JAMB Chemistry 1997 Question 25

Question 25

Energy Changes

2SO_2(g) + O_2(g) ⇌ 2SO_3(g). In the reaction above, the standard heats of formation of SO_2(g) and SO_3(g) are −297 kJ mol⁻¹ and −396 kJ mol⁻¹ respectively. The heat change of the reaction is

A.−99 kJ mol⁻¹
B.−198 kJ mol⁻¹
C.+198 kJ mol⁻¹
D.+683 kJ mol⁻¹.

Show the answer and explanation

Answer: B (−198 kJ mol⁻¹)

Explanation

ΔH = ΣΔH_f(products) − ΣΔH_f(reactants) = 2(−396) − 2(−297) = −792 + 594 = −198 kJ mol⁻¹ (O₂ is zero).