Chemical equilibrium

Forward and reverse reactions

• Not all chemical reactions occur in one direction.• They can go “forward” – to the right.• They can go in “reverse” – to the left.• Both the forward and reverse reactions have a

reaction rate.• Chemical equilibrium is reached when the rates of

the forward and reverse reactions are equal.

Le Chatelier’s Principle

• We can disrupt a reactions equilibrium by changing parts of the reaction or the conditions of the reaction. (change temperature, add reactants, add products, etc.)

• When we do this, the reaction has to make up for the difference, so it “shifts” the equilibrium to make up for the change.

• If it shifts right, we say that it favors the products or that it favors the forward reaction.

• If it shifts left, we say that it favors the reactants or that it favors the reverse reaction.

Effect of changing concentrationA + B C + D

If we increase the concentration of either A or B, the reaction will shift to the opposite side.In this case the equilibirum will shift to the right in favor of the products.Reason: The reaction needs to “use up” A and B and it does this by making more C and D.

EX: Which way will the equilibirum shift if we add more C? More D?

EX: Which way will the EQ shift if we remove A? Remove C?