This is Germain Henry Hess. In the study of thermochemistry, there is an important law from him named 'HESS' LAW'. This is a principle of conservation of energy.
Hess' Law states that the heat evolved or absorbed in a chemical reaction is the same whether the reaction takes place in one or in several steps. This is also known as the law of constant heat summation.
Hess's law is important because it provides a practical way to combine thermochemical equations for known 'step' reactions to get a thermochemical equation for some 'target' reaction. The basic procedure is :
1. Write all the thermochemical equations for the step reactions.
2. Write the balanced chemical equation for the target reaction.
3. Reverse step reactions so products/reactants match the target reaction.
4. Scale step reactions so products/reactants that don't appear in the target reaction will cancel out.
5. Add the step reactions.
6. Scale the resulting reaction so it matches the target reaction
Okay students, you have to apply this law. Please find an exercise from your textbook and try to do summation of step reactions to find the enthalpy change of the target reaction. Firstly, do the simple equation, focus on 2 step reactions.1. Write all the thermochemical equations for the step reactions.
2. Write the balanced chemical equation for the target reaction.
3. Reverse step reactions so products/reactants match the target reaction.
4. Scale step reactions so products/reactants that don't appear in the target reaction will cancel out.
5. Add the step reactions.
6. Scale the resulting reaction so it matches the target reaction
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