Hess’s Law is a principle in thermochemistry that states:
The total heat change of a chemical reaction is the same, no matter whether the reaction occurs in one step or multiple steps, as long as the initial and final conditions are the same.
Key points:
- Additivity of enthalpy changes:
- You can add the heat changes of individual steps to find the overall heat change of a reaction.
- Independent of pathway:
- The heat change depends only on the initial and final states of the reactants and products, not on how the reaction occurs.
- Applications:
- Calculating enthalpy changes for reactions that are difficult to measure directly.
- Determining heats of formation, combustion, and reaction for complex chemical reactions.
Example in simple terms:
If breaking compound A into B in one step releases 50 units of heat, and doing it in two steps releases 30 and 20 units respectively, the total heat change is still 50 units.
In short, Hess’s Law allows chemists to calculate heat changes by breaking reactions into simpler steps, making it a very useful tool in thermochemistry.