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Neutralisation Reaction

We attend parties, eat lots of good food and have some nice sugary drinks. We upset our stomachs by overloading them. Our digestive system avenges its ill-treatment by giving us heart burns, bloating and whatnot! We then rush to the physician or a pharmacist who recommends an antacid. On consuming the antacid, we feel relieved of the burning sensation.

But what is the science behind all of this? Our stomach produces gastric juice which has hydrochloric acid that aids in the effective digestion of food. Eating more food than required causes the production of more gastric juice to digest, leading to increased amounts of HCl in the stomach. This causes heartburn. Antacids like the milk of magnesia (Magnesium hydroxide-Mg(OH)2) are basic in nature that counteract the effect of acid in the stomach by neutralising it. This chemical reaction between an acid and a base is called neutralisation.

• We will look into the definition of Neutralisation.
• Then, we will head over to see the products produced in a neutralisation reaction.
• Later, we will see the chemical equation corresponding to the neutralisation reaction.
• Then, we will see some examples of neutralisation reactions.
• We will finish by discussing the different types of neutralisation reactions under which we will discuss the strengths of acids and bases.

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.

Remember that the pH scale goes from 1 to 14, with acids being below 7, and bases being above 7. pH 7 is termed the neutral point.

In brief, you can define the neutralisation reaction as:

A neutralisation reaction is a chemical double displacement between an acid and a base to produce salt and water.

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.

It is interesting to note that when an acid is dissolved in water, it releases a proton - $$H^+$$ which combines with water to form a hydronium ion $$H_3O^+$$.

Let us consider HCl( a strong acid) and NaOH(a strong base) to explain this concept of how hydronium ion is formed from HCl and what happens when it reacts with the $$OH^-$$.

$$HCl+H_2O\rightarrow H_3O_{(aq)}^++Cl_{(aq)}^-$$

$$NaOH_{(aq)}\rightarrow Na_{(aq)}^++OH_{(aq)}^-$$

$$H_3O^++OH^-\rightarrow 2H_2O$$

Adding up all these equations and cancelling the common ions(Out of the two water molecules on the RHS, only one remains while one cancels out with the water molecule on the LHS)

$$HCl+\cancel {H_2O}+NaOH_{(aq)}+\cancel {H_3O^+}+\cancel {OH^-}\rightarrow \cancel {H_3O_{(aq)}^+}+Cl_{(aq)}^-+Na_{(aq)}^++\cancel {OH_{(aq)}^-}+\cancel {2H_2O}$$

We get

$$HCl+NaOH\rightarrow NaCl_{(aq)}+H_2O$$

Further, removing the spectator ions

$$H^++\cancel {Cl^-}+\cancel {Na^+}+OH^-\rightarrow \cancel {Na^+}+\cancel {Cl^-}+H_2O$$

Thus, the net ionic equation is:

$$H^++OH^-\rightarrow H_2O$$

The net ionic equation remains the same, but one should remember that the protons from the acid combine with the water to form protonated water/hydronium ion.

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.

Let us consider the neutralisation reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH).

$$HCl+NaOH\rightarrow NaCl+H_2O$$

In the reaction above, you can see how a simple base and acid produce a water and a salt molecule. We can write out the ions for each reactant to see how the produced ions react with each other:

$$NaOH\rightarrow Na^++OH^-$$

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

We can then see clearly how each component comes together to form water and salt.

Remember that the most important for the balance of this reaction is that re components be equimolar, which means the same amount of reactants at the same concentration.

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.

In this example, we will examine how sulphuric acid (H2SO4) and sodium hydroxide (NaOH) behave in a neutralisation reaction. Here is the balanced reaction equation:

$$H_2SO_4+2NaOH\rightarrow 2H_2O+Na_2SO_4$$

Sulphuric acid- $$H_2SO_4$$ is a diprotic acid.

Can you see how when there is a diprotic acid, meaning that there is more than 1 hydrogen ion produced per molecule, the ratio between the acid and base is not 1:1? This is because you need twice the amount of base to counteract the diprotic acid.

In scenarios like this, you do not need an equimolar solution of acid and base, but rather a stoichiometric balance between the hydrogen and hydroxide ions.

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.

A strong acid or a strong base is a substance that dissociates/ionises completely in water to produce the corresponding ions.

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?

A weak acid or a weak base is a substance that doesn't fully dissociate into its corresponding ions when dissolved in water is called as a weak acid or a weak base.

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 pH 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 pH of the resulting solution.

 Type of neutralisation reaction Nature of the resulting solution Reason Strong acid vs strong baseHCl and NaOH Neutral- pH = 7 Both the acid and base dissociate completely. No free molecules in the solution.Complete dissociation of both acid and base gives a neutral solution Strong acid vs weak baseHCl and NH4OH Slightly acidic solution pH=3-6 Acid dissociates completely while the base doesn't. Hence, the resulting solution is slightly acidicThe concentration of free hydrogen ions is more while the weak base exists as molecules as it doesn't fully ionise. Strong base vs weak acidNaOH and CH3COOH Slightly basic solution pH = 8-11 The strong base dissociates completely while the weak acid doesn't. Hence, the resulting solution is slightly basic.The concentration of free hydroxide ions is more than the hydrogen ions in the solution. Weak acid vs weak baseCH3COOH and NH4OH It depends on the chemical properties of the reactants involved. However, generally, the equilibrium favours to stay on the LHS. Both the acid and the base are weak, they prefer to exist as molecules and dissociation is very difficult.

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.

Neutralisation Reactions - Key takeaways

• Neutralisation reactions occur between an acid and a base.
• The products are a salt and water.
• The generalised formula is: Acid + Base → Water + Salt.
• The strength of acids and bases is dependent on their extent of dissociation in solutions.
• There are 4 types of neutralisation reactions, and not all of them result in a neutral pH.

An example of a neutralisation reaction would be hydrochloric acid and sodium hydroxide. They react to form water and sodium chloride (a salt). HCl + NaOH => H2O + NaCl

There are 4 types of neutralisation reaction: strong acid and strong base, weak acid and strong base, strong acid and weak base, weak acid and weak base.

Neutralisation in a reaction is when an acid and a base combine to produce a salt and water, which results in the solution to be of neutral pH (pH = 7).

Neutralisation occurs when an acid and a base react to produce a neutral solution of pH 7.

The generalised equation for a neutralisation reaction is: Acid + Base => Salt + Water

Final Neutralisation Reaction Quiz

Question

What happens to the pH of a solution in a neutralisation reaction?

The pH goes to around 7.

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Question

During a neutralisation reaction, what are the two products?

Salt and Water

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Question

In what scenario can the pH of the resulting solution after neutralisation be above 7?

If the reactants are a strong base and a weak acid.

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Question

Give a generalised formula for a neutralisation reaction.

Acid + Base => Water + Salt

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Question

What determined the strength of an acid?

Its dissociation. If an acid completely dissociates in solution, then it is strong.

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Question

Is it possible that after an equimolar neutralisation reaction the pH be lower than 7?

Yes, if a diprotic acid is used or a weak base in the reactants.

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Question

WIll a neutralisation reaction occur between a weak acid and a weak base?

Yes.

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Question

Which ions of the reactant acid make up the salt in the products?

Anion

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Question

Which ion from the reactant base comprises the salt in the products?

Cation

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Question

What ratio between a diprotic acid and strong base do you need to get a pH of 7?

2:1 (diprotic acid:base)

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Question

How many types of neutralisation reactions are there?

4

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Question

Can you have a pH lower than 7 with a monoprotic acid?

Yes if the reactant base is a weak base.

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Question

What two ions comprise the water in the products in a neutralisation reaction?

The hydroxide ion and the hydrogen ion (hydronium ion)

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Question

Give an example of a strong acid and a weak acid.

Strong acid: HCl

Weak Acid: CH3COOH (ethanoic acid)

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Question

What condition regarding the concentration of a strong acid and a strong base needs to be met for the resulting solution to be pH 7?

The reactant solutions have to be equimolar, that means the same volume at the same concentration.

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