Enthalpy changes in chemical reactions

The enthalpy changes in common chemical reaction can be calculated from the heat of formation of compounds and elements involved in the corresponding reaction. Here is the oxidation reaction of methane, CH4 producing CO2 gas and H2O gas. Let’s calculate the heat of this reaction. This reaction can be written as the combination of three other reactions. Those are the formation reactions for CO2 gas, two H2O gas and CH4 gas. The above reaction is a combination of reaction I + II - reaction III. So, according to Hess’s law, the heat of reaction is delta Hf nough CO2 + 2 times delta Hf nought H2O - delta Hf nought CH4. In general, the heat of reaction is expressed like this.

It is the difference between delta H for the products and delta H for the reactants. What is delta H then? If the material involved is a compound, Delta H is delta Hf, the heat of formation at the corresponding temperature. If the material involved is a element, Delta H is just the enthalpy of that element at the corresponding temperature. Therefore, at 298K the standard condition, delta H is zero since it is at the reference state.