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4.2.2Additional lessons

Hess’s Law — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 74 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5The Law

Statement
  • Statement The enthalpy change of a reaction is the same regardless of the route taken, provided initial and final conditions are the same.
Reason
  • Reason Enthalpy is a state function, so it depends only on the start and end states, and energy is conserved.
Use
  • Use Reactions that are hard to measure directly can be found by combining reactions whose enthalpy changes are known.
Cycle
  • Cycle An enthalpy cycle shows the direct route as one arrow and the indirect route as arrows going through other substances.
Equality
  • Equality The enthalpy change of the direct route equals the sum of the changes along the indirect route.

Part 2 of 5Common Cycles

1
A formation cycle uses elements as the common point, so ΔH = ΣΔHf°(products) − ΣΔHf°(reactants).
2
A combustion cycle uses combustion products as the common point, so ΔH = ΣΔHc°(reactants) − ΣΔHc°(products).
3
Arrows pointing in the direction of the route give a positive contribution; moving against an arrow changes the sign.
4
Multiply a value by the number of moles shown in the balanced equation before adding.
5
Check that the cycle closes, meaning that both routes start and end at the same substances.
Data typeFormula for ΔH
FormationΣ products − Σ reactants
CombustionΣ reactants − Σ products

Part 3 of 5Rules for Manipulating Equations

Rules for Manipulating Equations
Reverse
  • Reverse Reversing a reaction changes the sign of ΔH, so −393.5 becomes +393.5.
Multiply
  • Multiply Multiplying all coefficients by a factor multiplies ΔH by the same factor.
Add
  • Add Adding reactions adds their enthalpy changes, and species on both sides cancel.
State
  • State State symbols must be the same for species that cancel.

Part 4 of 5Worked Example

Question Use ΔHc°(C₂H₄) = −1411, ΔHc°(H₂) = −286 and ΔHc°(C₂H₆) = −1560 kJ mol⁻¹ to find ΔH for C₂H₄(g) + H₂(g) → C₂H₆(g).

  1. Use the combustion formulaΔH = Σ ΔHc°(reactants) − Σ ΔHc°(products).
  2. Σ reactants = −1411 + (−286) = −1697.
  3. Σ products = −1560.
  4. ΔH = −1697 − (−1560) = −137 kJ mol⁻¹.

Answer ΔH = −137 kJ mol⁻¹

Part 5 of 5Exam Checklist & Tips

4.2.2 – Key Points for the Exam

  • Hess's law The overall enthalpy change is independent of the pathway.
  • State function A property that depends only on the current state of the system.
  • Enthalpy cycle A diagram connecting reactants and products by alternative routes.
  • Direct route The reaction path going straight from reactants to products.

Common Mistakes

  • Forgetting to change the sign when a reaction is reversed.
  • Using the formation formula for combustion data (or vice versa).
  • Not multiplying the enthalpy values by the number of moles.
  • Ignoring state symbols such as H₂O(l) and H₂O(g).

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