9.55, 9.57

[Ardo 2K]

Can someone please explain why in problem 9.55 we only use the delta H of formation of the products to find the Standard change in enthalpy of the reaction, but in 9.57 we subtract the the sum of the enthalpy of the formation of products by the sum of the enthalpy of formation of reactants. Don't we need to do that in both cases?

[Deap Bhandal L1 S1J]

In 9.55 delta H (formation) equals delta H (reaction) since reactants for each equation you produce are in their standard states. These reactions have a total delta H of zero so subtracting the reactants like you did in 9.57 is not necessary.