Acids and bases for AQA GCSE Chemistry

Acids and bases

This page covers the following topics:

1. Strong and weak acids
2. Carboxylic acids
3. Bases and alkalis
4. pH scale

Dissociation is a process of a substance splitting into ions within a solution. Strong acids dissociate fully to their respective ions. Weak acids dissociate 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

Acidic solutions have more H⁺ ions than OH⁻ ions. Alkaline solutions have more OH⁻ ions than H⁺ ions. Neutral solutions have the same amount of H⁺ ions and OH⁻ ions.

The pH is a measure of how acidic or alkaline a solution is. Neutral solutions have a pH of 7, acidic solutions have a pH of less than 7, alkaline solutions have a pH of more than 7. The pH is usually between 0 and 14 but occasionally can go beyond these values.

The pH value is linked to the concentration of H⁺ and OH⁻ ions. Diluting an acidic solution usually results in a pH value increasing as it becomes more neutral. Diluting an alkaline solution usually results in a pH value decreasing as it becomes more neutral. In fact, changing the concentration of H⁺ or OH⁻ ions in a solution by 10 times is equivalent to changing the pH by 1 unit.

pH can be measured using indicators or electronic devices like pH probes. Universal indicator changes its colours at different pH values allowing to get approximate pH values. For a universal indicator red, orange and yellow indicates an acidic solution, green - neutral, blue, indigo, violet - alkaline. pH probe can provide a more accurate reading of a pH of a solution.

pH scale

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

The concentration of an alkaline solution has been reduced by 1000 times by diluting it. Find the pH of the newly created solution if the pH of the initial solution is 12.

Diluting an alkaline solution usually results in a pH value decreasing as it becomes more neutral. Changing the concentration of H⁺ or OH⁻ ions in a solution by 10 times is equivalent to changing the pH by 1 unit.

Since the alkaline solution has been diluted, its pH has decreased. The concentration has been decreased by 1000 times; therefore, the pH has to change by 3. Combined together these statements result in the pH changing from 12 to 9.

9

The concentration of an alkaline solution has been reduced by 1000 times by diluting it. Find the pH of the newly created solution if the pH of the initial solution is 12.

4

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⁻.

5

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

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