From the statement of Hess law, the ΔH3 is calculated when others are known from; ΔH3 = ΔH4 -( ΔH1 + ΔH2).
Hess law of constant heat summation states that the total heat evolved or absorbed for a series of reactions is the sum of all the heat absorbed or evolved at all steps in the reaction sequence.
Let the heat for the overall reaction be ΔH4 while ΔH1, ΔH2 and ΔH3 designate the heat of reaction for reactions 1,2,3. It then follows that, the ΔH3 is calculated when others are known from; ΔH3 = ΔH4 -( ΔH1 + ΔH2).
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