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Hess's law – enthalpy cycle builder

ChemistryEnergetics & ThermodynamicsAges 15–16

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Four screens on Hess's law. Hess cycle: find ΔrH of a reaction through enthalpies of formation, enthalpies of combustion or mean bond enthalpies, as a cycle or an energy-level diagram, step by step. Combine equations: reverse, scale and add given equations until they give the target equation. Born–Haber cycle: find the lattice enthalpy of NaCl, KCl, LiF, MgO and MgCl₂. Lab: use a polystyrene-cup calorimeter to find, indirectly, the enthalpy of hydration of CuSO₄ or the enthalpy of formation of MgO.

Lesson: Enthalpy changes; enthalpies of formation and combustion, bond enthalpies; Hess's law; Born–Haber cycles

What it shows

Hess's law states that the enthalpy change of a reaction is the same whatever route is taken from reactants to products, because enthalpy is a state function. It lets you find enthalpy changes that cannot be measured directly. Going through the elements gives ΔrH = ΣΔfH(products) − ΣΔfH(reactants); going through the combustion products gives ΔrH = ΣΔcH(reactants) − ΣΔcH(products). Mean bond enthalpies only give an estimate. A Born–Haber cycle applies the same idea to the formation of an ionic solid to find its lattice enthalpy.

How to use

On Hess cycle, choose a Reaction and a Route, press Step to follow ΔH₁ and ΔH₂, and switch View to an energy-level diagram. On Combine equations, use + and − and Reverse until the sum matches the target. On Born–Haber, build the cycle with Step and predict the lattice enthalpy. In the Lab, add the solid, record both reactions and compare with the data-book value.

Parameters you can change

  • Screen Hess cycle, Combine equations, Born–Haber cycle, Calorimetry lab
  • Reaction (Hess cycle) C₂H₄(g) + H₂(g) → C₂H₆(g), 3C₂H₂(g) → C₆H₆(l), C₆H₁₂O₆(s) → 2C₂H₅OH(l) + 2CO₂(g), CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g), N₂(g) + 3H₂(g) → 2NH₃(g), H₂(g) + Cl₂(g) → 2HCl(g), CaCO₃(s) → CaO(s) + CO₂(g), Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g), C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l)
  • Route Via enthalpies of formation, Via enthalpies of combustion, Via bond enthalpies
  • View Cycle, Energy-level diagram
  • Combine-equations problem C(s) + ½O₂(g) → CO(g), C(s) + 2H₂(g) → CH₄(g), 2C(s) + H₂(g) → C₂H₂(g), N₂(g) + 2O₂(g) → 2NO₂(g), C₂H₄(g) + H₂(g) → C₂H₆(g)
  • Ionic compound (Born–Haber) NaCl, KCl, LiF, MgO, MgCl₂
  • Quantity to find Lattice enthalpy, Enthalpy of formation
  • Experiment Enthalpy of hydration of CuSO₄, Enthalpy of formation of MgO
  • Cup with lid

Questions to explore

  1. Why do the formation route and the combustion route give exactly the same ΔrH?
  2. Why does the value from mean bond enthalpies differ from the value from enthalpies of formation?
  3. Why is the lattice enthalpy of MgO so much larger than that of NaCl?

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