The Extent of Chemical Change – Equilibrium


We know that particles react together when they bump into each other with enough energy (Collision theory). Up until know we have only looked at reactions that can only go in 1 direction. Once a match is struck it cannot reform, once a cake is baked you cannot go back to the original ingredients – These are irreversible reactions.
Now we look at reversible reactions – reactions that can go back to their starting materials.

The Essentials

100 % reversible reactions

Deeper Understanding

You can predict what will happen in a reaction if you alter the conditions. This is known as Le Chatelier’s Principle.


If the concentration of a reactant is increased (A or B), more products will be formed until equilibrium is reached again. If the concentration of a product is decreased (C or D), more reactants will react until equilibrium is reached again.


If the temperature of a reaction is increased (heated) the reaction move in the endothermic reaction (more C & D made). If the temperature of a reaction decreases (cooled) the reaction moves in the exothermic directions (more A & B made)


If pressure is increased the reaction moves to where there are fewest gas molecules (A & B). If the pressure decreases it moves towards where there are more gas molecules (C & D)

Other Links

BBC Bitesize – Reversible Reactions

Review and Rate your Understanding

After revising the topic and completing the questions of the 100% sheet try the end of topic summary questions (answers given) and then print and complete the checklist to rate your understanding.


Have you learnt all the facts on the 100% sheet?

Have you completed the BBC Bitesize tutorial?

Have you been able to complete all the questions on the 100% sheet?

Let us know how you feel about this topic in the comments section below. Any questions you have, just ask

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