# Answer the following question. State Hess’s law of constant heat summation. Illustrate with an example. State its applications. - Chemistry

State Hess’s law of constant heat summation. Illustrate with an example. State its applications.

#### Solution

1. Hess’s law of constant heat summation:

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”.

2. Illustration:

• 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 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.
• To determine the overall equation of reaction, reactants and products in the individual steps are added or subtracted like algebraic entities.
• Consider the synthesis of NH3,

1.  2H2(g) + N2(g) → N2H4(g), triangle_"r" "H"_1^0 = + 95.4 kJ

2. N2H4(g) + H2(g) → 2NH3(g), triangle_"r" "H"_2^0 = - 187.6 kJ

3H2(g) + N2(g) → 2NH3(g), ΔrH° = - 92.2 kJ

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

3. Application of Hess’s law:
The Hess's law has been useful to calculate the enthalpy changes for the reactions with their enthalpies being not known experimentally.

Concept: Thermochemistry
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#### APPEARS IN

Balbharati Chemistry 12th Standard HSC for Maharashtra State Board
Chapter 4 Chemical Thermodynamics
Exercises | Q 4.03 | Page 88