The equilibrium constant K c for a particular reaction remains the same at constant temperature.

In this example, a moles of substance A react with b moles of substance B.

The products are c moles of substance C and d moles of substance D.

To work out the expression for K c, we look at the products first.

The square brackets show that we are using the concentration of substance C in mol dm -3.

We need to raise this to the power c.

We do the same thing for substance D. Notice that the two values will be multiplied together.

We need to raise this to the power a.

We do the same thing for substance B to complete the expression.

Let’s apply this to a chemical equation.

This is the equation for one of the reactions involved in the manufacture of sulphuric acid.

We use the product first.

Its concentration must be raised to the power of 2 – it must be squared.

Next, we look at the reactants. We use the concentration of SO 2 in mol dm -3.

Its concentration must be raised to the power of 2 – it must be squared.

Notice that the concentration of O 2 does not need to be raised to any power, because there is no number in front of it in the equation.

This is the chemical equation for the hydrolysis of ethyl ethanoate.

This is the corresponding expression for the equilibrium constant, K c.

The table shows experimental data for the equilibrium concentrations of each substance at 293 K.

Substitute the correct values for each equilibrium concentration.

Remember to raise numbers to the appropriate power, if necessary.

In this example, it is not necessary to raise any of the numbers to a power.

This is the value of K c for this chemical reaction, given the data in the table.

In this example, all the units of concentration cancel out, so K c has no units. If K c does have units, you must work them out.

What are the units of K c here?

Look at the concentration units.

We can cancel out these two concentrations.

We can also cancel out these two concentrations.

As all the concentrations cancel out, the value for K c in this example has no units.

This is the chemical equation for one of the stages in the manufacture of sulphuric acid.

What are the units of K c here?

Look at the concentration units.

We can cancel out these two concentrations.

This leaves us with a number 1 on top.

When the powers appear on the bottom like this, reverse the signs of the individual units.

This is the chemical equation for one of the stages in the manufacture of ammonia by the Haber process.

What are the units of K c here? It looks as if this could be difficult to work out, but there is a quick way to do it.

Look at the concentration units.

There are two concentration units on the top.

There are three concentration units on the bottom here.

There is one concentration unit on the bottom here.

There are four concentration units in total on the bottom.

Work out top minus bottom, in this case 2 minus 4, which equals –2.

Finally, multiply all powers of the individual units by –2.