Chemistry • Year 12 • Module 6 • Lesson 5
Strong vs Weak Acids & Bases: The Critical Distinction
Lock in the vocabulary, complete-dissociation lists, and arrow-notation rules before tackling exam-style questions.
1. Term–definition match
The twelve definitions below are shuffled. In the right-hand column write the matching term from this list: strong acid, weak acid, strong base, weak base, degree of dissociation, Ka, concentration, conductivity, equilibrium arrow (⇌), single arrow (→), spectator ion, strength vs concentration distinction. 12 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | An acid that ionises essentially completely in water; [H⁺] equals the initial acid concentration. | |
| 1.2 | An acid that ionises only partially in water; establishes a dynamic equilibrium with a small Ka value. | |
| 1.3 | A base that dissociates completely in water, producing [OH⁻] equal to the initial base concentration (or twice it for Ca(OH)₂). | |
| 1.4 | A base that only partially accepts protons from water; NH₃ is the most common HSC example. | |
| 1.5 | The fraction of acid or base molecules that have ionised in solution; near 1 for strong electrolytes, much less than 1 for weak. | |
| 1.6 | The equilibrium constant that quantifies the extent of ionisation of a weak acid; fixed at a given temperature regardless of concentration. | |
| 1.7 | The total amount of acid or base dissolved per litre of solution, expressed in mol/L; completely independent of strength. | |
| 1.8 | A measure of a solution’s ability to conduct electricity; strong acid/base solutions have higher values than weak acid/base solutions at the same concentration. | |
| 1.9 | The arrow notation used in ionic equations for strong acids and strong bases to indicate complete, irreversible ionisation. | |
| 1.10 | The arrow notation used in ionic equations for weak acids and weak bases to indicate partial, reversible ionisation. | |
| 1.11 | An ion present in solution that does not participate in the reaction and is not a proton donor or acceptor; e.g. Cl⁻ in HCl solution. | |
| 1.12 | The recognition that acid/base strength (Ka) and concentration (mol/L) are completely independent properties; a concentrated weak acid can have a lower pH than a dilute strong acid. |
2. Fill the blanks — strength, concentration, and arrow notation
Complete the paragraph using the word bank. Each word or symbol is used once only. 10 marks
Word bank: complete, equilibrium (⇌), independent, Ka, mol/L, partial, pH, single (→), strength, [H⁺]
Acid __________________ describes how extensively an acid ionises in water and is quantified by the dissociation constant __________________ . Concentration, measured in __________________ , describes the total moles of acid dissolved per litre. These two properties are completely __________________ of each other. A strong acid undergoes __________________ ionisation; its ionic equation therefore uses a __________________ arrow. A weak acid undergoes __________________ ionisation; its ionic equation uses an __________________ arrow. Because __________________ depends on both strength and concentration, the __________________ of a solution alone cannot tell you whether an acid is strong or weak.
3. True or false — with correction
Circle T or F. If false, write the correct version on the line below. 10 marks — 1 for T/F, 1 for the correction where needed
3.1 HF is a strong acid because it is a hydrogen halide, just like HCl, HBr and HI. T / F
3.2 Diluting HCl from 1 mol/L to 0.001 mol/L changes it from a strong acid to a weak acid. T / F
3.3 The correct ionic equation for acetic acid dissolving in water uses an equilibrium arrow: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq). T / F
3.4 Ca(OH)₂ is a weak base because limewater has a relatively low pH. T / F
3.5 For a 0.1 mol/L solution of NaOH, [OH⁻] = 0.1 mol/L because NaOH is a strong base that dissociates completely. T / F
4. Connect the terms — concept map
Draw labelled arrows between the six terms below to show causal or defining relationships. Each arrow must carry a brief linking phrase (e.g. “determines”, “is independent of”, “sets the value of”). Aim for at least 5 labelled arrows. 5 marks
Supplied terms: Ka · degree of dissociation · acid strength · concentration (mol/L) · [H⁺] · pH.
5. Function recall — short prose
Answer each in 1–2 sentences using precise lesson terms. 8 marks — 2 each
5.1 Why does 0.1 mol/L HCl have a lower pH than 0.1 mol/L CH₃COOH, even though both are at the same concentration?
5.2 What does the equilibrium arrow (⇌) communicate about a weak acid that a single arrow (→) would not?
5.3 Why must the ionic equation for Ca(OH)₂ show 2OH⁻ on the product side?
5.4 Why is HF considered a weak acid even though it is toxic and corrosive?
Q1 — Term–definition matches
1.1 strong acid • 1.2 weak acid • 1.3 strong base • 1.4 weak base • 1.5 degree of dissociation • 1.6 Ka • 1.7 concentration • 1.8 conductivity • 1.9 single arrow (→) • 1.10 equilibrium arrow (⇌) • 1.11 spectator ion • 1.12 strength vs concentration distinction.
Q2 — Cloze paragraph
Acid strength describes how extensively an acid ionises in water and is quantified by Ka. Concentration, measured in mol/L, describes total moles per litre. These two properties are completely independent of each other. A strong acid undergoes complete ionisation; its ionic equation uses a single (→) arrow. A weak acid undergoes partial ionisation; its ionic equation uses an equilibrium (⇌) arrow. Because [H⁺] depends on both strength and concentration, the pH of a solution alone cannot tell you whether an acid is strong or weak.
Q3 — True / false with correction
3.1 False. HF is a weak acid (Ka = 6.8 × 10⁻⁴). The H–F bond is unusually short and strong due to fluorine’s small atomic radius and very high electronegativity, making proton donation energetically unfavourable. HF must use ⇌ in its ionic equation.
3.2 False. HCl is a strong acid at any concentration. Dilution changes concentration, not Ka or degree of dissociation. 0.001 mol/L HCl is correctly described as a dilute strong acid.
3.3 True.
3.4 False. Ca(OH)₂ is a strong base — the dissolved fraction dissociates completely. The low [OH⁻] in limewater is due to low solubility (~0.02 mol/L at 25°C), not partial dissociation. Solubility and strength are independent properties.
3.5 True.
Q4 — Sample concept map arrows
- Ka → defines → acid strength
- acid strength → sets the value of → degree of dissociation
- degree of dissociation + concentration (mol/L) → together determine → [H⁺]
- [H⁺] → determines → pH
- concentration (mol/L) → is independent of → Ka
Award 1 mark per correctly labelled, directionally accurate arrow. Minimum 5 required.
Q5.1
HCl is a strong acid and ionises completely, so [H⁺] = 0.1 mol/L and pH = 1.0. CH₃COOH is a weak acid and ionises only partially (~1.3%), so [H⁺] ≈ 0.0013 mol/L and pH ≈ 2.9. Same concentration; different pH because of different degrees of dissociation.
Q5.2
The equilibrium arrow (⇌) indicates that the forward (ionisation) and reverse (recombination) reactions both occur at significant rates, establishing a dynamic equilibrium where most acid molecules remain intact. A single arrow (→) would falsely imply complete, irreversible ionisation.
Q5.3
Each formula unit of Ca(OH)₂ contains two hydroxide groups. Because Ca(OH)₂ is a strong base and dissociates completely, both OH⁻ ions are released per mole dissolved: Ca(OH)₂(aq) → Ca²⁺(aq) + 2OH⁻(aq). Therefore [OH⁻] = 2 × [Ca(OH)₂].
Q5.4
Acid strength (weak/strong) describes only the fraction of molecules that ionise in water, not the toxicity or hazard. HF has Ka = 6.8 × 10⁻⁴, so it ionises only partially — making it a weak acid by definition. Its extreme danger arises from the intact, uncharged HF molecules penetrating tissue and releasing F⁻ ions that bind Ca²⁺ and Mg²⁺, causing systemic effects. Weak acid does not mean safe.