All calculations
Gibbs free energy and feasibility
Thermodynamics
AQA 7405
ORIGINAL
dG = dH - T dS
Entropy is commonly in J/K/mol while enthalpy is in kJ/mol; align the units before multiplying by temperature.
Verified against AQA 7405 (2026 spec)
Work it, then mark it
Do each calculation on paper first, then reveal the mark scheme and tick the marks you actually earned — the same way you should mark past papers.
Q1
4 marks
A reaction has dH = +40.0 kJ/mol and dS = +120 J/K/mol. Calculate the minimum temperature for feasibility and dG at 350 K.
Do the calculation on paper first — then mark it.
Where the marks get lost
- Mixing joules and kilojoules.
- Treating a negative dG as proof that the reaction will be fast.
Exam tip: A negative dG predicts thermodynamic feasibility, not reaction rate; kinetics is a separate question.
Calculations are the most trainable marks in chemistry
They come up every paper and reward a clean method. Send me one you keep dropping marks on — your first lesson is free.