What is Hess's Law?
Hess's Law states that the total enthalpy change of a reaction is the same regardless of the pathway taken. Whether a reaction occurs in one step or multiple steps, ΔH(total) = ΔH₁ + ΔH₂ + ... + ΔHₙ. This law is crucial for calculating enthalpy changes of reactions that are difficult to measure directly.
Hess's Law allows us to calculate ΔH for any reaction by combining other reactions whose enthalpy changes are known. When combining reactions, reverse any that need reversing (flip the sign of ΔH) and multiply reactions by coefficients as needed. The sum of all ΔH values gives the overall ΔH.
- 1↓Target reactionA → D (enthalpy unknown)
- 2↓Known reactionsMultiple pathways with known ΔH values
- 3↓Arrange and reverseReverse, multiply, and add known reactions
- 4↓Sum the ΔH valuesΔH(total) = ΔH₁ + ΔH₂ + ...
- 5ResultΔH for target reaction is calculated
Try it: interactive calculator
Step-by-step worked examples
Given: (1) C + O₂ → CO₂, ΔH = −393 kJ/mol. (2) 2CO + O₂ → 2CO₂, ΔH = −566 kJ/mol. Find ΔH for: 2C + O₂ → 2CO.
Write target: 2C + O₂ → 2CO
Use reaction (1) twice, reverse (2) and divide by 2:
Reaction (1) × 2: 2C + 2O₂ → 2CO₂, ΔH = 2(−393) = −786 kJ/mol
Reverse (2)÷2: CO₂ → CO + ½O₂, ΔH = +566/2 = +283 kJ/mol
Add them: 2C + 2O₂ → 2CO₂ (ΔH = −786)
+ CO₂ → CO + ½O₂ (ΔH = +283)
Cancel O₂ and CO₂: 2C + O₂ → 2CO, ΔH = −786 + 283 = −503 kJ/molΔH₁ = −200 kJ/mol, ΔH₂ = −150 kJ/mol, ΔH₃ = +50 kJ/mol. Total ΔH?
ΔH(total) = ΔH₁ + ΔH₂ + ΔH₃ ΔH(total) = (−200) + (−150) + (+50) ΔH(total) = −300 kJ/mol
Reaction A has ΔH = −400 kJ/mol. If you reverse it, what is ΔH?
When reversing a reaction, flip the sign of ΔH. Reversed: ΔH = +400 kJ/mol This is useful when combining reactions and a particular pathway must go backwards.
Flashcards
Quick quiz
Q1.Hess's Law states ΔH is:
Q2.Reverse reaction: A → B, ΔH = −500 kJ/mol. New ΔH?
Q3.3A → 3B, ΔH = −600 kJ/mol. If you double the reaction:
Q4.Given ΔH₁ = −100, ΔH₂ = −200, ΔH₃ = +50 kJ/mol:
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Common mistakes
Hess's Law only works for single-step reactions. — Correct: It works for multi-step reactions; the total ΔH is the sum of individual ΔH values.
Reversing a reaction doesn't change ΔH. — Correct: Reversing flips the sign of ΔH.
Multiplying a reaction by 2 doesn't change ΔH. — Correct: ΔH is multiplied by the same factor as the reaction.
The enthalpy change depends on the reaction pathway. — Correct: Hess's Law shows ΔH is independent of pathway; it depends only on initial and final states.
FAQ
What is Hess's Law?
A law stating that the enthalpy change of a reaction is independent of the pathway taken — it depends only on the initial and final states of the reactants and products.
How do you use Hess's Law to find an unknown ΔH?
Arrange known reactions (reversing and/or multiplying as needed) so they sum to the target reaction. Then add their ΔH values.
What happens to ΔH when you reverse a reaction?
The sign reverses: if ΔH was −100 kJ/mol, reversing gives +100 kJ/mol.
Can Hess's Law be used for reactions at different temperatures?
Hess's Law applies at constant temperature. At different temperatures, the ΔH values change slightly due to heat capacity effects.