Chemistry • Year 11 • Module 4 • Lesson 8
Hess's Law
Lock in the core vocabulary, the path-independence principle, and the two manipulation rules before tackling calculations.
1. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: Hess's Law, state function, enthalpy, thermochemical equation, intermediate species, target equation, manipulation rule, pathway independence, sign change, scaling. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | The principle that the total enthalpy change for a reaction is the same regardless of the pathway taken from reactants to products. | |
| 1.2 | A thermodynamic property that depends only on the current state of a system (initial and final conditions), not on the history of how it got there. | |
| 1.3 | A measure of the heat content stored in chemical bonds; symbol H; change in H is measured as heat flow at constant pressure. | |
| 1.4 | A balanced chemical equation that also states the ΔH value and includes state symbols for all species. | |
| 1.5 | A species that appears in the intermediate steps of a Hess's Law calculation but cancels out when equations are added, leaving only the target species. | |
| 1.6 | The overall equation whose ΔH value is to be determined by combining two or more known equations. | |
| 1.7 | The operation of reversing a thermochemical equation, which requires the sign of ΔH to be multiplied by −1. | |
| 1.8 | The operation of multiplying all coefficients in a thermochemical equation by a factor n, which also multiplies ΔH by the same factor n. | |
| 1.9 | The quality of being independent of the route taken; in thermochemistry this means ΔH is fixed by the initial and final states alone. | |
| 1.10 | Any allowed algebraic manipulation applied to a thermochemical equation; Hess's Law permits only two: reversing the equation or multiplying it by a constant. |
2. True or false — with correction
For each statement circle T or F. If false, write the corrected version on the line below. 10 marks — 1 for T/F, 1 for each correction where needed
2.1 Hess's Law states that the enthalpy change depends on the number of intermediate steps taken to convert reactants to products. T / F
2.2 Enthalpy is a state function because it depends only on the initial and final states of the system, not the pathway. T / F
2.3 If a thermochemical equation is reversed, the magnitude of ΔH stays the same but the sign changes from positive to negative (or vice versa). T / F
2.4 Scaling a thermochemical equation by a factor of 3 changes only the coefficients of the equation; ΔH is unaffected. T / F
2.5 It is impossible to measure the standard enthalpy of formation of CO(g) directly in a calorimetry experiment because burning carbon always produces a mixture of CO and CO₂. T / F
3. Cloze — fill the blanks
Complete the paragraph by filling each blank with the correct word or phrase from the word bank. Each word is used once. 8 marks
Word bank: state function • First Law of Thermodynamics • pathway • Germain Hess • 1840 • intermediates • reversed • cancel
Hess's Law was discovered by Swiss-Russian chemist ______________ in ______________, more than fifty years before the electron was identified. The law states that the total enthalpy change of a reaction is independent of the ______________ taken. This follows directly from enthalpy being a ______________, a consequence of the ______________: energy is conserved. In practice, to find ΔH for a target reaction, chemists take known thermochemical equations, manipulate them by reversing and/or scaling, then add them together. Species that appear on both sides of the combined equations are called ______________. When equations are combined correctly, these species ______________ out, leaving only the species in the target equation. If an equation must be ______________ so that a species appears on the correct side, the sign of its ΔH must also be flipped.
4. Label the enthalpy cycle diagram
The energy-level diagram below shows two pathways from C(s) + O₂(g) to CO₂(g): the direct path (top arrow) and the two-step path via CO(g) (bottom arrows). Labels A–F are missing. Write each label in the table. 6 marks
| Label | Your answer |
|---|---|
| A | |
| B | |
| C | |
| D | |
| E | |
| F |
5. Function recall
Answer each in 1–2 precise sentences. 8 marks — 2 each
5.1 What does it mean to say enthalpy is a “state function”? Why does this make Hess’s Law work?
5.2 Why is it necessary to change the sign of ΔH when a thermochemical equation is reversed?
5.3 In a Hess’s Law calculation, what is the role of “intermediate species” and how do you know they have cancelled correctly?
5.4 Germain Hess published this law in 1840. Explain why his law only applies to enthalpy and not to quantities such as heat flow q.
6. Concept map — Hess’s Law connections
Draw labelled arrows between the six terms to show how they connect. Each arrow must carry a linking phrase (e.g. “enables”, “requires”, “is a type of”). Aim for at least 6 labelled arrows. 6 marks
Supplied terms: Hess’s Law • state function • enthalpy (ΔH) • pathway independence • thermochemical equation • intermediate species.
Q1 — Term–definition match
1.1 Hess’s Law • 1.2 state function • 1.3 enthalpy • 1.4 thermochemical equation • 1.5 intermediate species • 1.6 target equation • 1.7 sign change • 1.8 scaling • 1.9 pathway independence • 1.10 manipulation rule.
Q2 — True / false with correction
2.1 False. Correction: Hess’s Law states that the total enthalpy change is the same regardless of the number of intermediate steps — it is pathway-independent.
2.2 True.
2.3 True.
2.4 False. Correction: scaling by 3 multiplies both the coefficients and ΔH by 3 — they are inseparable.
2.5 True.
Q3 — Cloze
In order: Germain Hess • 1840 • pathway • state function • First Law of Thermodynamics • intermediates • cancel • reversed.
Q4 — Energy-level diagram labels
A: ΔH for the direct reaction C(s) + O₂(g) → CO₂(g) = −393.5 kJ mol⁻¹.
B: ΔH₁ for C(s) + ½O₂(g) → CO(g) = −110.5 kJ mol⁻¹.
C: ΔH₂ for CO(g) + ½O₂(g) → CO₂(g) = −283.0 kJ mol⁻¹.
D: Hess’s Law (path-independence principle).
E: Intermediate species (CO(g) + ½O₂(g) — appears in steps but not in the target equation).
F: ΔH(A) = ΔH(B) + ΔH(C) — i.e. −393.5 = −110.5 + (−283.0).
Q5 — Function recall
5.1 A state function depends only on the current state (initial and final conditions), not on the history or route taken. Because enthalpy is a state function, the ΔH for converting reactants to products is fixed regardless of how many steps are taken — this is exactly what Hess’s Law describes.
5.2 Reversing an equation means the reaction literally runs backward: what was exothermic (releases energy) now becomes endothermic (absorbs the same energy). Conservation of energy requires the magnitude to be identical but the direction to be opposite, so the sign of ΔH must change.
5.3 Intermediate species are species that appear in the sub-steps but not in the overall target reaction; they cancel when the sub-equations are added together. You know cancellation is correct when the species remaining on both sides of the combined equation exactly match the target equation — no more, no fewer.
5.4 Heat flow q depends on the pathway (e.g. whether work is done on the system, whether the process is at constant pressure or constant volume). Enthalpy is specifically defined so that ΔH = q at constant pressure, and it is a state function — independent of path. Hess’s Law relies on this path-independence, so it applies to ΔH but not to q in general.
Q6 — Sample concept map
A correct map should include arrows such as:
- enthalpy (ΔH) — is a → state function
- state function — implies → pathway independence
- pathway independence — is the basis of → Hess’s Law
- Hess’s Law — is applied using → thermochemical equation
- thermochemical equation — contains → enthalpy (ΔH)
- intermediate species — cancels when Hess’s Law equations are combined by → Hess’s Law
Any biologically valid linking phrases accepted. Award 1 mark per correctly labelled arrow with correct causal direction.