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
Exothermic reactions
- In exothermic reactions, ΔHreactionꝋ is negative
- If the ΔSsystemꝋ is positive:
- Both the first and second term will be negative
- Resulting in a negative ΔGꝋ so the reaction is feasible
- Therefore, regardless of the temperature, an exothermic reaction with a positive ΔSsystemꝋ will always be feasible
- If the ΔSsystemꝋ is negative:
- The first term is negative and the second term is positive
- At high temperatures, the -TΔSsystemꝋ will be very large and positive and will overcome ΔHreactionꝋ
- Therefore, at high temperatures ΔGꝋ is 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 ΔHreactionꝋ resulting in a negative ΔGꝋ
- 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
The diagram shows under which conditions exothermic reactions are feasible
Endothermic reactions
- In endothermic reactions, ΔHreactionꝋ is positive
- If the ΔSsystemꝋ is negative:
- Both the first and second term will be positive
- Resulting in a positive ΔGꝋ so the reaction is not feasible
- Therefore, regardless of the temperature, endothermic with a negative ΔSsystemꝋ will never be feasible
- If the ΔSsystemꝋ is positive:
- The first term is positive and the second term is negative
- At low temperatures, the -TΔSsystemꝋ will be small and negative and will not overcome the larger ΔHreactionꝋ
- Therefore, at low temperatures ΔGꝋ is 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 ΔHreactionꝋ resulting in a negative ΔGꝋ
- 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
The diagram shows under which conditions endothermic reactions are feasible