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JAMB Chemistry 2000 Question 28

Question 28

Energy Changes

0.46g of ethanol when burned raised the temperature of 50 g of water by 14.3 K. Calculate the heat of combustion of ethanol.
[C = 12, O = 16, H = 1, Specific heat capacity of water = 4.2 Jg−1K−1]

  1. A.+3000 kJ mol−1
  2. B.+300 kJ mol−1
  3. C.−300 kJ mol−1
  4. D.−3000 kJ mol−1
Show the answer and explanation

Answer: C (−300 kJ mol−1)

Explanation

Heat = 50 × 4.2 × 14.3 ≈ 3003 J. 0.46 g of ethanol (M = 46) is 0.01 mol, so 3003 ÷ 0.01 ≈ 300 kJ mol−1; combustion gives out heat, so ΔH = −300 kJ mol−1.