ヘスの法則

Enthalpy is a state function because its change depends only on the initial and final states of a system, not on the path taken between them. This means the enthalpy change for a reaction is the same whether it occurs in one step or multiple steps. This property makes Hess's Law valid and allows chemists to calculate enthalpy changes from other known reactions.

Thermochemical equations can be manipulated by reversing their direction, which reverses the sign of ΔH, or by multiplying coefficients by a factor, which multiplies ΔH by the same factor. These manipulations allow individual reaction steps to be combined so their sum equals the target reaction. The stoichiometric coefficients and enthalpy changes must be adjusted consistently to ensure the equations align properly.

When a chemical reaction is reversed, the enthalpy change (ΔH) has the same magnitude but opposite sign. For example, if a forward reaction is exothermic with ΔH = −100 kJ, the reverse reaction is endothermic with ΔH = +100 kJ. This relationship reflects the fact that energy released in one direction must be absorbed when the reaction proceeds in the opposite direction.

Enthalpy change is directly proportional to the quantities of reactants and products. When stoichiometric coefficients are multiplied by a factor, the enthalpy change must be multiplied by the same factor. For instance, if coefficients are doubled, ΔH is also doubled. This proportional relationship allows chemists to scale reactions to match the target equation.

Some reactions are difficult or impossible to measure directly because they occur under extreme conditions, involve unstable intermediates, or proceed too slowly or too rapidly for accurate measurement. For these reactions, Hess's Law provides an alternative by calculating the enthalpy change from other experimentally determined reactions that sum to the target reaction.

According to Hess's Law, the overall enthalpy change of a reaction equals the sum of the enthalpy changes of all individual steps. Intermediate products that appear in one step and are consumed in another cancel out, leaving only the net reactants and products. This additive property holds regardless of how many steps the reaction is divided into.

Select reactions whose steps, when combined, produce the target reaction. Arrange and manipulate the equations so that unwanted intermediates cancel out and the final products and reactants match the desired reaction. The stoichiometric coefficients and reaction directions must be adjusted so that when summed, they yield the target equation with the correct coefficients.