Neutralisation Reaction

Neutralisation Reaction Definition

Here we will go over the definition of a neutralisation reaction in chemical contexts.

What do you think about when you hear something is "neutralised"? To neutralise something usually means to bring it to a midpoint by minimizing its chemical or biological effect.

At its core, neutralisation reactions deal with the reactions that acids and bases have. When you combine an equal part of each you will get a neutralisation reaction. That means that they (the two chemical substances) will counterbalance each other's effects. This is because each of the reactants, an acid and the base, come from the opposite side of the pH scale. When both the solutions intermix with each other, their pH s cancels out, resulting in the neutralisation of the solution. The resulting pH depends on the strength of the acid and base that participated in the reaction.

In brief, you can define the neutralisation reaction as:

Fig. 1: Strengths of acids and bases-pH scale StudySmarter Originals

We shall discuss the products of the neutralisation reaction and how they are formed in the next section.

But it is important to remember that the most important thing in neutralisation reactions is that they require an acid and a base. This is very key to the nature of neutralisation reactions as they are the only reaction within acid-base chemistry that involve both acids and bases.

Neutralisation Reaction Products

From the definition, it is evident that salt and water are the products of a neutralisation reaction.

We can represent a neutralisation reaction as:

\begin{align} & HA + BOH\rightarrow BA + H_2O\\ & Acid+Base\rightarrow Salt+Water\end{align}

Observe the above equation. Can you understand how water is formed? Which substance donated the hydrogen ion\(H^+\) and which substance contributed the \(OH^-?\)

These questions will be answered in the next section where in we will write learn to write the ionic equation for neutralisation.

Neutralisation Reaction Equation

In this section, we will cover how an acid and a base come together chemically to neutralise each other and produce salt and water

Let us revisit the general molecular equations for acids and bases:

\begin{align} & HA + BOH\rightarrow BA + H_2O\\ & Acid+Base\rightarrow Salt+Water\end{align}

HA, the acid contains \(H^+\)- hydrogen ion and \(A^-\) anion. Similarly, BOH is an alkali(a base that is soluble in water) that has \(B^+\) cation and \(OH^-\) - hydroxide ions.

To determine which substance contributed to what ion of the water molecule. Let us rip the chemical species in the above equation into their corresponding ions. The resulting total ionic equation is:

\(H^+A^- + B^+ OH^-\rightarrow B^+A^- + H^+ OH^-\)

\(H^+\cancel A^- + \cancel B^+ OH^-\rightarrow \cancel B^+\cancel A^- + H_2O\)

The \(A^-\) and the \(B^+\) ions are called spectator ions because they appear the same on either side of the equation. Hence, if you remove them from your sight, you can see the net ionic equation which is as follows:

Net ionic equation

\(H^+ + OH^-\rightarrow H_2O\)

This is the reaction that takes place in almost every neutralisation reaction between the acid and base (strong acid and a strong base).

Now, we can say that the \(H^+\) ions are contributed by the acid, while the \(OH^-\) ions are contributed by the base. The resulting solution is a neutral solution whose pH will be 7 or moving towards 7 and this depends on the strengths of acids and bases that participated in the neutralisation as discussed earlier.

Recall the definition of neutralisation once again. We discussed that it is a double displacement chemical reaction. Here, the two reactants-acid \(HA\) and base \(BOH\) exchanged their ions to produce new products-salt and water. This justifies that neutralisation is a double displacement reaction.

Fig. 2: Neutralisation is a double displacement reaction, Archana Tadimeti, StudySmarter Originals

Neutralisation Reactions Examples

Here we will go over some possible neutralisation reaction examples.

First, we will go over a very basic neutralisation reaction, one that produces a common type of table salt.

The next example we will look at deals with how other types of acids, for example, diprotic acids (acids which give two hydrogen ions), behave in neutralisation reactions.

Neutralisation Reaction Types

Here we will go over the types of neutralisation reactions that occur in chemistry most commonly.

There are four types of neutralisation reactions, and they all relate to the strength of acids and bases.

Strength of Acids and Bases

So, what is strength when discussing acids and bases?

The strength of an acid or a base is determined by the dissociation of the compound in the solution.

We can define a strong acid or a strong base in the following way, based on their extent/capacity of dissociation.

So, what does that exactly mean? Take for example hydrochloric acid HCl. We know that in solution it completely dissociates to form chloride ions (Cl^-) and hydrogen ions (H⁺). This is because of its chemical composition and partly ionic behaviour. We can visualise this dissociation with the following formula:

$$ HCl\rightarrow H^++Cl^-$$

Notice that we used a one-directional arrow (\(\rightarrow\) ) rather than the two-directional half arrows ( \(\rightleftharpoons \) ) .This indicates that the reverse reaction is almost impossible and the dissociation is complete

Because hydrochloric acid dissociates completely, we term it a strong acid. The same can be applied to bases, where a strong base is a base that completely dissociates in solution to give a hydroxide ion and the resulting cation in solution.

But what does it mean for an acid or a base to be weak?

Generally, we describe organic acids as weak acids while most inorganic acids are strong.

An example of a weak acid would be ethanoic acid(organic acid), often referred to as acetic acid (CH₃COOH). It is a very common household acid (vinegar), and that is because it is relatively weak. In solution, it does not completely dissociate. This can be visualised by the following equation:

$$CH_3COOH\rightleftharpoons H^++CH_3COO^- $$

Can you see the equilibrium sign in the equation? This means that the system exists in equilibrium between the two states. The acid does not completely dissociate in solution to give hydrogen ions and the anion, but rather exists mostly as the full compound. The same can be applied to bases.

Types of Neutralisation Reactions

As mentioned earlier there are four different types of neutralisation reactions. They are as follows:

1. strong acid and strong base.
2. weak acid and strong base.
3. strong acid and weak base.
4. weak acid and weak base.

Recall that we mentioned the p^H of the resulting solution is based on the strengths of acids and bases that are participating in the reaction. Here, in the table below you will now understand how the nature of the reactants affects the p^H of the resulting solution.

In this article, you should have grasped a strong understanding of what acids and bases are, as well as what happens when you put the two together - a neutralisation reaction! Neutralisation pops up everywhere in science, and currently it helps us fight climate change and medical diseases.