0.6 Neutralization reaction

Neutralization reaction refers to acid and base reaction producing salt and water. The equivalent amounts of acid react with base to form equivalent amounts of salt and water. When the reaction is complete, acid and base are said to neutralize each other. Since water is produced, neutralization reaction is also referred as “water forming reaction”.

$$\text{acid}+\text{base}\to \text{salt}+\text{water}$$

Consider reaction like :

$$NaOH+HCl\to NaCl+H_{2}O$$

We can rewrite the reaction in ionic form as :

$$N{a}^{+}\left(aq\right)+O{H}^{-1}\left(aq\right)+H^{+}\left(aq\right)+C{l}^{-}\left(aq\right)\to N{a}^{+}\left(aq\right)+C{l}^{-}\left(aq\right)+H_{2}O$$

Essentially, neutralization reaction is double displacement reaction. Some definitions used in the study of neutralization reaction are :

Equivalence point : The state of acid base reaction when acid and base react in exact equivalent amounts as determined theoretically.

End point : The state of acid base reaction when acid and base react in exact equivalent amounts as determined practically (during titration). Clearly, end point volume measurement is slightly greater than that corresponding to equivalence point.

Neutral point : The state of acid-base reaction when product solution is neutral (pH=7).

The ionic salt formed from the reaction of strong acid $[H_{2}S{O}_{\mathrm{4,}}HCl,HN{O}_{\mathrm{3,}}{H}_{3}P{O}_4]$ and strong base $[NaOH,KOH,Mg{\left(OH\right)}_{\mathrm{2,}}Ca{\left(OH\right)}_2]$ are pH neutral. Also, water is pH neutral. As such, strong acid and strong base reaction yields neutral products or product solution. In this case, equivalence and neutral points are same.

However, salts formed with other combinations like “strong acid - weak base” and “weak acid - strong base” are not neutral. They produce salts, which are either acidic or basic in nature. As such, pH of the product solution is not 7. Here, equivalence point is not same as neutral point.

$$H_{2}S{O}_4\left(\text{strong acid}\right)+2N{H}_{4}OH\left(\text{weak base}\right)\to {\left(N{H}_4\right)}_{2}S{O}_4\left(\text{acidic salt}\right)+2H_{2}O$$

$$C{H}_{3}COOH\left(\text{weak acid}\right)+NaOH\left(\text{strong base}\right)\to C{H}_{3}COONa\left(\text{basic salt}\right)+H_{2}O$$

We refer acid-base reaction as neutralization reaction even though products are not neutral (pH=7). As a matter of fact, product solution is acidic (pH<3) for strong acid –weak base reaction. We need to add more of base solution beyond equivalence volume to make the product solution neutral. Similarly, product solution is basic (pH>7) for weak acid-strong base reaction. Here, we need to add more acid beyond equivalence volume to make the product solution neutral.

Titration

The acid-base reaction involves completion of reaction in equivalent proportion of solutes in the solutions. The experimental process to study proportion of reacting volumes and calculation of mass/ concentration of solution is known as “titration”. Completion of reaction is known by chemical indicator or by other means depending on the type of reaction involved. We shall study details of titration process separately.

Analyzing neutralization reaction

We treat neutralization reaction on similar footing as other reactions. Generally, it involves reaction between two solutions of certain concentrations. Clearly, it is helpful to describe analysis in terms of molarity or normality. For a generic consideration :

$$m{A}_1+n{A}_2\to p{A}_3+q{A}_4$$