CIE A Level Chemistry (9701) exams from 2022

Revision Notes

5.2.4 Reaction Feasibility

The Gibbs Equation: Reaction Feasibility

  • The Gibbs equation can be used to calculate whether a reaction is feasible or not

ΔG = ΔHreaction – TΔSsystem

  • When ΔG is negative, the reaction is feasible and likely to occur
  • When ΔG is positive, the reaction is not feasible and unlikely to occur

Worked example: Calculating entropy changes

Entropy - Worked example - Calculating entropy changes, downloadable AS & A Level Chemistry revision notes

Answer

  • Step 1: Calculate ΔSsystem

ΔSsystem= ΣΔSproducts – ΣΔSreactants

ΔSsystem= (2 x ΔS [CaO(s)]) –  (2 x ΔS [Ca(s)] + ΔS[O2(g)])

= (2 x 40.00) – (2 x 41.00 + 205.0)

= -207.0 J K-1 mol-1

  • Step 2: Convert ΔSto kJ K-1 mol-1

Gibbs equation reaction feasibility worked example eq

= -0.207

  • Step 3: Calculate ΔG

ΔG = ΔHreaction – TΔSsystem

= -635.5 – (298 x -0.207)

= -573.8 kJ mol-1

  •  Step 4: Determine whether the reaction is feasible
    Since the ΔG is negative the reaction is feasible and likely to occur
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