Oxides And Oxy-acids Of Chlorine : Hess's Law

If a reaction is carried out in stages, the algebraic sum of the amounts of heat evolved in the separate stages (heat absorbed being reckoned negative) is equal to the total evolution of heat when the reaction occurs directly.

This simple consequence of the Law of Conservation of Energy is known as Hess's Law of constant heat summation (1840). It enables one to calculate many heats of reaction which could not be determined directly.

Example. - Find the heat of formation per mol of carbon monoxide, CO, from solid carbon and gaseous oxygen, given the following data:

Heat of combustion of carbon to carbon dioxide: C + O₂=CO₂ + 94 k. cal.

Heat of combustion of carbon monoxide to dioxide: CO + ½O₂ = CO₂ + 67.8 k. cal.

By subtracting the second of these equations from the first, we find: Heat of formation of carbon monoxide: C + ½O₂ = CO + 26.2 k. cal.

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Oxides And Oxy-acids Of Chlorine
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