Thermochemical Equations

Most chemical reactions occur at atmospheric pressure. Under conditions of constant pressure, the heat change associated with the reaction, ΔQ, equals the change in enthalpy, ΔH, also called the enthalpy or heat of reaction.

Enthalpy of the reaction is the difference between the enthalpies of the products and the reactants. When the enthalpy of the products is greater than the enthalpy of the reactants, ΔH is positive. Such reactions absorb heat and are endothermic.

On the contrary, if the enthalpy of the reactants is greater than the enthalpy of the products, ΔH is negative. Such reactions release heat and are exothermic.

For any chemical reaction, the magnitude of the accompanying enthalpy change depends on the stoichiometric amounts of reactants and products, as indicated by the coefficients of the balanced equation.

A balanced chemical equation that includes phase labels and enthalpy of reaction, ΔH, is called a thermochemical equation.

Consider the combustion of methane – a primary source of fuel. Burning methane releases heat to the surroundings. The exothermic nature of the reaction is indicated by the negative enthalpy change in the thermochemical equation.

The equation for combustion shows that when one mole of methane gas reacts with 2 moles of oxygen gas to yield 1 mole of carbon dioxide gas and 2 moles of liquid water, 890.8 kilojoules of heat are released to the surroundings.

The molar ratio between the reactants or products and the heat of reaction can be used as conversion factors to calculate the heat exchanged during the reaction.

If a gas cylinder contains 25.5 kilograms of methane, and all of the methane in the cylinder undergoes combustion, what amount of heat will be produced? In general, the conceptual plan is to convert the mass to moles and then moles to the heat of reaction.

To begin, 25.5 kilograms is multiplied by 1000 to find the mass in grams. Then, the mass of methane is divided by its molar mass – 16.0 grams per mol, to yield 1594 moles of methane.

Finally, using the conversion factor between moles of methane and heat of reaction, 1594 moles of methane releases 14.2 times ten to the 6 kilojoules of heat - which is the heat of the reaction. The answer is negative since the reaction is exothermic as heat evolves in the reaction.