Using the standard enthalpies of formation, what is the standard enthalpy of reaction?
co(g) + h2o(g) -> co2(g) + h2(g)

ΔH = Enthalpy of formation of products - Enthalpy of formation of rectants.
ΔH = [ΔHf (CO2)(s) + ΔHf (H2)(g) ] − [ ΔHf (CO)(g) + ΔHf (H2O)(g) ] 

ΔHf (CO2) = −393.509 kJ/mol
ΔHf (H2) = 0 kJ/mol
ΔHf (CO) = −110.525 kJ/mol
ΔHf (H2O) = −241.818 kJ/mol
ΔH = [−393.509 + 0 ] − [ −110.525 + (−241.818) ] =
 −41.166 kJ/mol  , answer.