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JAMB Chemistry 1983 Question 27

Question 27

Energy Changes

In the reactions
(I) H2(g) + ½O2(g) → H2O(l); ΔH = −286 kJ
(II) C(s) + O2(g) → CO2(g); ΔH = −406 kJ
the equations imply that

  1. A.more heat is absorbed in (I)
  2. B.more heat is absorbed in (II)
  3. C.less heat is evolved in (I)
  4. D.reaction (II) proceeds faster than (I).
  5. E.reaction (I) proceeds faster than (II).
Show the answer and explanation

Answer: C (less heat is evolved in (I))

Explanation

Both ΔH values are negative, so both reactions give out heat; reaction (I) gives out 286 kJ, less than the 406 kJ of (II). ΔH says nothing about speed.