Acids – SS1 Chemistry Lesson Note (Second Term)

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

Behavioural objectives

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

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

How does an acid produce hydroxonium ions?

An acid produces hydroxonium ions when it dissolves in water, because the hydrogen ion joins a water molecule.

H⁺(aq) + H₂O(l) → H₃O⁺(aq)

• Hydrogen ion (H⁺): a hydrogen atom that has lost its electron, so it is a proton.
• Hydroxonium ion (H₃O⁺): the form in which the hydrogen ion exists in water.
• Proton donor: an acid gives H⁺ to another substance, such as water.

What are the classes of acids?

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

What are organic acids?

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

What are inorganic acids?

Inorganic acids are acids prepared from mineral elements or inorganic matter.

What are dilute and concentrated acids?

An acid is dilute or concentrated depending on the amount of water in the solution.

• Dilute acid: made by adding a large amount of water to a small amount of acid.
• Concentrated acid: made by adding only a small amount of water to a relatively large amount of acid.

• Safety rule: when diluting, pour the acid slowly into water, stirring all the time.
• Reason: adding water to concentrated acid can release heat and make the acid splash.

What is the strength of an acid?

The strength of an acid is how completely it ionises in water, and an acid is either strong or weak.

• Strength: depends on the extent of ionisation of the acid.
• Concentration: depends on the amount of acid dissolved in the solution.

What are strong acids?

Strong acids are acids that ionise completely in aqueous solution, so the solution has a high concentration of H⁺.

• Examples: HCl, H₂SO₄ and HNO₃.

H₂SO₄ → 2H⁺ + SO₄^2-

HNO₃ → H⁺ + NO₃^–

HCl → H⁺ + Cl^–

What are weak acids?

Weak acids are acids that ionise only slightly in aqueous solution, so the solution has a low concentration of hydrogen ions.

• Examples: ethanoic acid (CH₃COOH), H₂CO₃, H₃PO₄ and H₂SO₃.
• Equation sign: the double arrow (⇌) shows that ionisation is only partial.

H₂CO₃ ⇌ 2H⁺ + CO₃^2-

H₃PO₄ ⇌ 3H⁺ + PO₄^3-

CH₃COOH ⇌ H⁺ + CH₃COO^–

H₂SO₃ ⇌ 2H⁺ + SO₃^2-

What is the difference between strong and weak acids?

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.

• Ethanoic acid: has four hydrogen atoms, but only the one in the COOH group is replaceable.

What are the physical properties of acids?

The physical properties of acids are the features we can observe or test without a chemical reaction.

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.

• Warning: never taste laboratory acids. The sour taste is known from foods such as lime and vinegar.

How do indicators show an acid?

An indicator is a substance that changes colour to show whether a solution is acidic or alkaline.

What are the chemical properties of acids?

The chemical properties of acids are the reactions they undergo with metals, bases and trioxocarbonate (IV) salts.

How do acids react with metals?

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

Acid + metal → salt + hydrogen gas

2HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g)

H₂SO₄(aq) + Mg(s) → MgSO₄(aq) + H₂(g)

• Limit: this applies to metals above hydrogen in the reactivity series, using dilute acids such as HCl and H₂SO₄.
• No reaction: unreactive metals such as copper do not liberate hydrogen from dilute acids.

How do acids react with bases?

Acids react with bases to form salt and water only, and this reaction is called neutralisation.

Acid + base → salt + water

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

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

How do acids react 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

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

• Test for the gas: carbon (IV) oxide turns lime water milky.

How are acids prepared?

There are three common methods of preparing acids: dissolving an acid anhydride in water, combining constituent elements, and displacing an acid from its salt.

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: 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

Some acids form when hydrogen combines with a non-metal, and the gas formed is dissolved in water.

• Hydrochloric acid: hydrogen burns in chlorine, with activated charcoal as catalyst, to give HCl gas. The gas dissolves readily in water.
• Hydrobromic acid: hydrogen and bromine vapour are heated with platinum as catalyst to give hydrogen bromide. The gas dissolves readily in water.

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

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.

• Example (a): the more volatile hydrogen chloride is displaced from a metallic chloride by the less volatile concentrated tetraoxosulphate (VI) acid.
• Example (b): the weaker trioxoborate (III) acid, H₃BO₃, is displaced from borax, Na₂B₄O₇, by tetraoxosulphate (VI) acid.

NaCl(s) + H₂SO₄(l) → NaHSO₄(s) + HCl(g)

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

What are the uses of acids?

Acids are used in the home, in industry and in the body.

• Ethanoic acid: gives vinegar its sour taste and is used in food.
• Citric acid: gives lime and lemon their sour taste and is used to flavour foods and drinks.
• Tetraoxosulphate (VI) acid: used in car batteries and in making fertilisers.
• Trioxonitrate (V) acid: used in making fertilisers.
• Hydrochloric acid: present in the stomach, where it helps digestion.
• Carbonic acid: present in fizzy drinks.

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

Define basicity, state the basicity of H₃PO₄ and CH₃COOH, list three physical properties of acids, and describe one method of preparing an acid with an equation.