Neutralization (chemistry) - Resultant PH

Resultant PH

Neutralization reactions do not necessarily imply a resultant pH of 7. In the case that a strong acid and strong base participate in a neutralization reaction, the resultant pH will be 7. For example, the strong acid, HCl, and the strong base, NaOH, react to give water and a salt, NaCl:

HCl + NaOH → H₂O + NaCl

Since there is no net change in the concentrations of either H₃O+ or OH-, the end pH is 7.

If a weak acid and a strong base participate in a neutralization reaction, the resultant pH will be greater than 7. For example, the weak acid, CH₃COOH, and the strong base, NaOH, react to give water, Na+, and acetate, CH₃COO-:

CH₃COOH + NaOH → Na+ + H₂O + CH₃COO-

Na+ behaves as a spectator ion. However, acetate is a weak base that hydrolyzes water to give OH- ions.

CH₃COO- + H₂O ⇌ CH₃COOH + OH-

Thus, the resultant solution is basic.

If a weak base and a strong acid participate in a neutralization reaction, the resultant pH will be less than 7. For example, the weak base, CN-, and the strong acid, HCl, react to give Cl- and hydrocyanic acid, HCN:

CN- + HCl → Cl- + HCN

Cl- behaves as a spectator ion. However, hydrocyanic acid is a weak acid that hydrolyzes water to give H₃O+ ions.

HCN + H₂O ⇌ CN- + H₃O+

Thus, the resultant solution is acidic.

Finally, if a weak acid and a weak base participate in a neutralization reaction, the resultant pH will depend on the relative strength of the acid and base reactants. For example, the weak base, CN-, and the weak acid, CH₃COOH, react to give HCN and CH₃COO-. Because CH₃COOH (pKa=4.75) is a stronger acid than HCN (pKa=9.2), the equilibrium is driven to the right, assuming equimolar initial concentrations of the weak acid and weak base.

CN- + CH₃COOH ⇌ HCN + CH₃COO-

The acetate ions further react with water to give acetic acid and OH-.

CH₃COO- + H₂O ⇌ CH₃COOH + OH-

In this particular example, the resultant solution is basic. However, this is not a general rule for all neutralization reactions between a weak acid and a weak base.