Explain how a problem can be solved using Hess's Law.

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justaguide | College Teacher | (Level 2) Distinguished Educator

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Hess's law gives a way of calculating the total change in enthalpy for an equation if the reaction can be split into a number of steps and the enthalpy change for each step is known.

Stated differently, it means that enthalpy changes are additive in nature. And the difference in the enthalpy change in the formation of products and the enthalpy change in the formation of the reactants is the enthalpy change of the reaction. As an example consider the following reaction:

PCl5 --> PCl3 + Cl2

We need to know the enthalpy change in the reaction. We are given that

P4 + 6Cl2 --> 4 PCl3, enthalpy change = -2439 kJ
and 4PCl5 --> P4 + 10 Cl2, enthalpy change = 3438 kJ

Adding the two equations we get:
P4 + 6Cl2 + 4PCl5 --> 4 PCl3 + P4 + 10 Cl2

Eliminate the common terms and simplify
PCl5 --> PCl3 + Cl2

The enthalpy change is (-2439 + 3438)/4 = 249.75 kJ

The original reaction PCl5 --> PCl3 + Cl2 has an enthalpy change of 249.75 kJ, the same as what we got.

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