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Reaction Feasibility: Temperature Changes (CIE A Level Chemistry)

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Francesca

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Francesca

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Feasibility & Temperature Changes

  • 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 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
    • The reaction is more feasible at low temperatures, as the second term will not be large enough to overcome ΔHreaction resulting in a negative ΔG

  • This corresponds to Le Chatelier’s principle which states that for exothermic reactions an increase in temperature will cause the equilibrium to shift position in favour of the reactants, i.e. in the endothermic direction
    • In other words, for exothermic reactions, the products will not be formed at high temperatures
    • The reaction is not feasible at high temperatures

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 less 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 again corresponds to Le Chatelier’s principle which states that for endothermic reactions an increase in temperature will cause the equilibrium to shift position in favour of the products
    • In other words, for endothermic reactions, the products will be formed at high temperatures
    • The reaction is therefore feasible

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

The diagram shows under which conditions endothermic reactions are feasible

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Francesca

Author: Francesca

Expertise: Head of Science

Fran studied for a BSc in Chemistry with Forensic Science, and since graduating taught A level Chemistry in the UK for over 11 years. She studied for an MBA in Senior Leadership, and has held a number of roles during her time in Education, including Head of Chemistry, Head of Science and most recently as an Assistant Headteacher. In this role, she used her passion for education to drive improvement and success for staff and students across a number of subjects in addition to Science, supporting them to achieve their full potential. Fran has co-written Science textbooks, delivered CPD for teachers, and worked as an examiner for a number of UK exam boards.