Question

In an exothermic reaction, is the heat of the reation less than, greater than, or equal to zero?

In an endothermic reaction, is the heat of reaction less than, greater than, or equal to zero?

Answer 1

Exothermic reaction - Energy is released - so it is less than zero. [Negative value]

Endothermic reaction - Energy is absorbed - greater than zero [Positive value]

Examples -

Exothermic Reactions
C(s) + O2(g) → CO2(g); ΔH = –393.5 kJ
H2(g) + 1/2 O2(g) → H2O(l); ΔH = –285.8 kJ

Endothermic Reactions
N2(g) + O2(g) → 2NO(g); ΔH = +180.5 kJ > 0
C(s) + 2S(s) → CS2(l); ΔH = +92.0 kJ > 0