College Board-Aligned Original Notes
AP Chemistry 6.7: Hess's law
Add reactions and enthalpy changes to find an overall reaction enthalpy.
Aligned to Thermochemistry from the current College Board AP Chemistry course outline. Exam weighting for this unit: 7%-9% of the multiple-choice score range listed by College Board.
What To Know
- Enthalpy is a state function, so path does not matter.
- Reactions can be reversed or multiplied to match a target equation.
- The same operations must be applied to delta H values.
Detailed Notes
Hess's law is part of Unit 6: Thermochemistry. The main skill is to add reactions and enthalpy changes to find an overall reaction enthalpy. Before answering, decide whether the prompt is asking for a particulate explanation, a mathematical setup, a graph interpretation, or a connection between more than one representation.
The first idea to keep straight is that enthalpy is a state function, so path does not matter. In the same topic, remember that reactions can be reversed or multiplied to match a target equation. A complete AP answer also uses the fact that the same operations must be applied to delta H values. These ideas should be tied to specific particles, charges, attractions, energy changes, or measured quantities rather than stated as isolated facts.
For calculations or symbolic work, anchor the solution with delta Hoverall = sum delta Hsteps. Define what each quantity represents, substitute values with units, and check whether the sign, magnitude, charge balance, atom balance, or equilibrium direction makes chemical sense for this topic.
If a reaction is reversed, change the sign of delta H. In a free-response explanation, state the chemistry concept first, show the relevant equation or representation, and then explain how the evidence supports the conclusion for hess's law.
Key Vocabulary
Hess's law
The rule that reaction enthalpy can be found by adding enthalpy changes of steps.
Path independence
The idea that a state function's change does not depend on the route taken.
Target equation
The overall reaction equation that step equations are manipulated to produce.
Enthalpy cycle
A set of related reactions used to calculate an unknown enthalpy change.
Useful Relationships
Worked Study Approach
If a reaction is reversed, change the sign of delta H.
Common Mistakes
- Using a memorized rule without explaining the chemical reason behind it.
- Forgetting to conserve atoms, charge, energy, or units when the topic involves calculations.
- Mixing up particle-level explanations with macroscopic observations.
Quick Practice
How would you explain Hess's law in one sentence?
Use the focus statement above, then add one particle-level or mathematical detail.
What evidence would support an AP-style answer on this topic?
Use a balanced equation, diagram, graph, table, numerical setup, or particulate model depending on the prompt.