## Chemistry (4th Edition)

By reversing reaction 1 and adding it with reaction 2, we get the required reaction. Therefore, $\Delta H_{rxn}$ is obtained by changing the sign of $\Delta {H}$ for reaction 1 (as we reversed reaction 1) and then adding it with $\Delta H$ for reaction 2. That is, $\Delta H_{rxn}=1640.1\,kJ/mol-2940.1\,kJ/mol$ $=-1300\,kJ/mol$