Question

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

Answer 1

Hess’s law of constant heat summation states that, “Overall the enthalpy change for a reaction is equal to sum of enthalpy changes of individual steps in the reaction”.

Explanation:

1. The enthalpy change for a chemical reaction is the same regardless of the path by which the reaction occurs. Hess’s law is a direct consequence of the fact that enthalpy is a state function. The enthalpy change of a reaction depends only on the initial and final states and not on the path by which the reaction occurs.
2. To determine the overall equation of reaction, reactants and products in the individual steps are added or subtracted like algebraic entities.
3. Consider the synthesis of NH₃,

1.	\[\ce{2H_{2(g)} + N_{2(g)} -> N2H_{4(g)}}\],	`Delta_"r""H"_1^0` = +95.4 kJ
2.	\[\ce{N2H_{4(g)} + H_{2(g)} -> 2NH_{3(g)}}\],	`Delta_"r""H"_2^0` = −187.6 kJ
	\[\ce{3H_{2(g)} + N_{2(g)} -> 2NH_{3(g)}}\],	∆rH° = – 92.2 kJ

The sum of the enthalpy changes for steps (1) and (2) is equal to enthalpy change for the overall reaction.