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Hess's law


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If reactants can be converted into products by a series of reactions, the sum of the heats of these reactions (with due regard to their sign) is equal to the heat of reaction for direct conversion from reactants to products. More generally, the overall energy change in going from reactants to products does not depend on the route taken. The law can be used to obtain thermodynamic data that cannot be measured directly. For example, the heat of formation of ethane can be found by considering the reactions:2C(s)+3H2(g)+3½O2(g) → 2CO2(g)+3H2O(l) The heat of this reaction is 2ΔHC+3ΔHH, where ΔHC and ΔHH are the heats of combustion of carbon and hydrogen respectively, which can be measured. By Hess's law, this is equal to the sum of the energies for two stages:2C(s)+3H2(g) → C2H6(g) (the heat of formation of ethane, ΔHf) and C2H6(g)+3½O2 → 2CO2(g)+3H2O(l) (the heat of combustion of ethane, ΔHE). As ΔHE can be measured and as ΔHfHE=2ΔHc+3ΔHH ΔHf can be found. Another example is the use of the Born-Haber cycle to obtain lattice energies. The law was first put forward in 1840 by the Swiss-born Russian chemist Germain Henri Hess (1802–50). It is sometimes called the law of constant heat summation and is a consequence of the law of conservation of energy.

2C(s)+3H2(g)+3½O2(g) → 2CO2(g)+3H2O(l)

2C(s)+3H2(g) → C2H6(g)

C2H6(g)+3½O2 → 2CO2(g)+3H2O(l)

ΔHfHE=2ΔHc+3ΔHH

Subjects: Chemistry — Physics.


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