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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 was fully aware of the importance of his own contribution. In 1842, Hess proposed the law of thermoneutrality, which states that no heat is evolved in the exchange reactions of neutral salts in aqueous solution. A full explanation would only be given 45 years later, in terms of electrolytic dissociation, by the Swedish chemist Svante ...
From the first law of thermodynamics, =, where W is the work done by the system. When only expansion work is possible for a process we have Δ U = Q V {\displaystyle \Delta U=Q_{V}} ; this implies that the heat of reaction at constant volume is equal to the change in the internal energy Δ U {\displaystyle \Delta U} of the reacting system.
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.
Hess's law, in physical chemistry: the total enthalpy change during the complete course of a reaction is the same whether the reaction is made in one step or in several steps. Hick's law, in psychology, describes the time it takes for a person to make a decision as a function of the number of possible choices.
The sum of the Gibbs energy changes for subsequent reductions (e.g. from O 2 to H 2 O 2, then from H 2 O 2 to H 2 O) is the same as the Gibbs energy change for the overall reduction (i.e. from O 2 to H 2 O), in accordance with Hess's law.
Some Democrats are dismissing the forthcoming DOGE push to cut wasteful government spending. Others in the party aren't totally writing off what Elon Musk and Vivek Ramaswamy are selling.
The law distinguishes two principal forms of energy transfer, heat and thermodynamic work, that modify a thermodynamic system containing a constant amount of matter. The law also defines the internal energy of a system, an extensive property for taking account of the balance of heat and work in the system. Energy cannot be created or destroyed ...