The formula for ΔU°rxn is: ΔU°rxn = H - PV. Assuming ideal gas, it could also be: ΔU°rxn = H - RT.
For the first reaction:
ΔU°rxn = (-511.3 kJ/mol) - (0.008314 J/mol-K)(25+273 K) = -513.78 kJ/mol
For the second reaction:
ΔU°rxn = (-184.6 kJ/mol) - (0.008314 J/mol-K)(25+273 K) = -187.08 kJ/mol