?
Choose one answer.
Correct Not quite: the answer is To reduce energy lost to the surroundings
Mark scheme
C
Model answer
C: it reduces energy lost to the surroundings by air movement.
Burning 0.85 g of ethanol raised the temperature of 100 cm3 of water by 18 °C.
Calculate the temperature rise per gram of ethanol.
Mark scheme
- 18 ÷ 0.85 = 21 °C/g (1)
Model answer
18 ÷ 0.85 = 21.2 °C/g (to 3 significant figures).
Give two variables that must be controlled.
Mark scheme
any two from:
- volume / mass of water (1)
- distance from flame to can (1)
- size of wick / flame (1)
- type of can (1)
Model answer
The volume of water and the distance between the flame and the can.
Describe the trend in the results and explain it.
Mark scheme
- temperature rise per gram increases as the number of carbon atoms increases (1)
- larger molecules release more energy when burned / more bonds made (1)
Model answer
The temperature rise per gram increases from ethanol to pentanol, because larger alcohol molecules release more energy when they burn.
The energy released measured in this experiment is less than the true value. Suggest two reasons.
Mark scheme
any two from:
- energy lost to the surroundings / heating the air (1)
- energy used to heat the can (1)
- incomplete combustion (1)
- alcohol evaporates from the wick (1)
Model answer
Much of the energy heats the surrounding air and the can, and some of the alcohol burns incompletely.
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