6.1.6Additional lessons
Strong and Weak Acids and Bases — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 82 of 98
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Strength Depends on Ionisation
Strength describes how far a substance ionises, while concentration describes how much is dissolved.
Strong acid
- Strong acid A strong acid ionises completely in water, shown with a single arrow, for example HCl → H⁺ + Cl⁻.
Weak acid
- Weak acid A weak acid ionises only partially, so an equilibrium is set up, for example CH₃COOH ⇌ CH₃COO⁻ + H⁺.
Strong base
- Strong base A strong base such as NaOH or KOH dissociates fully to give a high concentration of OH⁻.
Weak base
- Weak base A weak base such as ammonia accepts protons only partly, so NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
Part 2 of 6Examples
Examples
Strong acids
- Strong acids HCl, HBr, HI, HNO₃ and H₂SO₄ are strong acids in water.
Weak acids
- Weak acids Ethanoic acid, other carboxylic acids and carbonic acid are weak acids.
Strong bases
- Strong bases Group 1 hydroxides such as NaOH and KOH are strong bases.
Weak bases
- Weak bases Ammonia and amines such as ethylamine are weak bases.
Part 3 of 6Comparing at Equal Concentration
pH
- pH A strong acid has a lower pH than a weak acid of the same concentration because [H⁺] is higher.
Conductivity
- Conductivity A strong acid has greater electrical conductivity because it contains more mobile ions.
Rate with metals
- Rate with metals A strong acid reacts faster with magnesium or carbonates because [H⁺] is higher.
Amount neutralised
- Amount neutralised Equal volumes of equal concentration need the same amount of base, because the weak acid ionises further as H⁺ is removed.
| Property (0.10 mol dm⁻³) | HCl | CH₃COOH |
|---|---|---|
| pH | 1.0 | about 2.9 |
| Conductivity | High | Low |
| Rate with Mg | Fast | Slower |
Part 4 of 6Reasoning Through a Comparison
1
State whether the acid is strong or weak using its degree of ionisation.
2
Link the degree of ionisation to the concentration of H⁺ in the solution.
3
Use the [H⁺] to explain the pH, conductivity or reaction rate observed.
4
Remember that moles of base needed in a titration depend on moles of acid, not on strength.
Part 5 of 6Worked Example
Question Explain why 0.10 mol dm⁻³ ethanoic acid has a higher pH than 0.10 mol dm⁻³ hydrochloric acid.
- HCl is a strong acid and ionises completely, so [H⁺] = 0.10 mol dm⁻³ and pH = 1.0.
- Ethanoic acid is weak and only partly ionises, so [H⁺] is much less than 0.10 mol dm⁻³.
- A lower [H⁺] corresponds to a higher pH.
Answer Ethanoic acid is only partially ionised, so it has a lower [H⁺] and therefore a higher pH than HCl.
Part 6 of 6Exam Checklist & Tips
6.1.6 – Key Points for the Exam
- Strong acid An acid that completely ionises in aqueous solution.
- Weak acid An acid that only partially ionises, forming an equilibrium.
- Degree of ionisation The fraction of dissolved molecules that have ionised.
- Concentration The amount of solute per unit volume, which is independent of strength.
Common Mistakes
- Confusing strong with concentrated; a dilute solution of a strong acid is still strong.
- Writing a weak acid ionisation with a single arrow.
- Saying a weak acid is less able to neutralise a base; equal moles neutralise equal moles of base.
- Claiming a weak acid has no H⁺ ions; it has fewer than a strong acid of equal concentration.
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