Acids – SS1 Chemistry Lesson Note (Second Term)

An acid is a substance which in an aqueous solution produces a hydroxonium ion (H₃O⁺) or hydrogen ion (H⁺) as the only positive ion. Acids can also be called proton donors.

Behavioural objectives

By the end of the lesson, students should be able to:

• define an acid;
• classify acids as organic or inorganic and give examples of each;
• distinguish between dilute and concentrated acids, and between strong and weak acids;
• state the basicity of common acids;
• list the physical and chemical properties of acids;
• describe the methods of preparing acids.

How does an acid produce hydroxonium ions?

An acid can also be defined as a substance which produces hydroxonium ions when it dissolves in water:

H⁺(aq) + H₂O(l) → H₃O⁺(aq)

What are the classes of acids?

There are two classes of acids: organic acids and inorganic acids.

Organic acids

Organic acids occur as natural products in plants and animal material.

Inorganic acids

Inorganic acids can be prepared from mineral elements or inorganic matter.

What are dilute and concentrated acids?

An acid can be dilute or concentrated, depending on the amount of water added. A dilute acid is produced when a large amount of water is added to a small amount of acid. A concentrated acid is produced when only a small amount of water is added to a relatively large amount of acid.

What is the strength of an acid?

The strength of an acid can be either weak or strong.

Strong acids

Strong acids ionise completely in an aqueous solution, so the solution has a high concentration of H⁺. Examples are HCl, H₂SO₄ and HNO₃.

H₂SO₄ = 2H⁺ + SO₄^2-

HNO₃ = H⁺ + NO₃^–

HCl = H⁺ + Cl^–

Weak acids

Weak acids ionise or dissociate only slightly or partially in an aqueous solution, so the solution has a low concentration of hydrogen ions. Examples are ethanoic acid (CH₃COOH), H₂CO₃, H₃PO₄ and H₂SO₃.

H₂CO₃ = 2H⁺ + CO₃^2-

H₃PO₄ = 3H⁺ + PO₄^3-

CH₃COOH = H⁺ + CH₃COO^–

H₂SO₃ = 2H⁺ + SO₃^2-

What is the basicity of an acid?

The basicity of an acid is the number of replaceable hydrogen ions, H⁺, in one molecule of the acid. For example:

1. Hydrochloric acid is monobasic.
2. Tetraoxosulphate (VI) acid is dibasic.
3. Tetraoxophosphate (V) acid is tribasic.
4. Ethanoic acid is monobasic.

What are the physical properties of acids?

1. They have a sour taste.
2. They turn blue litmus paper red.
3. They are corrosive, especially strong acids.
4. They conduct electricity in aqueous solution.

What are the chemical properties of acids?

Reaction with metals

Acids react with metals to liberate hydrogen gas and form a salt of the metal.

Acid + metal → salt + hydrogen gas

For example:

2HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g)

H₂SO₄(aq) + Mg(s) → MgSO₄(aq) + H₂(g)

Reaction with bases

Acids react with soluble bases to form salt and water only. This reaction is known as neutralisation.

Acid + base → salt + water

For example:

H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)

2HCl(aq) + CaO(s) → CaCl₂(aq) + H₂O(l)

Reaction with trioxocarbonate (IV) salts

Acids react with trioxocarbonate (IV) salts to liberate carbon (IV) oxide, and form a salt and water.

Acid + trioxocarbonate (IV) → salt + water + CO₂

For example:

2HCl(aq) + Na₂CO₃(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)

How are acids prepared?

Acids can be prepared by the following methods.

Dissolving an acid anhydride in water

An acid anhydride is an oxide of a non-metal that dissolves in water to produce the corresponding acid. Examples are SO₂, CO₂, NO₂ and SO₃.

SO₂(g) + H₂O(l) → H₂SO₃(aq)

CO₂(g) + H₂O(l) → H₂CO₃(aq)

SO₃(g) + H₂O(l) → H₂SO₄(aq)

Combination of constituent elements

(a) Burning hydrogen in chlorine, with activated charcoal as the catalyst, yields HCl gas. The gas dissolves readily in water to give hydrochloric acid.

H₂(g) + Cl₂(g) → 2HCl(g) (catalyst: activated charcoal)

(b) Heating hydrogen gas and bromine vapour, with platinum as the catalyst, produces hydrogen bromide. The gas dissolves readily in water to form hydrobromic acid.

H₂(g) + Br₂(g) → 2HBr(g) (catalyst: platinum)

Displacement of an acid from its salt

A weak or more volatile acid can be displaced from its salt by a stronger or less volatile acid. For example:

(a) The more volatile hydrogen chloride is displaced from a metallic chloride by the less volatile concentrated tetraoxosulphate (VI) acid.

NaCl(s) + H₂SO₄(aq) → NaHSO₄(aq) + HCl(aq)

(b) The weaker trioxoborate (III) acid is displaced from borax by tetraoxosulphate (VI) acid.

Na₂B₄O₇(s) + H₂SO₄(aq) + 5H₂O(l) → Na₂SO₄(aq) + 4H₃BO₃(aq)

Here Na₂B₄O₇ is borax and H₃BO₃ is trioxoborate (III) acid.

Evaluation

1. Define an acid.
2. Name two organic acids and two inorganic acids, and give a source or constituents of each.
3. What is the difference between a strong acid and a weak acid? Give one example of each.
4. State the basicity of hydrochloric acid and tetraoxosulphate (VI) acid.
5. Write the equation for the reaction between hydrochloric acid and sodium trioxocarbonate (IV).

Assignment

1. Define basicity and state the basicity of H₃PO₄ and CH₃COOH.
2. List three physical properties of acids.
3. Describe two methods of preparing acids, with an equation for each.