The Feasibillity of a Reaction (OCR A Level Chemistry)

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The Feasibility of a Reaction

  • 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
  • Feasible and spontaneous are fairly similar terms to describe reactions
    • Feasible tends to be used to describe reactions which are energetically favourable, so reactions that should go
    • Spontaneous tends to be used to describe reactions that go of their own accord

Worked example

Calculate the Gibbs free energy change for the following reaction at 298 K and determine whether the reaction is feasible.

2Ca (s) + O(g) → 2CaO (s)         ΔH = -635.5 kJ mol-1

S[Ca(s)] = 41.00 J K-1 mol-1

S[O2(g)] = 205.0 J K-1 mol-1

S[CaO(s)] = 40.00 J K-1 mol-1

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

   = -207.0 J K-1 mol-1÷ 1000 = -0.207 kJ mol-1

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

Feasibility and Temperature 

  • The feasibility of a reaction can be affected by the temperature
  • The Gibbs equation will be used to explain what will affect the feasibility of a reaction for exothermic and endothermic reactions

Entropy - Gibbs Equation, downloadable AS & A Level Chemistry revision notes

Exothermic reactions

  • In exothermic reactions, ΔHreaction is negative
  • If the ΔSsystem is positive:
    • Both the first and second term will be negative
    • Resulting in a negative ΔG so the reaction is feasible
    • Therefore, regardless of the temperature, an exothermic reaction with a positive ΔSsystem will always be feasible

  • If the ΔSsystem is negative:
    • The first term is negative and the second term is positive
    • At very high temperatures, the -TΔSsystemwill be very large and positive and will overcome ΔHreaction
    • Therefore, at high temperatures ΔGis positive and the reaction is not feasible

  • Since the relative size of an entropy change is much smaller than an enthalpy change, it is unlikely that TΔS > ΔH as temperature increases
  • These reactions are therefore usually spontaneous at normal conditions

Entropy - Feasibility of Exothermic Reactions, downloadable AS & A Level Chemistry revision notes

The diagram shows under which conditions exothermic reactions are feasible

Endothermic reactions

  • In endothermic reactions, ΔHreaction is positive
  • If the ΔSsystem is negative:
    • Both the first and second term will be positive
    • Resulting in a positive ΔG so the reaction is not feasible
    • Therefore, regardless of the temperature, endothermic with a negative ΔSsystem will never be feasible

  • If the ΔSsystem is positive:
    • The first term is positive and the second term is negative
    • At low temperatures, the -TΔSsystemwill be small and negative and will not overcome the larger ΔHreaction
    • Therefore, at low temperatures ΔGis positive and the reaction is not feasible
    • The reaction is more feasible at high temperatures as the second term will become negative enough to overcome the ΔHreaction resulting in a negative ΔG

  • This tells us that for certain reactions which are not feasible at room temperature, they can become feasible at higher temperatures
    • An example of this is found in metal extractions, such as the extraction if iron in the blast furnace, which will be unsuccessful at low temperatures but can occur at higher temperatures (~1500 oC in the case of iron)

Entropy - Feasibility of Endothermic Reactions, downloadable AS & A Level Chemistry revision notes

The diagram shows under which conditions endothermic reactions are feasible

A summary table of free energy conditions

Free Energy summary Table, downloadable AS & A Level Chemistry revision notes

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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.