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2.7.2Inorganic Chemistry

Acids, Bases and Protons — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions

5 min read 4 parts Lesson 37 of 52

📚 Edexcel International GCSE Science Double Award 4SD0 - Chemistry Hub / Topical Revision
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Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
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Part 1 of 4Acids as Proton Donors

2.7.2 Acids, Bases and Protons

In aqueous solution an acid releases hydrogen ions, which are protons, into the water.

Proton donor
  • An acid is a substance that donates a proton, H⁺, to another substance in a reaction.
Ionisation
  • Hydrochloric acid ionises completely in waterHCl(g) → H⁺(aq) + Cl⁻(aq), making the solution acidic.
Sulfuric acid
  • Sulfuric acid releases hydrogen ions into solution, which is why it reacts with bases to form sulfates.
Acidic solution
  • The concentration of hydrogen ions in an acidic solution is greater than in pure water.

Part 2 of 4Bases as Proton Acceptors

A base accepts a proton, and the bases that dissolve in water are called alkalis.

Bases as Proton Acceptors
Proton acceptor
  • A base is a substance that accepts a proton, H⁺, from an acid in a reaction.
Alkali ionisation
  • Sodium hydroxide dissolves and releases hydroxide ions: NaOH(s) → Na⁺(aq) + OH⁻(aq).
Alkaline solution
  • Hydroxide ions accept protons from acids, which makes the solution alkaline and gives a pH above 7.
Ammonia
  • Ammonia accepts a proton to form ammonium ions: NH₃ + H⁺ → NH₄⁺, so it behaves as a base.

Part 3 of 4Acid and Base Equations

Writing ionic equations shows which particles are responsible for acidic and alkaline behaviour.

1
Write the symbol equation for the ionisation of the acid or alkali, including the state symbols for the solution.
2
Use the symbol (aq) for a substance dissolved in water and (s) or (l) for substances that are solid or liquid before dissolving.
3
Check the charges on both sides to make sure the equation balances for both atoms and charge.
4
Compare the equations for an acid and an alkali to see that acids release H⁺ and alkalis release OH⁻.

Part 4 of 4Exam Checklist & Tips

2.7.2 – Key Points for the Exam

  • Proton A hydrogen ion, H⁺, which is a single proton with no electron.
  • Acid A proton donor that produces H⁺ ions in aqueous solution.
  • Base A proton acceptor that neutralises an acid to form a salt and water.
  • Alkali A base that dissolves in water to produce hydroxide ions.
  • Ionisation The process in which a compound separates into ions when it dissolves or reacts.

Common Mistakes

  • Describing acids as producing hydrogen gas in solution; acids produce hydrogen ions, not hydrogen molecules.
  • Saying that all bases are alkalis; only soluble bases form alkaline solutions.
  • Writing HCl(aq) as the formula of the acid in an equation;
    • it is the ionisation that produces H⁺(aq) and Cl⁻(aq).
  • Forgetting the state symbols, which are needed to show whether a substance is dissolved, solid or liquid.

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