Question

Answer the following question.

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

Answer 1

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

1. \[\ce{2H_{2(g)} + N_{2(g)} → N2H_{4(g)}, Δ_r H^0_1 = + 95.4 kJ}\]
2. \[\ce{N2H_{4(g)} + H_{2(g)} → 2NH_{3(g)}, Δ_r H^0_2 = -187.6 kJ}\]
                                                                                             
   \[\ce{H_{2(g)} + N_{2(g)} → 2NH_{3(g)}, Δ_rH^0 = - 92.2 kJ}\]

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