Question

Is the pH at the equivalence point always equal to 7? Explain your answer based on your experimental observations. Include a discussion of equivalence point pH values for strong versus weak acids.

Answer 1

The equivalence point is defined as when you have an equal amount of acid and base. It you have a strong acid and a strong base, the pH at the equivalence point should be pretty close to 7, but if you have a weak acid or a weak base involved, there is not reason why the equivalence point should be at pH 7. You would have a conjugate base or acid that is reasonably basic or acidic at the equivalence point.

Let's consider an example where you are titrating acetic acid with sodium hydroxide. You are correct in saying that when equal amount of substance of acid and base is added, the acid will be completely neutralized. The reaction is:

CH3COOH + NaOH ⟶ CH3COONa + H2O

Hence, at the equivalence point, the conical flask will only contain CH3COONa (which is a salt) and H2O. However, acids and bases are not the only substances which have a pH different from 7. Some salts are also able to be acidic or basic. In the case of CH3COONa, it is a basic salt. The reason why is because CH3COONa actually consists of CH3COO− (a base) and Na+ (an acid). Since CH3COO− is the conjugate base of a weak acid, it is strong enough to be able to react with water to produce OH− ions:

CH3COO− + H2O ↽⇀ CH3COOH + OH−

Meanwhile, since Na+ is the conjugate acid of a strong base, it won't be strong enough to react with water.

Therefore, at the equivalence point, even though there is no acid or base present, [OH−]>[H3O+], hence the pH will be >7.