Catalysts and reversible Reactions

Activation Energy is needed to start a reaction. The activation energy is the minimum amount of energy needed for bonds to break and a reaction to start. 

If the energy input is less than the activation energy there won't be enough energy and nothing will happen.

Catalysts lower the activation energy. A catalyst is a substance which changes the speed of a reaction, without being used up in the reaction. Catalysts lower the activation energy needed for reactions to happen by providing alternative routes.

A Reversible Reaction Can go both ways.

A Reversible Reaction is one where the products of the reaction can themselves react to produce the original Reactants.

Recersible Reactions will reach Dynamic Equillibrium. If a reversible reaction takes place in a closed system then a state of equillibrium will always be reached (A 'closed system' just means that none of the reactants or products escaped).

Equillibrium means that the relative %  quantaties of reactants and products will reach a certain balance and stay there. It is infact a dynamic equillibrium, which means that reactions are taking place in both directions, but the otherall effect is nil because the foward and reverse reactions cancel each other out. The Reactions are taking place at exactly the same rate in both directions.