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Hess, Germain Henri

law heat chemistry reaction

(1802–50) Swiss–Russian chemist; pioneer of thermochemistry.

When Hess was 3 years old his father, a Swiss artist, became tutor to a rich Moscow family and the boy moved from his birthplace (Geneva) to Russia. He was there for the rest of his life, taking a medical degree at Tartu in 1825 and then visiting in Stockholm. The visit was only for a month, but its influence was permanent. From 1830 he studied the heat evolved in chemical reactions, as a route to the understanding of ‘chemical affinity’. Rather little had been done in thermo-chemistry since the work by . Hess’s Law (the law of constant heat summation) of 1840 states that the heat change accompanying a chemical reaction depends only on the final and initial states of the system and is independent of all intermediate states. The law enables the heat of reaction to be calculated in a case where direct measurement is impractical. Hess’s Law follows from the law of conservation of energy, but the latter was not clearly understood in 1840.

Hess researched in other areas, and did much for the development of chemistry in Russia, where he taught in St Petersburg.

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