Reversible reactions, equilibrium and Le chatelier’s principle

What is a reversible reaction?

Reaction where the products of the reaction can react to produce the original reactants.

Forward and backwards reactions…..

Can take place at different rates

what is Equilibrium

When the amounts of reactants and products reach a balance - their concentrations stop changing.

Equilibrium….

Will always be reached,

Takes place in a closed system.

Equilibrium process

At the start of the reaction the forward reaction is fast as there’s lots of reactants and no products (backward reaction wouldn’t have started yet), as reactants turn to products the forward reaction slows down and the backwards reaction speeds up. After a while the rates will even out. (Forward and backwards reaction going at same speed so conc. of reactants and products won’t change)

When reactions at equilibrium it doesn’t mean the amount of products are equal…..

•if equilibrium lies to the right the concentration of products is greater than reactants

•if equilibrium lies to the left, the concentration of reactants is greater than products.

Energy transfer

•If reaction is endothermic in one direction it will be ectothermic in the other direction.

•energy absorbed by the endothermic reaction is equal to energy released by exoteric reaction.

Le chatelier’s principle

If you change the conditions of a reversible reaction at equilibrium, the system will try to counteract the change. (Can alter yield of reaction)

raise temperature

The yield of the endothermic reaction will increase and yield of expthermic will decrease.

Decrease temperature

If you reduce the temperature, the yield of the exothermic reaction will increase and the yield of the endothermic reaction will decrease.

Raise pressure

Favours reaction which produces less volume (the fewest number of gas molecules)

Lowering pressure

Favours reaction which produces more volume (the greatest number of gas molecules)

Concentration

If you change concentration of either reactants or products the system will no longer be at equilibrium. So system will respond to bring itself back to equilibrium.

Increasing concentration

The system tries to decrease concentration by making more products

Decreasing concentration

The system tries to increase concentration by reducing the amount of reactants.