Reactions of Acids

This section explains the reaction of acids covering, reactions of acids with metals, neutralisation of acids and the preparation of salts, making salts and the preparation of soluble salts. 

Reactions of Acids with Metals

When acids react with metals, they produce hydrogen gas and a salt. This is an example of a displacement reaction where the metal displaces the hydrogen ions in the acid.

Example: Reaction of Magnesium with Hydrochloric Acid

When magnesium reacts with hydrochloric acid (HCl), the products are magnesium chloride (MgCl₂) and hydrogen gas (H₂):

$$Mg (s) + 2HCl (aq) \rightarrow MgCl_2 (aq) + H_2 (g)$$ 

In this reaction, magnesium (Mg) reacts with hydrochloric acid to form magnesium chloride (MgCl₂) and hydrogen gas (H₂). The hydrogen ions from the acid (H⁺) are displaced by the magnesium ions (Mg²⁺), producing hydrogen gas.

Neutralisation of Acids

Neutralisation is the reaction between an acid and a base to produce a salt and water. In this process, the hydrogen ions (H⁺) from the acid react with the hydroxide ions (OH⁻) from the base to form water (H₂O).

General Neutralisation Reaction:

$$H^+ (aq) + OH^- (aq) \rightarrow H_2O (l)$$ 

For example, when hydrochloric acid reacts with sodium hydroxide (a base), the products are sodium chloride (NaCl) and water:

$$HCl (aq) + NaOH (aq) \rightarrow NaCl (aq) + H_2O (l)$$ 

This neutralisation reaction shows that the acid (HCl) and the base (NaOH) neutralise each other to form a salt (NaCl) and water.

Preparation of Salts

Salts are formed during reactions between acids and metals, bases, or carbonates. The salt produced depends on the acid and the substance with which it reacts. The general steps involved in preparing salts vary depending on the method used.

• From an Acid and a Metal: When a metal reacts with an acid, a salt is formed, as shown in the reaction between magnesium and hydrochloric acid. The salt produced is based on the acid used. In this case, magnesium chloride (MgCl₂) is the salt formed.
• From an Acid and a Base (Neutralisation): When an acid reacts with a base, the resulting salt depends on the acid and the base involved. For example:

$$H_2SO_4 (aq) + 2NaOH (aq) \rightarrow Na_2SO_4 (aq) + 2H_2O (l)$$ 

In this reaction, sulfuric acid (H₂SO₄) reacts with sodium hydroxide (NaOH) to form sodium sulphate (Na₂SO₄) and water.

• From an Acid and a Carbonate: When an acid reacts with a carbonate, carbon dioxide gas (CO₂), water, and a salt are produced. For example:

$$2HCl (aq) + Na_2CO_3 (aq) \rightarrow 2NaCl (aq) + H_2O (l) + CO_2 (g)$$ 

Here, hydrochloric acid (HCl) reacts with sodium carbonate (Na₂CO₃) to form sodium chloride (NaCl), water, and carbon dioxide gas.

Making Salts and Preparation of Soluble Salts

To make salts in the laboratory, a common method is to react an acid with a suitable base, metal, or carbonate. If you want to prepare soluble salts, you need to follow certain steps:

• Method 1: Using an Acid and an Insoluble Base (e.g. metal oxide)

If the base is insoluble, you can add the base to the acid until no more base reacts (i.e., the reaction stops when all the acid is neutralised). The mixture is then filtered to remove any unreacted base. The filtrate contains the dissolved salt, which can be obtained by evaporating the water.

Example: Preparation of Copper Sulphate

• Add excess copper oxide (CuO) to dilute sulphuric acid (H₂SO₄).
• Heat the mixture and filter to remove the unreacted copper oxide.
• The filtrate contains copper sulphate (CuSO₄), which can be obtained by evaporating the water.
• Method 2: Using an Acid and a Soluble Base (e.g. alkali)

If the base is soluble (such as sodium hydroxide), the acid is neutralised by adding the base until the solution reaches the desired pH. After neutralisation, the salt solution is evaporated to produce crystals of the salt.

Example: Preparation of Sodium Chloride

• Mix hydrochloric acid (HCl) with sodium hydroxide (NaOH) in a beaker.
• Add the sodium hydroxide until the solution is neutral (use pH paper or a pH meter to check).
• Evaporate the water from the solution to obtain crystals of sodium chloride (NaCl).
• Method 3: Using an Acid and a Carbonate

If an acid reacts with a carbonate, the reaction produces a salt, water, and carbon dioxide. The solution is filtered to remove any excess solid carbonate, and the salt solution can be evaporated to obtain the salt.

Example: Preparation of Sodium Carbonate

• Add excess sodium carbonate (Na₂CO₃) to dilute hydrochloric acid (HCl).
• Filter the mixture to remove the excess carbonate.
• Evaporate the solution to obtain sodium chloride (NaCl).

Summary of Key Points:

• Acid + Metal → Salt + Hydrogen gas.
• Acid + Base → Salt + Water (Neutralisation).
• Acid + Carbonate → Salt + Water + Carbon dioxide.
• Salts can be prepared by reacting acids with metals, bases, or carbonates.
• Soluble salts can be obtained by evaporating the water from the solution after neutralisation or reaction with a metal or carbonate.

Understanding these reactions and processes is crucial for your GCSE Chemistry studies and for mastering the concept of acid-base reactions and salt formation.