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Hess's law of constant heat summation, also known simply as Hess's law, is a relationship in physical chemistry and thermodynamics [1] named after Germain Hess, a Swiss-born Russian chemist and physician who published it in 1840. The law states that the total enthalpy change during the complete course of a chemical reaction is independent of ...
Hess's law states that the sum of the energy changes of all thermochemical equations included in an overall reaction is equal to the overall energy change. Since Δ H {\displaystyle \Delta H} is a state function and is not dependent on how reactants become products as a result, steps (in the form of several thermochemical equations) can be used ...
Hess' law of constant heat summation (1840): The energy change accompanying any transformation is the same whether the process occurs in one step or many. [3] These statements preceded the first law of thermodynamics (1845) and helped in its formulation. Thermochemistry also involves the measurement of the latent heat of phase transitions.
For many substances, the formation reaction may be considered as the sum of a number of simpler reactions, either real or fictitious. The enthalpy of reaction can then be analyzed by applying Hess' law, which states that the sum of the enthalpy changes for a number of individual reaction steps equals the enthalpy change of the overall reaction.
The law was named for chemist and physicist Robert Boyle, who published the original law in 1662. The French physicist Edme Mariotte discovered the same law independently of Boyle in 1676. 1671: Newton–Raphson method – Joseph Raphson (1690), Isaac Newton (Newton's work was written in 1671, but not published until 1736).
A similar calculation applies for any metal other than lithium and/or any non-metal other than fluorine. [6] The word cycle refers to the fact that one can also equate to zero the total enthalpy change for a cyclic process, starting and ending with LiF(s) in the example. This leads to
Germain Henri Hess (Russian: Герман Иванович Гесс, romanized: German Ivanovich Gess; 7 August 1802 – 12 December [O.S. 30 November] 1850) was a Swiss-Russian chemist and doctor who formulated Hess' law, an early principle of thermochemistry.
In thermochemistry, the enthalpy of solution (heat of solution or enthalpy of solvation) is the enthalpy change associated with the dissolution of a substance in a solvent at constant pressure resulting in infinite dilution.