The gas-phase decomposition of nitrous oxide,
N2O(g) → N2(g) + ½ O2(g), ΔH ≈ +163 kJ mol−1,
is strongly endothermic. A chemist instead proposes the net reaction
N2O(g) + H2(g) → N2(g) + H2O(g).
In one or two sentences, explain qualitatively why introducing H2 is expected to lower the overall enthalpy change of the process in terms of bond breaking and bond formation.