Enthalpies of Chemical Changes

Exothermic reactions decrease the system's enthalpy, releasing energy to the surroundings. Endothermic reactions increase the system's enthalpy, absorbing energy from the surroundings. Energy diagrams visually represent these differences, with the gap between reactant and product lines showing the enthalpy change for each reaction type.

The standard enthalpy change equals the sum of standard enthalpies of formation of products minus the sum of standard enthalpies of formation of reactants, each multiplied by stoichiometric coefficients. Standard enthalpy of formation for elements in their reference states is zero by definition. This calculation applies under standard conditions, typically 298.15 K or 25°C.

Hess's law states that the overall enthalpy change of a multi-step reaction equals the sum of enthalpy changes for individual steps. Since enthalpy is a state function, the total energy change depends only on initial and final states, not the reaction pathway. Thermochemical equations must be manipulated appropriately—multiplying, dividing, or reversing reactions—to sum to the target reaction.

Multiplying a chemical equation by a factor changes its enthalpy change by the same factor. Reversing a reaction flips the sign of its enthalpy change; an exothermic forward reaction becomes endothermic in reverse. These manipulations are essential when using Hess's law to combine reactions into a desired overall equation.

Standard enthalpy of formation is the enthalpy change when one mole of a compound forms from its elements in their reference states under standard conditions. By definition, the standard enthalpy of formation for elements in their reference states equals zero. This value serves as a reference point for calculating reaction enthalpies using tabulated data.

Enthalpy is a state function because its value depends only on the initial and final states of a system, not on the path taken between them. This property enables Hess's law: the enthalpy change for any reaction can be calculated by combining enthalpies of other reactions, regardless of whether the intermediate steps actually occur.

Stoichiometric coefficients directly scale enthalpy values in calculations. When computing standard enthalpy change, each standard enthalpy of formation is multiplied by its corresponding stoichiometric coefficient. Doubling reactant or product quantities doubles the enthalpy change; halving them halves the enthalpy change proportionally.