Acids and alkalis for OCR A-level Chemistry

Acids and alkalis

This page covers the following topics:

1. Strong and weak acids
2. Carboxylic acids
3. Bases and alkalis
4. Brønsted-Lowry acids and bases

Dissociation is a process of a substance splitting into ions within a solution. Strong acids dissociate/ionise fully to their respective ions. Weak acids dissociate/ionise only partially creating an equilibrium between the acid molecules and their respective ions. Both weak and strong acids produce H⁺ ions within solutions.

Examples of strong acids include hydrochloric acid (HCl), sulphuric acid (H₂SO₄), nitric acid (HNO₃). Dissociation of hydrochloric acid is a one-way reaction: HCl → H⁺ + Cl⁻.

Examples of weak acids include carbonic acid (H₂CO₃), phosphoric acid (H₃PO₄), and many organic acids. Dissociation of carbonic acid is a reversible reaction: H₂CO₃ ⇌ H⁺ + HCO₃⁻.

Strong and weak acids

Carboxylic acids are weak organic acids that end with −COOH. In the homologous series of carboxylic acids there are multiple short- and long-chain molecules:
• COOH → methanoic acid
• CH₃COOH → ethanoic acid (vinegar)
• CH₃CH₂COOH → propanoic acid
• CH₃CH₂CH₂COOH → butanoic acid

Carboxylic acids dissolve well (especially the shorter ones) in water and have relatively high melting and boiling points due to intermolecular forces called hydrogen bonding. Carboxylic acids have a variety of uses, such as household cleaning products (vinegar) and pharmaceuticals.

Carboxylic acids

Bases are substances (usually metal hydroxides and metal oxides) that react with acids to form a salt and water. Common bases include sodium oxide (Na₂O), calcium oxide (CaO), and magnesium oxide (MgO).

Alkalis are soluble bases (usually soluble metal hydroxides). Common alkalis include sodium hydroxide (NaOH), potassium hydroxide (KOH), barium hydroxide (Ba(OH)₂), and ammonia (NH₃). Alkaline solutions contain hydroxide ions (OH⁻).

NaOH → Na⁺ + OH⁻
Ba(OH)₂ → Ba²⁺ + 2OH⁻
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

Bases and alkalis

Brønsted–Lowry acids are compounds with particles that donate protons (hydrogen ions). Brønsted–Lowry bases are compounds with particles that accept protons (hydrogen ions). This transfer of protons happens when acids and bases react together. Some compounds like water can behave both like an acid and a base.

After a reaction between an acid and a base occurs, the initial base becomes a conjugate acid and the previous acid becomes a conjugate base. The conjugate acid and the conjugate base together are called a conjugate acid-base pair.

Brønsted-Lowry acids and bases

1

Which of the following acids are weak?

• phosphoric acid
• hydrochloric acid
• sulphuric acid
• nitric acid
• carbonic acid

Examples of strong acids include hydrochloric acid (HCl), sulphuric acid (H₂SO₄), nitric acid (HNO₃). Examples of weak acids include carbonic acid (H₂CO₃), phosphoric acid (H₃PO₄), and many organic acids.

• phosphoric acid → weak acid
• hydrochloric acid → strong acid
• sulphuric acid → strong acid
• nitric acid → strong acid
• carbonic acid → weak acid

phosphoric acid, carbonic acid

Which of the following acids are weak?

• phosphoric acid
• hydrochloric acid
• sulphuric acid
• nitric acid
• carbonic acid

2

Define bases.

Bases are substances (usually metal hydroxides and metal oxides) that react with acids to form a salt and water.

substances that react with acids to form a salt and water

Define bases.

3

Although H₂SO₄ is considered a strong acid, the second order dissociation of HSO₄⁻ losing its hydrogen resembles a weak acid dissociation. Provide a dissociation reaction equation for HSO₄⁻.

Dissociation is a process of a substance splitting into ions within a solution. Weak acids dissociate/ionise only partially creating an equilibrium between the acid molecules and their respective ions. Due to resembling the dissociation of a weak acid, dissociation of HSO₄⁻ is a reversible reaction.

HSO₄⁻ ⇌ H⁺ + SO₄²⁻

Although H₂SO₄ is considered a strong acid, the second order dissociation of HSO₄⁻ losing its hydrogen resembles a weak acid dissociation. Provide a dissociation reaction equation for HSO₄⁻.

4

Which of the following compounds are bases?
• CO₂
• CaO
• Ba(OH)₂
• SiO₂
• NH₃

Bases are substances (usually metal hydroxides and metal oxides) that react with acids to form a salt and water. Common bases include sodium oxide (Na₂O), calcium oxide (CaO), magnesium oxide (MgO).

CaO, Ba(OH)₂, NH₃

Which of the following compounds are bases?
• CO₂
• CaO
• Ba(OH)₂
• SiO₂
• NH₃

5

Which particle is a conjugate base in the reaction provided?

H₃PO₄ + H₂O ⇌ H₂PO₄⁻ + H₃O⁺

Brønsted–Lowry acids are compounds with particles that donate protons (hydrogen ions). After a reaction between an acid and a base occurs, the previous acid becomes a conjugate base. Since initially H₃PO₄ is an acid, H₂PO₄⁻ is a conjugate base.

H₂PO₄⁻

Which particle is a conjugate base in the reaction provided?

H₃PO₄ + H₂O ⇌ H₂PO₄⁻ + H₃O⁺

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