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Thermodynamics (A-level only)
This A-level-only topic extends energetics into lattice enthalpy, entropy and thermodynamic feasibility.
9
Objectives
45
Flashcards
44
Questions
90 min
Study time
AQAA LevelChemistryPhysical chemistry
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9 objective pages available
Born-Haber cycles (A-level only)4 objectives
- Construct Born-Haber cycles for ionic compounds.
- Calculate lattice enthalpy from Born-Haber data.
- Compare experimental and theoretical lattice enthalpies.
- Use lattice enthalpy comparisons to infer covalent character.
Entropy and Gibbs free energy (A-level only)5 objectives
- Explain entropy as a measure of dispersal of energy or disorder.
- Calculate entropy changes from standard entropy data.
- Use ΔG = ΔH - TΔS to calculate Gibbs free energy change.
- Predict whether a reaction is feasible from Gibbs free energy.
- Explain why feasibility does not guarantee an observable reaction rate.
Key terms
Born–Haber cycleLattice enthalpylattice enthalpyBorn-Haber cycleExperimental Lattice EnthalpyTheoretical Lattice Enthalpycovalent characterentropyGibbs free energyentropy changestandard entropy dataGibbs free energy change
Exam tips
- Constructing Born-Haber Cycles: When constructing a Born-Haber cycle, start by identifying the enthalpy changes involved in the formation of the ionic compound from its elements. Use Hess's law to relate these changes.
- Calculating Lattice Enthalpy: Use the Born-Haber cycle to calculate lattice enthalpy by summing the enthalpy changes for each step.
Common mistakes
- Incorrect Born-Haber Cycle Construction: To correctly construct a Born-Haber cycle, include all necessary enthalpy changes: the enthalpy of formation, sublimation, ionization, and electron affinity. For example, use the formula: ΔH_f = ΔH_sublimation + ΔH_ionization + ΔH_electron_affinity + ΔH_lattice. Substitute the values for each enthalpy change, perform the calculations, and ensure all steps are represented in the cycle.
- Incorrect Lattice Enthalpy Calculation: To fix this, remember that lattice enthalpy is exothermic and should be treated as a negative value in calculations. Use the formula: Lattice Enthalpy = ΔHf (formation enthalpy) - (ΔHsub + ΔHionization + ΔHelectron + ΔHbond). Substitute the values correctly, ensuring to apply the negative sign for lattice enthalpy. For example, if ΔHf = -400 kJ/mol, ΔHsub = +100 kJ/mol, ΔHionization = +200 kJ/mol, ΔHelectron = -100 kJ/mol, and ΔHbond = +50 kJ/mol, the calculation would be: Lattice Enthalpy = -400 - (100 + 200 - 100 + 50) = -400 - 250 = -650 kJ/mol. Thus, the lattice enthalpy is -650 kJ/mol.
Practice preview
- What is the first step in constructing a Born-Haber cycle for an ionic compound?
- In a Born-Haber cycle, which value represents the energy required to separate one mole of an ionic solid into its gaseous ions?
- Which of the following is NOT typically included in a Born-Haber cycle?
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