The balanced chemical equation for the combustion of methane is:
CH4(g) + 2 O2(g) -> CO2(g) + 2 H2O(l)
The standard enthalpy of formation (ΔHf) for methane is -74.8 kJ/mol, for carbon dioxide it is -393.5 kJ/mol, and for water it is -285.8 kJ/mol. The standard enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states.
To calculate the standard enthalpy of reaction (ΔHrxn), we need to use the equation:
ΔHrxn = ΣΔHf (products) - ΣΔHf (reactants)
Plugging in the values for the reactants and products, we get:
ΔHrxn = (2 * -285.8 kJ/mol) + (-393.5 kJ/mol) - (-74.8 kJ/mol)
ΔHrxn = -890.2 kJ/mol
This reaction is exothermic because the value of ΔHrxn is negative. Exothermic reactions release heat, while endothermic reactions absorb heat.