Topic 6: Chemical equillibria, Le Chatlier’s principle and Kc
Chemical equillibria and Le Chatlier’s principle
Some reactions are reversible, where the reactants react to form products, which can then
react to reform the original reactants. It is represented using a double arrow.
In each reaction system, one reaction will be endothermic and the opposite reaction will be
exothermic.
Dynamic equillibrium
Reversible reactions will eventually reach dynamic equilibrium. This is when the forward and
backward rate of reaction is the same and remains at a constant rate. At this point, the
concentrations of the reactants and products are constant and appear not to change.
Dynamic equilibrium can only occur in closed systems where no substances can get in or out
in order to influence the reactions.
Le Chatlier’s principle
This theory is used to predict how an equilibrium mixture will change if the reaction
conditions are altered. It states:
When a system is subject to change,
the system will alter to lessen the effect of that change.
It is therefore a useful principle to consider in industry so that the reaction conditions can be
altered to maximise the yield of a desired product.
Changing temperature
Increasing temperature favours the endothermic reaction (+ve ∆H) as the excess heat needs
to be removed from the system to lessen the effect of the initial increase. It will increase the
yield of the endothermic products. Likewise, decreasing temperature favours the exothermic
reaction (-ve ∆H) as heat needs to be gained and the yield of the exothermic products will be
increased.
Chemical equillibria and Le Chatlier’s principle
Some reactions are reversible, where the reactants react to form products, which can then
react to reform the original reactants. It is represented using a double arrow.
In each reaction system, one reaction will be endothermic and the opposite reaction will be
exothermic.
Dynamic equillibrium
Reversible reactions will eventually reach dynamic equilibrium. This is when the forward and
backward rate of reaction is the same and remains at a constant rate. At this point, the
concentrations of the reactants and products are constant and appear not to change.
Dynamic equilibrium can only occur in closed systems where no substances can get in or out
in order to influence the reactions.
Le Chatlier’s principle
This theory is used to predict how an equilibrium mixture will change if the reaction
conditions are altered. It states:
When a system is subject to change,
the system will alter to lessen the effect of that change.
It is therefore a useful principle to consider in industry so that the reaction conditions can be
altered to maximise the yield of a desired product.
Changing temperature
Increasing temperature favours the endothermic reaction (+ve ∆H) as the excess heat needs
to be removed from the system to lessen the effect of the initial increase. It will increase the
yield of the endothermic products. Likewise, decreasing temperature favours the exothermic
reaction (-ve ∆H) as heat needs to be gained and the yield of the exothermic products will be
increased.
