One very important feature of chemical reactions is that all chemical reactions are reversible. As a chemical reaction occurs, there is a rate associated with the forward reaction. However, there is also a rate associated with the reverse reaction. Once equilibrium is achieved, the forward rate and the reverse rate are equal. The reaction does not stop as the equilibrium is dynamic. To express the equilibrium, the equilibrium constant expression (Kc) is used.

For the above balanced equation, the Kc expression can be seen below.

The equilibrium constant expression can be found by multiplying the equilibrium concentrations of the products raised to their coefficient powers and dividing by the equilibrium concentration of the reactants raided to their coefficient powers.

When writing equilibrium constant expressions, some exceptions do occur.

Exceptions: Do not include any pure substances as their concentrations are assumed to be 1. This includes all solids, water (l), water (s) and any compound in its pure liquid form.

Write the equilibrium expressions for the two examples below:

Equation #1

Equation #2

Notice that Equation #2 is the reverse of Equation #1. Notice also that the equilibrium constant expression for Equation #2 is the reciprocal of the equilibrium constant expression for Equation #1.

Some additional comments:

1. If you flip an equation:



2. If you add equations together:



3. If you multiply an equation by a coefficient n:

Example: Calculate the K for the reaction of H and Br atoms to give HBr if given the following:

In the previous example, you learned how to calculate equilibrium constants from existing equilibrium constants. The equations were assumed to be at equilibrium. What if the reactions aren't at equilibrium?

For nonequilibrium conditions, K is replaced by the reaction quotient, Q.

But, what does Q have to do with K? Depending on the relationship between Q and K, one can predict the direction the reaction will shift until equilibrium is achieved. There are three conditions which should be considered.

1. Q = K, then the reaction is at equilibrium.
2. Q > K, then the reaction is not at equilibrium and some products will be converted to reactants.
3. Q < K, then the reaction is not at equilibrium and some reactants will be converted to products.

The equilibrium constants may also be used to calculate the equilibrium concentrations of the reactants and products in a chemical equation. Follow the steps below to determine the equilibrium concentrations.

1. Write the balanced equation
2. Write the equilibrium constant expression
3. Write the initial concentrations of each species
4. Show the change in concentrations (don’t forget coefficients) using x
5. Solve for x
6. Calculate the equilibrium concentrations

Let's try a problem: The equilibrium constant for the dissociation of iodine is 3.76 E3 at 1000 K. Suppose 1.00 mol of iodine is placed in a 2.00-L flask at 1000 K. What are the concentrations of iodine and the iodine atom when the system comes to equilibrium?

1. Write the balanced equation

2. Write the equilibrium constant expression

3. Write the initial concentrations of each species

[I] = 0 M

4. Show the change in concentrations (don’t forget coefficients) using x

5. Solve for x

6. Calculate the equilibrium concentrations