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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, “Overall the enthalpy change for a reaction is equal to sum of enthalpy changes of individual steps in the reaction”.
Explanation:
- 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 reaction, reactants and products in the individual steps are added or subtracted like algebraic entities.
- Consider the synthesis of NH3,
| 1. | \[\ce{2H_{2(g)} + N_{2(g)} -> N2H_{4(g)}}\], | `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.
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