Hess’s law states that the evolved or absorbed heat in a chemical reaction is the same whether the reaction takes one step or several steps. It is also known as Hess’s law of constant heat summation.
The Swiss-Russian chemist Germain Henri Hess discovered the aforementioned law while researching the quantities of heat released in various chemical reactions. In his 1840 publication, he notes the experiment he conducted:
- He first measures the heat released when concentrated sulphuric acid is added to ammonia.
- He then measures the heat released when sulphuric acid is diluted with water after which he adds ammonia to the diluted acid and measures the heat released.
From this, he notes that the heat released when concentrated sulphuric acid was added to ammonia was roughly the same as the heat released when diluted sulphuric acid was added to the same volume of ammonia. From these observations, he deduced that the quantity of heat released after a chemical reaction has taken place is constant, whether the reaction takes place directly, or indirectly and on different occasions. Here is his original tabulation of the results of the aforementioned experiment (first column: concentration of the acids, second column: heat released after adding them to ammonia, third column: heat released after adding the acids to water, fourth column: total heat released):