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Chemistry Revision Guides
A-Level Revise Aid: This resource is designed to support your revision and may contain errors. If you find a discrepancy with your class teaching, your teacher is correct -- please let us know at alevelrevise@scott.scottrix.co.uk.

Energetics

Year 1 / ASYear 2 / A-Level All Boards (AQA, Edexcel, OCR, WJEC, CCEA) AQA

A-Level Chemistry revision: Energetics. Learning objectives, key points, worked examples and practice questions across AQA, Edexcel, OCR, WJEC and CCEA.

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πŸ“Œ Key Points

Key Fact: Standard conditions: 100 kPa, 298 K, 1 mol dm⁻^3; DeltaH_f deg elements = 0
Key Fact: Hess: DeltaH_total = Ξ£DeltaH_steps (path independent); Cycle: formation = atomisation + ionisation + electron affinity + lattice
Key Fact: Born-Haber: DeltaH_f deg = DeltaH_atom + Ξ£IE + Ξ£EA + DeltaH_lattice (for ionic compounds)
Key Fact: Bond enthalpies: DeltaH = Ξ£(bonds broken) - Ξ£(bonds formed); mean values, gaseous only
Key Fact: Solution: DeltaH_soln = DeltaH_lattice + DeltaH_hydration; hydration exothermic
Key Fact: Energy level diagrams: reactants -> products; exo: products lower; endo: products higher

🎯 Learning Objectives

  • Define standard enthalpy changes: formation, combustion, neutralisation, atomisation
  • Apply Hess's Law to calculate enthalpy changes
  • Use Born-Haber cycles for lattice enthalpies
  • Calculate enthalpy changes from bond enthalpies
  • Understand enthalpy of solution and hydration
  • Use mean bond enthalpies and their limitations
  • Interpret energy level diagrams

πŸ’‘ Worked Example

Exam-Style Question

Question: Use Born-Haber cycle to calculate lattice enthalpy of NaCl given: DeltaH_f deg (NaCl) = -411, DeltaH_at(Na)=+107, DeltaH_at(Cl)=+122, IE(Na)=+496, EA(Cl)=-349 kJ/mol

Model Answer:

DeltaH_f = DeltaH_at(Na) + DeltaH_at(Cl) + IE(Na) + EA(Cl) + LE. -411 = 107 + 122 + 496 - 349 + LE -> LE = -411 - 376 = -787 kJ/mol. Lattice enthalpy = +787 kJ/mol (endothermic to break)

❓ Practice Questions

Questions:

  • Define standard enthalpy of combustion
  • Calculate DeltaH for reaction using bond enthalpies: Hβ‚‚ + Clβ‚‚ -> 2HCl
  • Why are mean bond enthalpies less accurate than formation data?
  • Explain trend in hydration enthalpy down Group 1
  • Sketch energy profile for exothermic reaction with catalyst

🎬 Video Resources

πŸ“„ Past Papers & Exam Resources

πŸ”— Further Reading & Resources