Energy changes — WJEC A-Level Chemistry
In summary: Energy changes is a key topic in WJEC A-Level Chemistry. Key exam tip: Always check units for entropy (J K-1 mol-1) and enthalpy (kJ mol-1) before using the Gibbs equation
Exam Tips for Energy changes
- Always check units for entropy (J K-1 mol-1) and enthalpy (kJ mol-1) before using the Gibbs equation
- Draw clear energy cycles to avoid sign errors in Hess's law or Born-Haber calculations
- Remember that ΔG must be negative for a reaction to be feasible
- Use the correct state symbols when defining enthalpy changes
- Be prepared to explain why a reaction becomes feasible at higher temperatures based on the TΔS term
Common Mistakes
- Confusing lattice formation enthalpy with lattice breaking enthalpy
- Incorrectly assigning signs to enthalpy values in Born-Haber cycles
- Failing to convert units (e.g., J to kJ) when calculating Gibbs free energy
- Misinterpreting the effect of temperature on the feasibility of reactions
- Assuming all spontaneous reactions must be exothermic
Marking Points
- Definition of enthalpy changes of atomisation, lattice formation, lattice breaking, hydration, and solution
- Construction and interpretation of Born-Haber cycles
- Relationship between lattice breaking enthalpy, hydration enthalpies, and enthalpy of solution
- Definition of entropy as a measure of particle freedom
- Calculation of entropy change using absolute entropy values
- Application of the Gibbs free energy equation (ΔG = ΔH - TΔS)
- Interpretation of ΔG values regarding reaction spontaneity
- Explanation of spontaneous endothermic processes using entropy changes
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