What exactly is a neutralisation reaction?

Question

I personally understood a neutralisation reaction as a reaction for which the pH after the reaction gets closer (or becomes) 7 (neutral pH). But the definition in my textbook says that :

A neutralisation reaction is a reaction during which an acid reacts with a base to form a salt and water only.

I understand the part :

A neutralisation reaction is a reaction during which an acid reacts with a base

since, given that the acid is an H+ donor and the base and H+ acceptor, they will react to form water and bring the pH back to 7. But why should they react to form water and a salt only ? The reaction of metal carbonates with acids also produce carbon dioxide gas along with a salt and water and if metal carbonates are also bases, then when they react with acids, aren’t these also neutralisation reactions? Furthermore, even if metals are not bases, they will react with acids to form a salt and hydrogen gas, which will also cause H+ concentration to decrease and pH to become closer to 7. So why isn’t this also a neutralisation reaction?

Can someone please give a better definition of a neutralisation reaction and clarify which of the above is or is not a neutralisation reaction, and a reason for why? Thanks

Answer 1

There are multiple definitions of acid and base. If one picks the one by Brønstedt and Lowry, than indeed the acid is the proton donor, the base the proton acceptor; and the transfer of the proton from the acid to the base is the neutralization reaction.

However, e.g. hydrochloric acid isn't just water and $\ce{H+}$, and lye isn't only water and $\ce{OH^-}$. For charge compensation, the former solution equally contains $\ce{Cl^-}$, and the later $\ce{Na+}$ ions. In this neutralization reaction yields water, $\ce{H2O}$. Simultaneously, $\ce{Na+}$ and $\ce{Cl^-}$ however remain untouched and in the solution which (at equivalence point of acid and base) is like a neutral aqueous solution containing (dissociated) salt.

In the case of neutralizing e.g., an aqueous solution $\ce{HCl}$ with $\ce{Na2CO3}$, the product of neutralization is not water, but (formally) $\ce{H2CO3}$. It is one of the properties of carbonic acid that this one is in a chemical equilibrium

$$\ce{H2CO3 <=>>[H2O] CO2 ^ + H2O}$$

where the subsequent decomposition releasing gaseous $\ce{CO2}$ is favoured. Like the earlier case, $\ce{Na+}$ by the base and $\ce{Cl^-}$ of the acid are not affected by the neutralization reaction, nor alter the pH value of the solution. They remain in aqueous solution.

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Note, it is by convention that the equivalence point for the neutralization of $\ce{OH^-}$ by a strong base (e.g., $\ce{NaOH}$) with $\ce{H^+}$ by a strong acid (e.g., $\ce{HCl}$) in water at ambient conditions with $c(\ce{OH^-}) = c(\ce{H^+}) = \pu{10E-7 mol/L}$ (measured quantities) is set as $\text{pH} = 7$ (a derived scale). It is special case though; for the neutralization of e.g., a weak acid (e.g., acetic acid), the equivalence point need not be at $\text{pH} = 7$. In case of acids like $\ce{H2SO4}$ and $\ce{H3PO4}$, there are multiple equivalence points, too (link to a program to predict them).