IC S2 K2Interpret the equilibrium constant expression from the chemical equation of equilibrium reactions.

P3 Process and present information from secondary sources to calculate K from equilibrium conditions

The equilibrium constant (Kc) is a number that indicates the relationship between the concentrations of reactants and products at equilibrium

The equilibrium constant Kc 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.

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.

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

To work out the expression for Kc, 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 SO2 in mol dm-3.

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

Notice that the concentration of O2 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 one of the stages in the manufacture of sulphuric acid.

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

Calculating the equilibrium constant

Kc = [COCl2]

[CO] [Cl2]

Kc = [COCl2]

[CO] [Cl2]

Kc = [COCl2]

[CO] [Cl2]

Kc = [COCl2]

[CO] [Cl2]

Kc = 3.2 = 20

0.40 0.40

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

This is the corresponding expression for the equilibrium constant, Kc.

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.

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

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

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

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