Common ion effect
In an ion balance, the addition of chemical species may contemplate ions that already exist in the system or not.
If the added ion already exists in equilibrium (common ion), its behavior will be like adding any substance that already exists in the reaction.
If a species that the system does not have is added and it reacts with some equilibrium present, we should study the effect of decreasing the concentration of this second substance. If the added substance does not react at equilibrium, adding it will not modify the system. See the following example:
According to the side that the balance shifts, the solution changes color.
See what can happen if it is added to this reaction:
- HCl addition (aq) - HCl acid ionizes to form the H + cation ion (aq). It therefore causes an increase in the concentration of this ion in the system. Shifts the balance to the right until the added excess is consumed. The H + ion (aq) is the common ion to the system.
- NaOH addition - the NaOH base dissociates to form the OH- (aq) anion ion which reacts with the H + cation (aq), causing water formation and decreasing the H + (aq) concentration. Shifts the balance to the left to reset the amount removed from this ion. The OH- (aq) ion is the unusual ion to equilibrium.