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Question
State and explain Hess’s law of constant heat summation.
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Solution
Hess’s law of constant heat summation states that, “The change in enthalpy for a reaction is the same whether the reaction takes place in one or a series of steps.”
The Hess’s law is a direct consequence of the fact that the enthalpy is a state function, and so the enthalpy change depends only on the initial and final states of the system and not on the path by which the reaction takes place.
Example: The conversion of A to C can take place directly
in a single step.
A → C, ΔH° = ΔH1
The reaction can also proceed in two steps, for which the ΔH° values are known.
Step (1): A → B, ΔH° = ΔH2
Step (2): B → C, ΔH° = ΔH3
______________________________
Overall: A → C, ΔH° = ΔH2 + ΔH3
According to Hess’s law, ΔH1 = ΔH2 + ΔH3. The sequence of steps is represented in the figure.
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