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Hess' law

Germain Henri Hess (1802 - 1850)Germain Henri Hess (1802 - 1850) is important primarily for his thermochemical studies, which began in 1839; his other work was of less importance. The key paper was published in 1840. The contributions of Hess to heat can be summed up in what is called the law of Hess, which is an empirical law. It is explained by thermodynamic theory, which holds that enthalpy is a state function. Chemists have made great use of the law of Hess in establishing the heats of formation of compounds which are not easily formed from their constituent elements.

BASICALLY Hess' Law states "the heat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps" Also known as the law of constant heat summation.

All it means is that no matter how many steps the chemical reaction proceeds through, the total heat evolved or absorbed is the added heat values of each step and will always be a constant figure for that process.

For example: When phosphoric acid (a tri-hydrogen acid) is neutralised with a base, the hydrogens are neutralised in 3 steps.

H3P04 + NaOH -> NaH2PO4 + H2O, this is step one, and will give X amount of heat.

NaH2PO4 + NaOH -> Na2HPO4 + H2O, this is step two, and will give Y amount of heat.

Na2PO4 + NaOH -> Na3PO4 +H2O, this is step three, and will give Z amount of heat.

Therefore X+Y+Z will give the total heat or the constant summation of heat.

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