Chemical Equilibrium

A reversible reaction with reactants A & B and products C & D is represented by:

A is reacting with B to produce C and D. At the same time, C is reacting with D to produce A and B.

In a closed system (where energy, but not matter can be exchanged with the surroundings), this reaction will eventually reach a state of ‘chemical equilibrium’.

Chemical equilibrium occurs when the rate of forward reaction (how quickly A and B are producing C and D) is equal to the rate of reverse reaction (how quickly C and D are producing A and B). This means that the concentration of reactants and products will remain constant over time.

A system in state of chemical equilibrium can be thought as a ‘pushing contest’ between two people trying to move a box in opposite directions, and where each person is pushing with equal force…

Person 1 is trying to push the box to the right, while Person 2 is trying to push it to the left, but because they are each pushing with equal force, the box will not move in either direction.

Similarly, in a system at equilibrium, the reactants are trying to push the equilibrium position to the right, while the products are trying to push it to the left, but, because the rates of forward and reverse reaction are equal, the equilibrium position will not move in either direction.

So, even though reactions are still occurring on the molecular level, there will be no observable, net change to a system at equilibrium.