A2-Level Equilibrium

By using the concentrations of reactants and products at equilibrium, K_c can be determined.

For example:

1.0 moles of each A and B₂ are allowed to reach equilibrium at temperature, T. 0.5 moles of C are present at equilibrium, determine K_c.

Write out the K_c expression.

If we know the starting amounts of A and B₂, and the final amount of C, we can find the amounts at equilibrium.

Two moles of C are produced for every one mole of each A and B₂ that react. Therefore, if 0.5 moles of C are present at equilibrium, 0.25 moles of each A and B₂ must have reacted.

One mole of each A and B₂ are present at the start. If 0.25 moles of each react together, 0.75 moles of each are left over. This means that at equilibrium we have 0.75 moles of each A and B₂, and 0.5 moles of C.

These values can now be used in the K_c expression to find the value of K_c.

Here, the number of moles of A, B₂ and C are used instead of concentrations. This is because the volume of the system does not change, so the volumes cancel out.

There are no units for K_c in this example because the units for concentration also cancel out.