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JAMB Chemistry 1991 Question 28

Question 28

Energy Changes

1.1 g of CaCl2 dissolved in 50 cm3 of water caused a rise in temperature of 3.4°C. The heat of reaction, ΔH for CaCl2 in kJ per mole is [Ca = 40, Cl = 35.5, specific heat of water is 4.18 JK−1].

  1. A.−71.1
  2. B.−4.18
  3. C.+71.1
  4. D.+111.0
Show the answer and explanation

Answer: A (−71.1)

Explanation

Heat given out = 50 × 4.18 × 3.4 ≈ 711 J. Moles of CaCl2 = 1.1 ÷ 111 ≈ 0.01, so ΔH ≈ −0.711 ÷ 0.01 ≈ −71 kJ/mol. It is negative because the temperature rose.