Neutralisation of Acids

What happens when an acid reacts with a base?

When an acid reacts with a base, a neutralisation reaction occurs
- These reactions are exothermic
Bases have pH values above 7
Many bases are insoluble in water but the ones that do dissolve in water are called alkalis
They thus form an alkaline solution
Examples of alkalis are soluble metal hydroxides such as NaOH and Ca(OH)₂
In all acid-base neutralisation reactions, salt and water are produced:

acid + base ⟶ salt + water

If the base is a metal carbonate, carbon dioxide is also produced:

acid + carbonate ⟶ salt + water + carbon dioxide

The identity of the salt produced depends on the acid used and the positive ions in the base
Hydrochloric acid produces chlorides, sulfuric acid produces sulfate salts and nitric acid produces nitrates

Table salt or sodium chloride

Sodium chloride is commonly known as table salt and can be formed from a neutralisation reaction

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Reactions of Acids with Metal Oxides and Metal Hydroxides

Metal oxides and metal hydroxides act as bases
When they react with acid, a neutralisation reaction occurs
In all acid-base neutralisation reactions, salt and water are produced

The following are some specific examples of reactions between acids and metal oxides:

2HCl + CuO ⟶ CuCl₂ + H₂O

H₂SO₄ + Na₂O⟶ Na₂SO₄ + H₂O

2HNO₃ + MgO ⟶ Mg(NO₃)₂ + H₂O

The following are some specific examples of reactions between acids and metal hydroxides:

2HCl + 2LiOH ⟶ 2LiCl + H₂O

H₂SO₄ + 2NaOH ⟶ Na₂SO₄ + 2H₂O

HNO₃ + KOH ⟶ KNO₃ + H₂O

Reactions of Acids with Metal Carbonates

Acids will react with metal carbonates to form the corresponding metal salt, carbon dioxide and water
These reactions are easily distinguishable from acid – metal oxide/hydroxide reactions due to the presence of effervescence caused by the carbon dioxide gas
- We can test for the presence of carbon dioxide gas by bubbling it through limewater
- If the limewater turns milky or cloudy carbon dioxide is present

The following are some specific examples of reactions between acids and metal carbonates:

2HCl + Na₂CO₃ ⟶ 2NaCl + H₂O + CO₂

H₂SO₄ + CaCO₃⟶ CaSO₄ + H₂O + CO₂

2HNO₃ + MgCO₃ → Mg(NO₃)₂ + H₂O + CO₂

Examiner Tip

For a neutralisation reaction to occur, water must be formed.
- Therefore, the reaction between an acid and a metal is not a neutralisation reaction.
If an acid-base reaction effervesces (fizzes), then the base must be a metal carbonate as carbon dioxide gas is produced.

Explaining Neutralisation Using Ions

Higher Tier

Acids are a source of hydrogen ions, H⁺
Bases (or alkalis) are a sources of hydroxide ions, OH^–
When they react together in a neutralisation reaction, the H⁺ ions react with the OH^– ions to produce water
This is the net ionic equation of all acid-base neutralisations and is what leads to a neutral solution, since water has a pH of 7:

H⁺(aq) + OH^–(aq)⟶ H₂O (l)

Not all reactions of acids are neutralisations
- For example when a metal reacts with an acid, although a salt is produced there is no water formed so it does not fit the definition of neutralisation
Neutralisation is very important in the treatment of soils to raise the pH as some crops cannot tolerate pH levels below 7
This is achieved by adding bases to the soil such as limestone and quicklime

Examiner Tip

Make sure you learn this equation for neutralisation, H⁺ (aq) + OH^– (aq) → H₂O (l), including state symbols, as it is a common question that is asked in exams.

Neutralisation of Acids (WJEC GCSE Chemistry: Combined Science)

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