AP Chemistry Unit 6 Study Notes

AP Chemistry 6.7: Hess's Law

Reverse, scale, and combine equations to calculate an overall enthalpy change.

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.

Study these notes

Start with each main idea, then follow the indented explanations and worked examples. Try the next calculation before reading its answer.

Organized from the provided Unit 6 study document. Further study: Khan Academy.

Hess's Law

  • Hess's law states that if a reaction can be written as the sum of several steps, the enthalpy change for the overall reaction equals the sum of the enthalpy changes for those steps.

    • In other words:
    • ΔHoverall = ΔH₁ + ΔH₂ + ΔH₃ + ...
    • This works because enthalpy is a state function.

  • A state function depends only on the initial and final states, not on the pathway used to get between them. Khan Academy's current Hess's law lesson emphasizes this pathway independence.

Why Hess's Law Works

  • Imagine traveling from the first floor of a building to the fifth floor.

    • You could go directly.
    • Or you could go:
    • Floor 1 → Floor 2 → Floor 4 → Floor 5.
    • Regardless of the route, the overall elevation change between Floor 1 and Floor 5 is the same.
    • Enthalpy works similarly.
    • If reactants begin at one energy level and products finish at another, the overall ΔH depends only on those initial and final states.
The Two Major Hess's Law Rules
The Two Major Hess's Law RulesIf You Reverse a Reaction

  • Reverse the sign of ΔH.

    • If:
    • A → B ΔH = +80 kJ
    • then:
    • B → A ΔH = −80 kJ
The Two Major Hess's Law RulesIf You Multiply a Reaction

  • Multiply ΔH by the same factor.

    • If:
    • A → B ΔH = +80 kJ
    • multiply the equation by 2:
    • 2A → 2B
    • then:
    • ΔH = +160 kJ

  • If you multiply by 1/2:

    • ½A → ½B
    • then:
    • ΔH = +40 kJ
Solving Hess's Law Problems

  • Suppose you want:

    • A + C → D
    • and are given:
    • Reaction 1: A → B ΔH₁ = +50 kJ
    • Reaction 2: B + C → D ΔH₂ = −120 kJ

  • Add the reactions:

    • A → B B + C → D
    • B appears on both sides and cancels.
    • Overall:
    • A + C → D

  • Add the enthalpies:

    • ΔH = +50 + (−120)
    • ΔH = −70 kJ
A More Complicated Example

  • Target:

    • 2C → 2A
    • Given:
    • A → B ΔH = +30 kJ
    • C → B ΔH = −40 kJ
    • We want C to become A.

  • Reverse the first equation:

    • B → A ΔH = −30 kJ

  • Keep the second:

    • C → B ΔH = −40 kJ
    • Add:
    • C → B B → A
    • B cancels:
    • C → A
    • ΔH:
    • −40 + −30 = −70 kJ
    • But the target is:
    • 2C → 2A

  • Double everything:

    • ΔH = −140 kJ
How to Approach Hard Hess's Law Problems

  • Start with the target reaction.

  • Look for a substance that appears only once among the given equations. Decide whether its equation must be reversed so the substance appears on the correct side.

  • Then adjust coefficients.

  • Do not calculate ΔH until you have correctly manipulated the reactions.

  • Finally, add the equations and verify that all unwanted species cancel.

  • The resulting equation must match the target exactly.

Hess's Law and Intermediate Species

  • Species that appear during the hypothetical pathway but cancel from the overall equation are similar to intermediate steps.

    • They do not appear in the final target equation.
    • This is completely fine. Hess's law does not require the actual reaction to physically occur using the exact listed pathway; it uses the fact that enthalpy depends only on initial and final states.
Hess's Law and Intermediate SpeciesCommon Mistakes

  • If you reverse the chemical equation, you must reverse the sign of ΔH.

  • If you multiply coefficients by 2, you must multiply ΔH by 2.

  • Do not multiply ΔH just because you are adding equations. Only scale it when the corresponding reaction is scaled.

  • Always check that the final chemical equation matches the target.

Hess's Law and Intermediate SpeciesRemember This

  • Hess's law is basically equation algebra:

    • flip reaction → flip ΔH sign
    • multiply reaction → multiply ΔH
    • add reactions → add ΔH