Question

state le chatelier principle and discuss its importance to buffer solutions

Answer 1

Le Chatlier principle states that "When any system at equilibrium is disturbed by a change in temperature, pressure and concentration of reactants and/or products the system responds to the change by readjusting itself to counteract the changes applied to a certain extent, thereby establishing a new equilibrium".

Buffer solutions are made up of salt of a weak acid and the weak acid or salt of a weak base and the weak base with the former type being an acidic buffer and the latter being a basic buffer.

Weak acids and bases exist in equilibrium with their deprotonated and protonated forms respectively in solutions. Acidic buffers will contain the salt's cation, protons from the weak acid and the conjugate base of the weak acid. The conjugate base of the weak acid is unstable and tries to maintain an equilibrium with its protonated form i.e the weak acid. This results in the abstraction of protons from water, rendering the medium hydroxide ions. Similarly, basic buffers contain hydroxide ions from the weak base, salt's anion which is neutral and the conjugate acid of the weak base from the salt. The latter to stabilize itself will abstract hydroxide ions from the medium releasing protons. Thus it can be seen that buffers generate both protons and hydroxide ions in the medium and the pH of the buffer depends on the final concentration of both ions after internal neutralizations.

If an acid is added to an acidic buffer, the protons generated by the acid will disturb the equilibrium in the buffer which the buffer counteracts by forming more of the weak acid in the buffer, thus releasing more hydroxide ions from salt's dissociation. If base is added, more of the weak acid will be neutralized and the salt formed will prevent dissociation of the salt already in the buffer, thus decreasing concentration of hydroxide ions.

If an acid is added to a basic buffer, the protons generated by the acid will be neutralized by the weak base in the buffer, resulting in formation of greater quantity of the weak base's salt. This will inhibit the dissociation of the salt already present in the buffer thus preventing the formation of the weak conjugate acid which will release protons in the medium by hydroxide abstraction. Also, if a base is added to a basic buffer, the increase in concentration of hydroxide ions will result in the decreased dissociation of the weak base to counteract the increased hydroxide concentration. When the base is less dissociated, more of the salt in the buffer will dissociate. This will result in the release of protons into the medium thus counteracting the addition of base by neutralizing the hydroxide with its protons.

Thus it can be observed that the equilbrium established in a buffer changes dynamically upon acid or base addition to dissociate and associate the salt or acid/base in it to maintain the pH, obeying Le Chatlier's principle.