Chemistry • Year 12 • Module 6 • Lesson 12
Ka, pKa & Comparing Acid Strengths
Apply Ka and pKa to real data, compare acid strengths across Australian contexts, and interpret an inline Ka comparison bar chart.
1. Interpret Ka data — five common weak acids
The table below shows Ka values for five weak acids important in Australian chemistry contexts. Use it to answer questions 1.1–1.5. 10 marks
| Acid | Chemical formula | Ka (25 °C) | Australian context |
|---|---|---|---|
| Hydrofluoric acid | HF | 6.8 × 10⁻⁴ | Industrial fluorination processes; aluminium smelting |
| Acetic (ethanoic) acid | CH₃COOH | 1.8 × 10⁻⁵ | AWRI wine acid research; food preservation |
| Carbonic acid | H₂CO₃ | 4.3 × 10⁻⁷ | Great Barrier Reef ocean acidification |
| Hydrocyanic acid | HCN | 6.2 × 10⁻¹⁰ | Gold cyanidation at Kalgoorlie mines (WA) |
| Water | H₂O | 1 × 10⁻¹⁶ | Universal solvent; reference point for Kw |
1.1 Rank the five acids from strongest to weakest. Write the ranking using chemical formulae. 2 marks
1.2 Calculate the pKa of HCN (Ka = 6.2 × 10⁻¹⁰). Show your working. 2 marks
1.3 Calculate Kb for the acetate ion (CH₃COO⁻), the conjugate base of acetic acid. Use Ka × Kb = Kw. Show your working. 2 marks
1.4 A student claims: “HCN has a very small Ka so it is a very dangerous acid.” Identify the error in the student's reasoning and explain why small Ka does not mean the acid is safe. 2 marks
1.5 The AWRI (Australian Wine Research Institute) measures titratable acidity in wine, which is dominated by tartaric acid (Ka1 = 1.0 × 10⁻³) and acetic acid (Ka = 1.8 × 10⁻⁵). Which acid contributes more [H⁺] at equal concentrations? Justify using Ka values. 2 marks
2. Interpret the pKa comparison bar chart — acid strength across a range
The chart below plots the pKa values of eight acids on a horizontal axis. A lower bar (shorter bar, smaller pKa) represents a stronger acid. Use it to answer questions 2.1–2.4. 8 marks
Data from NIST Webbook and lesson table. Strong acid pKa values are approximate.
2.1 Using the chart, identify the strongest and weakest weak acids shown (exclude the two strong acids HClO₄ and H₂SO₄). 2 marks
2.2 The chart shows H₂CO₃ (pKa 6.37) in the context of the Great Barrier Reef. Ocean acidification increases dissolved CO₂, forming more H₂CO₃. Using Ka and the equation [H⁺] = √(Ka × c), explain qualitatively what increasing c(H₂CO₃) does to coral reef pH. 2 marks
2.3 HCN has pKa = 9.21 and is used in gold cyanidation at Kalgoorlie. A student argues that because HCN has a very large pKa it is barely an acid and therefore safe to handle in water. Identify the flaw in this reasoning. 2 marks
2.4 H₂SO₄ and HF are both on the chart. Despite HF having a much higher pKa (3.17 vs ≈−3), HF is notorious for deeper tissue burns than dilute H₂SO₄. Suggest one reason, using the concept of conjugate base, why HF is uniquely hazardous despite being a weaker acid. 2 marks
3. Cause-and-effect chain — predicting reaction direction from Ka values
Fill in the effect boxes. The first step is completed as a model. 5 marks
Scenario: HF(aq) reacts with CN⁻(aq) ⇌ HCN(aq) + F⁻(aq). Ka(HF) = 6.8 × 10⁻⁴; Ka(HCN) = 6.2 × 10⁻¹⁰.
(model — completed)
3.1 Overall outcome: Will the equilibrium HF + CN⁻ ⇌ HCN + F⁻ lie predominantly to the left, to the right, or be approximately 50/50? Justify with Ka values. 2 marks
4. Apply to scenario — carbonic acid and Great Barrier Reef bleaching
In 2016 and 2022, mass coral bleaching events on the Great Barrier Reef were linked to both elevated sea surface temperatures and increasing ocean acidity. The dissolved CO₂ system is governed by: CO₂(aq) + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻, with Ka1(H₂CO₃) = 4.3 × 10⁻⁷ and Ka2(HCO₃⁻) = 4.7 × 10⁻¹¹. 6 marks
4.1 Calculate the ratio Ka1/Ka2 and explain what this tells you about the relative contribution of the two ionisation steps to ocean [H⁺]. 2 marks
4.2 Calculate Kb for CO₃₂⁻ (the conjugate base of HCO₃⁻). Use this Kb value to predict whether a Na₂CO₃ solution would be acidic, basic or neutral, and explain why. 2 marks
4.3 Predict and justify the pH change in a small, enclosed coral-reef tidal pool if photosynthesis by algae significantly lowers dissolved CO₂ during daylight. Use Ka1 in your reasoning. 2 marks
Q1 — Ka data table
1.1 HF > CH₃COOH > H₂CO₃ > HCN > H₂O (largest Ka = strongest).
1.2 pKa(HCN) = −log(6.2 × 10⁻¹⁰) = 10 − log(6.2) = 10 − 0.79 = 9.21.
1.3 Kb(CH₃COO⁻) = Kw / Ka = (1.0 × 10⁻¹⁴) / (1.8 × 10⁻⁵) = 5.6 × 10⁻¹⁰. This is a very weak base.
1.4 The error is conflating acid strength (Ka) with toxicity. HCN is toxic because of its CN⁻ ion binding to cytochrome c oxidase in mitochondria — this biochemical mechanism is unrelated to Ka. Many substances with small Ka are highly toxic (e.g. HCN, picric acid). Ka measures ionisation extent, not biological danger.
1.5 Tartaric acid (Ka1 = 1.0 × 10⁻³) contributes more [H⁺] than acetic acid (Ka = 1.8 × 10⁻⁵) at equal concentrations, because Ka(tartaric)/Ka(acetic) ≈ 56 — tartaric acid ionises ≈56 times more at the same concentration. The AWRI monitors both because tartaric dominates total acidity.
Q2 — Bar chart interpretation
2.1 Strongest weak acid: HF (pKa 3.17 — shortest bar). Weakest weak acid (excluding H₂O): HCO₃⁻ (pKa 10.33 — longest bar among the weak acids).
2.2 Increasing c(H₂CO₃) increases the product (Ka × c) inside the square-root expression for [H⁺]. Therefore [H⁺] increases and pH decreases. Ocean acidification causes coral reef pH to fall, reducing the CO₃₂⁻ and HCO₃⁻ available for shell-building and increasing dissolution of calcium carbonate reef structures.
2.3 A large pKa means a small Ka — HCN ionises very little in water. However, hazard is determined by toxicity at low concentration, not by Ka. Even nanomolar concentrations of CN⁻ are lethal. The student is conflating acid strength (Ka / pKa) with chemical or biological hazard, which are unrelated quantities.
2.4 HF's conjugate base F⁻ is a small, highly charge-dense anion that rapidly chelates Ca₂⁺ ions in bone and tissue, causing systemic hypocalcaemia — even in small topical doses. H₂SO₄ produces SO₄₂⁻, a weaker chelator of Ca₂⁺. So HF's tissue damage is driven by the reactivity of its conjugate base, not the acid's ionisation constant.
Q3 — Cause-and-effect chain
Effect 2: On the product side, HCN (weaker acid) and F⁻ (weaker conjugate base) are both weaker than HF and CN⁻ respectively. The products side therefore has the weaker acid and weaker base.
Effect 3: Equilibrium lies toward the side with the weaker acid and weaker base — the products side (HCN + F⁻) is strongly favoured.
3.1 The equilibrium lies predominantly to the right (products favoured). Ka(HF)/Ka(HCN) = 6.8 × 10⁻⁴ / 6.2 × 10⁻¹⁰ ≈ 1.1 × 10⁶ — the equilibrium constant for the reaction is approximately 10⁶, meaning products are overwhelmingly favoured.
Q4 — Great Barrier Reef scenario
4.1 Ka1/Ka2 = (4.3 × 10⁻⁷)/(4.7 × 10⁻¹¹) ≈ 9,100 (≈ 10⁴). The first ionisation produces ≈9,100 times more H⁺ than the second at equivalent conditions. In practice, the second ionisation is negligible for ocean chemistry pH calculations; only Ka1 drives the acidification.
4.2 Kb(CO₃₂⁻) = Kw/Ka2 = (1.0 × 10⁻¹⁴)/(4.7 × 10⁻¹¹) = 2.1 × 10⁻⁴. This is a moderately strong weak base. Na₂CO₃ produces CO₃₂⁻ in solution; because CO₃₂⁻ has a sizeable Kb it accepts H⁺ from water to produce OH⁻ and HCO₃⁻, making the solution basic (pH > 7). This is why washing soda (Na₂CO₃) is alkaline.
4.3 Photosynthesis consumes CO₂, shifting CO₂(aq) + H₂O ⇌ H₂CO₃ to the left (Le Chatelier). Less H₂CO₃ is present, so Ka1 = [H⁺][HCO₃⁻]/[H₂CO₃] is no longer at equilibrium — [H₂CO₃] has decreased, meaning the denominator is smaller relative to the numerator, so to restore equilibrium the reaction shifts further left, consuming H⁺ and HCO₃⁻. Net result: [H⁺] decreases and pH rises during peak photosynthesis. This well-documented daytime pH rise in tidal pools can reach 0.5–1 pH unit in shallow, enclosed systems.