Hess, Germain Henri (1802–1850)

Swiss-born Russian chemist, a pioneer in the field of thermochemistry. The law of constant heat summation is named after him.

Hess was born in Geneva, but his family emigrated to Russia. Hess studied at the University of Dorpat (now Tartu, Estonia), and in Stockholm, Sweden, under chemist Jöns Berzelius. Returning to Russia, he took part in a geological expedition to the Urals before setting up a medical practice in Irkutsk. In 1830 he settled in St Petersburg, where he held various academic appointments, becoming professor at the Technological Institute.

Hess's law was published 1840 and states that the heat change in a given chemical reaction depends only on the initial and final states of the system and is independent of the path followed, provided that heat is the only form of energy to enter or leave the system. Every chemical change is either endothermic (absorbing heat) or exothermic (evolving heat). Hess's law is in fact an application of the law of conservation of energy, but this was not formulated until 1842.

In 1842 Hess proposed his second law, the law of thermoneutrality, which states that in exchange reactions of neutral salts in aqueous solution, no heat effect is observed.