Neutralisations: The chemical reaction between an acid and a base is generally known as a neutralisation reaction. What are the products of such a reaction? Here is the answer, the products of such reactions are salt and water. In this article, let us see how these acids and bases react and discuss several examples of such reactions in detail.

Neutralisation Reaction

When an acid reacts with a base, salt and water are formed as the products. The salt formed can be acidic, basic, or neutral. Acidic salts are formed when strong acids react with a weak base. Basic salts are formed when a strong base reacts with a weak acid. When strong acids react with strong bases, neutral salts are formed.

The general form of such reaction is:

\({\rm{Acid + Base}}\, \to \,{\rm{Salt + water}}\)

When acid is treated with a base during such a reaction, the base neutralises the acid and destroys its acidity. Since an acid and a base neutralise each other’s effect, the reaction between an acid and a base to form salt and water is known as a neutralisation reaction. Neutralizing an acid with a soluble base is known as titration or acid base-titration. 

For example, when hydrochloric acid reacts with sodium hydroxide solution, then a neutralisation reaction takes place to form sodium chloride and water. The sodium chloride thus formed is a salt that is neither acidic nor basic. It is neutral in nature. 

Let us see what happens when sodium hydroxide reacts with hydrochloric acid. That is,

\(\mathrm{NaOH}+\mathrm{HCl} \rightarrow \mathrm{NaCl}+\mathrm{H}_{2} \mathrm{O}\) or

\(\mathrm{H}^{+}+\mathrm{Cl}^{-}+\mathrm{Na}^{+}+\mathrm{OH}^{-} \rightarrow \mathrm{Na}^{+}+\mathrm{Cl}^{-}+\mathrm{H}_{2} \mathrm{O}\) or

\(\mathrm{H}^{+}+\mathrm{OH}^{-} \rightarrow \mathrm{H}_{2} \mathrm{O}\)

All acids produce \(\mathrm{H}^{+}\) ions, and all bases provide \(\mathrm{OH}^{-}\) ions in an aqueous solution. During the neutralisation of an acid with a base or vice versa, \(\mathrm{H}^{+}\) ions (from acid) and \(\mathrm{OH}^{-}\) ions (from the base) combine to produce \(\mathrm{H}_{2} \mathrm{O}\) molecules.

Let us study the above neutralisation reaction using an experiment.

Materials Required: Dilute sodium hydroxide solution, hydrochloric acid solution (the concentration of dilute hydrochloric acid and sodium hydroxide must be the same), a test tube, a dropper, and phenolphthalein indicator.

Procedure: About \(5 \,\text {ml}\) of dilute sodium hydroxide is taken in a test tube. Two drops of phenolphthalein indicator are added to it. Now, add the dilute hydrochloric acid solution dropwise using a dropper (stir the test tube gently during the addition of each drop). Continue adding it until the pink colour of the solution disappears. Now, add a drop of sodium hydroxide solution into it and shake the test tube to mix the solution. Again, add a drop of dilute hydrochloric acid and shake the test tube to mix the solution.

The experiment can be picturized as,

Let us see what the observations found during the experiment are: 

Observations

1. When two drops of phenolphthalein are added to the test tube containing sodium hydroxide solution, the solution in the test tube turns pink. 
2. The pink colour of the solution disappears gradually by the dropwise addition of hydrochloric acid due to the neutralisation of sodium hydroxide by hydrochloric acid.
3. When sodium hydroxide is added again, the solution turns pink.
4. By the repeated addition of sodium hydroxide and hydrochloric acid one after the other, we can see the reappearance and disappearance of the pink colour. 
5. When we touch the test tube immediately during the neutralisation reaction, we can feel that the test tube is hot due to heat evolved during the reaction.

Have you arrived at any conclusions from the observations? Well, let us discuss them.

Conclusion

Phenolphthalein generally gives pink colour in basic solutions. Therefore, it has shown pink colour in dilute sodium hydroxide solution. By the addition of dilute hydrochloric acid solution, it neutralises the basic nature of dilute sodium hydroxide. The addition of an extra drop of dilute hydrochloric acid makes the solution acidic. This makes the phenolphthalein colourless. 

On the other hand, the addition of dilute sodium hydroxide neutralises dilute hydrochloric acid, making the solution basic again. So, the pink colour appears again. Therefore, we can say that acids react with bases to neutralize the effect. 

During a neutralisation reaction, heat is evolved. This shows that neutralisation reactions are generally exothermic reactions. 

Other Examples of Neutralisation Reactions

1. The Reaction of Sodium Hydroxide with Sulphuric Acid

Sodium hydroxide reacts with sulphuric acid to form sodium sulphate and water as the products. The chemical reaction for the reaction can be given as,

\(2 \mathrm{NaOH}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{Na}_{2} \mathrm{SO}_{4}+2 \mathrm{H}_{2} \mathrm{O}\)

Similarly, other bases like potassium hydroxide, magnesium hydroxide, etc., also react with other acids to form the corresponding salts and water as the products.

2. The Reaction of Metal Oxides with Acids

Metals oxides are generally basic (alkaline) in nature. Therefore, a metal oxide generally reacts with acids to form corresponding salts and water as the products. That is,

\({\rm{Metal\, Oxide + Acid }}\, \to \,{\rm{Salt + water}}\)

For example, copper (II) oxide is a metal oxide. It reacts with hydrochloric acid to form copper chloride and water as the products. The chemical equation for the reaction can be given as,

Therefore, the reaction between a metal oxide and an acid is an example of a neutralisation reaction. This shows the basic nature of metal oxides. 

Just like metal oxides, metal hydroxides are also basic (alkaline) in nature. Therefore, the acids also react with metal hydroxides to form salt and water as the products. Hence, the reaction between an acid and a metal hydroxide is also a kind of neutralisation reaction. For example, the antacid, which is called milk of magnesia used to treat indigestion, is a metal hydroxide called magnesium hydroxide that reacts with the excess hydrochloric acid present in the stomach and neutralises it. 

3. The Reaction of Non-metal Oxides with Bases

Non-metal oxides are generally acidic in nature. They react with bases to form salt and water as the products. Such reactions are also examples of neutralisation reactions. That is,

\({\rm{Non\, Metal\, Oxide + Base }} \to {\rm{ Salt + water}}\)

For example, sodium hydroxide reacts with an acidic oxide like carbon dioxide to give sodium carbonate and water. The chemical equation for the reaction can be given as,

\(2 \mathrm{NaOH}+\mathrm{CO}_{2} \rightarrow \mathrm{Na}_{2} \mathrm{CO}_{3}+\mathrm{H}_{2} \mathrm{O}\)

Similarly, calcium hydroxide reacts with carbon dioxide to form calcium carbonate and water as the products. The chemical equation for the reaction can be given as,

\(\mathrm{Ca}(\mathrm{OH})_{2}+\mathrm{CO}_{2} \rightarrow \mathrm{CaCO}_{3}+\mathrm{H}_{2} \mathrm{O}\)

Neutralisation in Everyday Life

Several neutralisation reactions are employed in our everyday life. Let us discuss them.

1. In the Treatment of Acidity or Indigestion

Acidity or indigestion is caused due to the presence of excess hydrochloric acid in our stomach. This can be neutralised by adding an antacid to it. Antacids are generally basic substances that are used to treat stomach acidity. For example, the milk of magnesia (magnesium hydroxide) is an antacid that can neutralise the hydrochloric acid in the stomach. Hence, it gives relief from the pain caused due to the acidity. Other such antacids used are calcium carbonate (Tums), aluminium hydroxide (Amphojel), etc.

2. In Soil Treatment:

Plants generally grow well in neutral soil, and they do not grow in soil with more acidity or basicity. 

Soils become acidic due to the excessive use of chemical fertilisers. This makes the soil acidic. Such type of soil can be treated using the bases like calcium oxide (quicklime). As a result, the soil is neutralized. If the soil is too basic, it can be neutralized by adding organic manure (they can release acids to the soil). 

3. For Treating Ant bite or Bee’s Bite

When ant or bee bites, they inject formic acid to the skin. This causes burning and swelling. This can be neutralized by rubbing with a mild base like baking soda or calamine solution (zinc carbonate). This neutralizes the formic acid injected by the ants or bees and gives relief from the pain caused by it.

4. In the Neutralisation of Factory Wastes

Certain wastes discharged from the factories contain acids. If these wastes are discharged into the water, the acids present in them will kill the fishes and other aquatic organisms that live in water. Therefore, such wastes should be treated with basic substances to neutralize the acid present in the wastes before discharging them into the water bodies.

Summary

We are now familiar with the neutralisation reaction. An acid reacts with a base to form salt and water as the products during a neutralization reaction. For example, a base like sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water as the products. Neutralisation reactions are generally exothermic. Such type of reactions is employed in our everyday life also. Such real-life examples of neutralisation reaction and more explanation about the neutralisation reaction are explained in this article. 

FAQs on Neutralisations

Q.1. Why neutralisation reaction is exothermic?
Ans: Neutralisation reactions involve the release of energy (or heat). This is the reason why neutralisation is an exothermic reaction.

Q.2. Is neutralisation between acid and base endothermic or exothermic?
Ans: Neutralisation reaction involves the release of energy. Therefore, it is considered an exothermic reaction.

Q.3. What does one mean by exothermic and endothermic reactions? Give examples.
Ans: The reactions that involve the release of energy (or heat) is called as an exothermic reaction. For example, neutralisation reactions are generally exothermic. The reactions that involve the absorption of energy (or heat) are called endothermic reactions. For example, evaporation is an endothermic reaction.

Q.4. What are two examples of neutralisation?
Ans: The two examples of a neutralisation reaction are:
1. The reaction between sodium hydroxide and hydrochloric acid.
2. The reaction between magnesium hydroxide and hydrochloric acid.

Q.5. What does neutralisation mean?
Ans: When an acid is treated with a base, the base neutralises the acid and destroys its acidity. Since an acid and a base neutralise each other’s effect, the reaction between an acid and a base to form salt and water is called a neutralisation reaction.

Study Nature of Acids and Bases Here

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