Thermodynamics

The bond enthalpies of heavy hydrogen, $\mathrm{O}-\mathrm{O}$ and $\mathrm{D}-\mathrm{O}$ are $+400,+498$ and $+490 \mathrm{kJmol}^{-1}$, respectively. The $\Delta_r H^{\circ}$ of the reaction to produce $\mathrm{D}_2 \mathrm{O}$ is