Final answer:
The heat of formation for water (H2O) can be omitted when calculating the reaction enthalpy for the combustion of methane.
Step-by-step explanation:
The reaction enthalpy for the combustion of methane can be calculated by using the heat of formation values for the reactants and products involved in the reaction. In the combustion equation of methane, CH4 + 2O2 → CO2 + 2H2O, the heat of formation for methane (CH4), oxygen (O2), and carbon dioxide (CO2) are needed to calculate the reaction enthalpy. However, the heat of formation for water (H2O) can be omitted since it is a by-product of the reaction and not a reactant.