CIE A Level Chemistry

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Syllabus Edition

First teaching 2020

Last exams 2024

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5.2.4 Reaction Feasibility

Test Yourself

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 ΔGis positive, the reaction is not feasible and unlikely to occur

Worked example

Worked example: Determining the feasibility of a reaction

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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