Consider the fusion reaction ${ }_{1}^{2} \mathrm{H}+{ }_{1}^{2} \mathrm{H} \rightarrow{ }_{2}^{3} \mathrm{He}+{ }_{0}^{1} \mathrm{n} .$ (a) Estimate the barrier energy by calculating the repulsive electrostatic potential energy of the two ${ }_{1}^{2} \mathrm{H}$ nuclei when they touch. (b) Compute the energy liberated in this reaction in $\mathrm{MeV}$ and in joules. (c) Compute the energy liberated per mole of deuterium, remembering that the gas is diatomic, and compare with the heat of combustion of hydrogen, about $2.9 \times 10^{5} \mathrm{~J} / \mathrm{mol}$