Hess’s Law and Enthalpy Changes
Quick Notes
- Hess’s Law: The overall enthalpy change for a reaction is the same, regardless of the pathway taken.
- Enthalpy is a state function – it depends only on the initial and final states, not the steps in between.
- Use Hess’s Law to calculate ΔH for a target reaction by adding or subtracting other known reactions.
- When reversing a reaction, change the sign of ΔH.
- When multiplying a reaction, multiply ΔH by the same factor.
Full Notes
What Is Hess’s Law?
Hess’s Law states:
“The total enthalpy change for a chemical reaction is the same, no matter how many steps the reaction is carried out in.”
This means if you know the enthalpy changes of several reactions, you can combine them to find the ΔH of a new reaction, even if it cannot be measured directly.
Why It Works: Enthalpy as a State Function
A state function is a property that depends only on the current state of a system (not how it got there).
Since enthalpy is a state function, ΔH depends only on the start and end points, not the reaction path.
Hess’s Law and Enthalpy Cycles
Hess’s Law is useful when directly measuring an enthalpy change is difficult. Instead, we use enthalpy cycles to calculate it indirectly.
This is especially useful for enthalpy of formation and combustion changes – HL students see Hess Cycles for Combustion and Formation Enthalpies.
Constructing and Using Hess’s Cycles
- Identify known enthalpy values.
- Draw an enthalpy cycle showing the different reaction pathways.
- Apply Hess’s Law equation to calculate the unknown enthalpy change.
Summary
- Hess’s Law states the overall enthalpy change is independent of pathway.
- Enthalpy is a state function, depending only on start and end states.
- Hess cycles allow indirect calculation of ΔH using experimental data.