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Hess’s law of constant heat summation - Chemistry

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State and explain Hess’s law of constant heat summation.

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Solution

It states that, “The amount of heat evolved or absorbed in a chemical change is the same whether the process takes place in one step or in several steps”. (i.e. it follows 1st Law of Thermodynamics)

For example, carbon can be oxidised to CO2 either directly or in two different steps as given below:

I st method C(s)+O2(g) →CO2(g)ΔH=-94.3kcal

II methode C(s)+(1/2)O2(g)→ CO(g)ΔH1=-26.0kcal

CO(g)+(1/2)O2(g)ΔH2=-68.3kcal

According to Hess’s law ΔH must be equal to  ΔH1+ΔH2 which is true

Concept: Chemical Thermodynamics and Energetic - Hess’ Law of Constant Heat Summation
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