A2-Level Equilibrium

Calculating Equilibrium Constant, Kc


By using experimental data to find the concentrations of reactants and products at equilibrium in a reaction, Kc can be determined.


For example, for the reaction



1.0 moles of A and B  are allowed to reach equilibrium at temperature, T. 0.5 moles of C are formed, determine Kc.


Write out the Kc expression.


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




Two moles of C is produced for every one mole of A and B that react. Therefore, if 0.5 moles of C has been formed at equilibrium, 0.25 moles of A and B must have reacted.




One mole of A and B is present at the start, if 0.25 moles of each react together, 0.75 moles are left over. At equilibrium we have 0.75 moles of A and B and 0.5 moles of C.


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




Here, the number of moles of A, B 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 also no units for Kc in this example because the units for concentration also cancel out.