KEY2CHEM

Equilibrium and Reaction Rates

A reaction is said to be at equilibrium when the rate of the forward reaction is equal to the rate of the reverse reaction. If these rates are not balanced, the system will shift in the direction that brings it to equilibrium. For example, if $$rate_{forward} > rate_{reverse}$$, the system will proceed in the forward direction to generate additional products. This in turn will increase the rate of the reverse reaction, allowing the rates to become equal to each other.

Example 1.

For a system at equilibrium, which statement is true?

A. The rate of the reverse reaction is zero.

B. The concentrations of reactants and products are equal to each other.

C. The rates of the forward and reverse reactions are equal to each other.

Solution

C. The rates of the forward and reverse reactions are equal to each other.

At equilibrium, the rates of the forward and reverse reactions are balanced (equal to each other).

Example 2.

A reaction has a forward rate that is faster than the reverse rate. In which direction will the reaction proceed to reach equilibrium?

A. forward

B. reverse

C. neither

Solution

A. forward

Since the forward rate is faster than the reverse rate, the forward reaction will proceed to a greater extent to achieve equilibrium. As the forward reaction proceeds, the concentration of reactants is depleted and the forward reaction slows down. As the concentration of products increases, the reverse reaction rate increases.

Example 3.

Increasing the concentration of products _______ the rate of the reverse reaction.

A. does not change

B. increases

C. decreases

Solution

B. increases

Increasing the concentration of products increases the rate of the reverse reaction.