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4.2.2Additional lessons

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

5 min read 6 parts Lesson 52 of 98

📚 IB Chemistry SL 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 6The Law

Hess's law is the basis of all enthalpy cycle calculations.

Statement
  • Statement The enthalpy change for a reaction is the same whichever route is taken, provided the initial and final conditions are the same.
Reason
  • Reason It follows from the conservation of energy, since a different total would let energy be created or destroyed by choosing a route.
State function
  • State function Enthalpy depends only on the state of the substances, not on the way they reached that state, so path does not matter.
Usefulness
  • Usefulness It allows the enthalpy change of a reaction that cannot be measured directly to be found from changes that can be measured.

Part 2 of 6Cycles and Diagrams

Cycles and Diagrams
Enthalpy cycle
  • Enthalpy cycle A triangle or box links the reactants and products by a direct route and an indirect route through intermediate substances.
Energy level diagram
  • Energy level diagram Horizontal levels show the enthalpy of each species, with vertical arrows showing the enthalpy changes between them.
Direction of arrows
  • Direction of arrows Following an arrow in its direction uses its ΔH, and travelling against it uses the value with the sign reversed.
Equal totals
  • Equal totals The sum of ΔH values along one route must equal the sum along the other, which is how the unknown value is found.

Part 3 of 6Solving a Problem

1
Write the target equation whose ΔH is required, and write down the equations for which ΔH is known.
2
Rearrange the given equations so that each substance appears on the correct side, reversing an equation and its sign if necessary.
3
Multiply equations and their ΔH values if a different number of moles is needed.
4
Add the equations together and cancel substances which appear on both sides until only the target remains.
5
Add the adjusted ΔH values to give the enthalpy change for the target reaction.

Part 4 of 6Why Indirect Routes Are Needed

Why Indirect Routes Are Needed
Slow reactions
  • Slow reactions Some reactions take too long to measure, so an alternative route using faster reactions is used.
Side products
  • Side products The reaction of carbon with limited oxygen forms both CO and CO₂, so ΔH for CO alone cannot be measured directly.
Hazardous reactions
  • Hazardous reactions Reactions which are dangerous to carry out can be analysed using safer steps.
Unstable compounds
  • Unstable compounds Compounds that decompose easily can be studied through their combustion or formation data.

Part 5 of 6Worked Example

Question Given C(s) + O₂(g) → CO₂(g), ΔH = −394 kJ mol⁻¹, and CO(g) + ½O₂(g) → CO₂(g), ΔH = −283 kJ mol⁻¹, find ΔH for C(s) + ½O₂(g) → CO(g).

  1. Keep the first equation as writtenC(s) + O₂(g) → CO₂(g), ΔH = −394.
  2. Reverse the second equationCO₂(g) → CO(g) + ½O₂(g), ΔH = +283.
  3. Add them; CO₂ cancels and one half of the oxygen cancels, leaving C(s) + ½O₂(g) → CO(g).
  4. ΔH = −394 + 283 = −111 kJ mol⁻¹.

Answer ΔH = −111 kJ mol⁻¹

Part 6 of 6Exam Checklist & Tips

4.2.2 – Key Points for the Exam

  • Hess's law The enthalpy change of a reaction is independent of the route taken between the same reactants and products.
  • Enthalpy cycle A diagram combining direct and indirect routes so that an unknown ΔH can be calculated.
  • State function A property that depends only on the current state of the system and not on how it was reached.
  • Conservation of energy Energy cannot be created or destroyed, only transferred or changed in form.

Common Mistakes

  • Forgetting to change the sign of ΔH when an equation is reversed.
  • Not multiplying ΔH when the equation is multiplied by a factor.
  • Starting from a cycle that does not have the same reactants and products as the target reaction.
  • Leaving intermediate species uncancelled when adding equations.

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