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pH change in sodium ethanoate formation
When sodium hydroxide solution is added to ethanoic acid, sodium ethanoate solution is formed according to the equation:
|
NaOH(aq) + CH3COOH(aq) ⇒ CH3COONa(aq) + H2O(l) |
At the beginning of this process we still have some unreacted ethanoic acid mixed up with the sodium ethanoate solution which is forming. If you checked the suggested link you will know that this is a common example of a buffer solution. Consequently, the pH change is gradual when sodium hydroxide solution is first added. However, after a certain volume of sodium hydroxide solution has been added, the buffer becomes exhausted, and the pH rises rapidly.
Even when an exactly equal amount of alkali (10 cm3) has been added to the ethanoic acid it will not be neutral. This is because we have made the salt, sodium ethanoate. The salt of a weak acid (ethanoic acid) and a strong base (sodium hydroxide) is always slightly alkaline in solution. This is because the anion of a weak acid (in this case, the ethanoate ion) is a conjugate base. It is difficult to remove H+ from a weak acid to form the anion. Consequently, the anion tends to recombine with H+ (from the water) leaving the solution alkaline:
|
CH3COO-(aq) + H2O(l)
|
