We can use hess's law to calculate enthalpy changes that cannot be measured. one such reaction is the conversion of methane to ethylene: 2ch4(g)⟶c2h4(g) 2h2(g) part a calculate the δh∘ for this reaction using the following thermochemical data: ch4(g) 2o2(g)⟶co2(g) 2h2o(l) δh∘