Combustion. Heats of Reaction. Bond Energies

All hydrocarbons are attacked by oxygen at elevated temperatures and, if oxygen is in excess, complete combustion occurs to carbon dioxide and water  

For the combustion of 1 mole of methane at 25°, we find by experiment (corrected from constant volume to constant pressure, if necessary) that the reaction is exothermic by 212.8 kcal. This statement can be expressed as follows  

The symbol (g) denotes that the reactants and products are in the gaseous state except for the water, which is liquid (/). If we wish to have AH° with gaseous water H20 (g) as the product we have to make a correction for the heat of vaporization of water (10.5 kcal mole-1 at 25°)  

2In this book we use kilocalories in place of the presently recommended (SI) joules for units of energy. As of the date of writing, it is not clear just how general the use of the joule will become among chemists. To convert calories to joules (or kcal to kJ), multiply by 4.184. 

3You may wonder how a reaction, such as combustion of methane, can occur at 25°. The fact is that the reaction can be carried out at any desired temperature. The important thing is that the AH° value we are talking about here is the heat liberated or absorbed when you start with the reactants at 25° and finish with the products at 25°. As long as AH0 is defined this way, it does not matter at what temperature the reaction actually occurs. Standard states for gases are 1 atm partial pressure. Standard states for liquids or solids usually are the pure liquid or solid at 1 atm external pressure. 

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