Heat changes in chemical reactions are often measured in the laboratory under conditions in which the reacting system is open to the atmosphere. In that case, the system is at a constant pressure. Enthalpy \(\left( H \right)\) is the heat content of a system at constant pressure. Chemists routinely measure changes in enthalpy of chemical systems as reactants are converted into products. The heat that is absorbed or released by a reaction at constant pressure is the same as the enthalpy change, and is given the symbol \(\Delta H\). Unless otherwise specified, all reactions in this material are assumed to take place at constant pressure. Enthalpies of reactions can be measured experimentally using calorimetry.
The change in enthalpy of a reaction is a measure of the differences in enthalpy of the reactants and products. The enthalpy of a reaction can be estimated using bond energies. Energy is required to break bonds and energy is released when bonds form. The quantity of the energy involved in breaking and forming a particular bond is the same, but the sign is different to indicate if the process is exothermic or endothermic. By convention, when energy is released by a system, the sign is negative and when energy is added to a system the sign is positive. The enthalpy of reaction can be estimated by comparing the energy needed to break all of the bonds of the reactants to the energy released when all of the bonds in the products form. If more energy is released when bonds form than was needed to break the bonds of the reactants, the overall reaction is exothermic. If more energy is needed to break bonds than is released when the bonds form, then the reaction is endothermic.
Several factors influence the enthalpy of a system. Enthalpy is an extensive property, determined in part by the amount of material we work with. The state of reactants and products (solid, liquid, or gas) influences the enthalpy value for a system. The direction of the reaction affects the enthalpy value. A reaction that takes place in the opposite direction has the same numerical enthalpy value, but the opposite sign.
Enthalpy is related to the heat of reaction.
CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.