Topic 17.2: The equilibrium law

17.2.1 Solve homogeneous equilibrium problems using the expression for Kc.

There are three different types of equilibrium questions the IBO can ask.


Calculating the equilibrium constant from initial and equilibrium concentrations.

If we know the equilibrium concentations of all reactants and products in a reaction, we can simply substitute these into the equilibrium expression to calculate Kc. The first step in such a calculation is always to write the equilibrium expression from the chemical equation.

Once you have your equation, just plug in the numbers of the equilibrium concentrations and you will receive Kc. If you were given only the initial concentration, we just need to convert it back to the equilibrium concentration for it to work.


Calculating equilibrium concentration from the equilibrium constant

If we know the value of Kc and the equilibrium concentrations of all but one of the components, we can calculate the remaining equilibrium concentration simply by substituting the values into the equilibrium expression.

Finding out what x is equal to would show the equilibrium concentrations. To find x, simply equate the Kc value with the equation containing the x. Simple maths and some rearrangement will provide the answer for x.