Thermochemical Equations

A thermochemical equation is a balanced chemical equation that includes phase labels (solid, liquid, gas, aqueous) and the enthalpy change, ΔH, for the reaction. Phase labels are essential because the physical state of reactants and products affects the amount of heat released or absorbed. For example, forming liquid water releases more heat than forming water vapor from the same reactants.

The sign of ΔH in a thermochemical equation indicates whether a reaction is exothermic or endothermic. A negative ΔH value means the reaction is exothermic and releases heat to the surroundings. A positive ΔH value means the reaction is endothermic and absorbs heat from the surroundings. The magnitude of ΔH shows how much heat is involved.

The enthalpy change is an extensive property that depends on the amounts of reactants and products involved in the reaction. The stoichiometric coefficients in a balanced equation represent the molar ratios of substances. If you double all coefficients, you must also double the ΔH value because twice as much reactant produces twice as much heat.

The molar ratio between reactants or products and the heat of reaction serves as a conversion factor. First, convert the mass of a substance to moles using its molar mass. Then, multiply moles by the ΔH value per mole to find total heat exchanged. For example, 1594 moles of methane combusting at -890.8 kJ/mol releases 14.2 × 10⁶ kilojoules of heat.

At constant pressure, the heat change associated with a reaction, ΔQ, equals the change in enthalpy, ΔH. This relationship allows chemists to measure enthalpy changes experimentally using constant pressure calorimetry. The enthalpy of reaction represents the difference between the enthalpies of products and reactants under these conditions.

Methane combustion is a classic exothermic reaction where one mole of methane gas reacts with two moles of oxygen gas to produce carbon dioxide and liquid water, releasing 890.8 kilojoules of heat. The negative ΔH value in the thermochemical equation indicates heat is released to the surroundings, making it an ideal example of how energy is quantified in chemical reactions.

Physical states matter because the same reaction can release different amounts of heat depending on whether products are solids, liquids, or gases. For instance, hydrogen and oxygen forming liquid water releases 286 kJ/mol, but forming water vapor releases only 242 kJ/mol. Including phase labels ensures the thermochemical equation accurately represents the specific reaction conditions and energy change.