Hesss Law Definition - Chemistry Glossary

Hesss law states that the energy change in an overall chemical reaction is equal to the sum of the energy changes in the individual reactions comprising it. In other words, the enthalpy change of a chemical reaction (the heat of reaction at constant pressure) does not depend on the pathway between the initial and final states. The law is a variation of the first law of thermodynamics and conservation of energy. Importance of Hesss Law Because Hesss law holds true, its possible to break a chemical reaction into multiple steps and use the standard enthalpies of formation to find the overall energy of a chemical reaction. Standard enthalpy tables are compiled from empirical data, usually acquired using calorimetry. Using these tables, its possible to calculate whether or not a more complex reaction is thermodynamically favorable or not. Applications of Hesss Law In addition to calculating the enthalpy of a reaction rather than directly measuring it, Hesss law is used to: Find electron affinities based on theoretical lattice energy.Calculate heat change of phase transitions.Calculate heat change when a substance changes allotropes.Find the heat of formation of an unstable intermediate in a reaction.Find the lattice energy of ionic compounds. Sources Chakrabarty, D.K. (2001). An Introduction to Physical Chemistry. Mumbai: Alpha Science. pp. 34–37. ISBN 1-84265-059-9.Leicester, Henry M. (1951). Germain Henri Hess and the Foundations of Thermochemistry. The Journal of Chemical Education. 28 (11): 581–583. doi:10.1021/ed028p581