7.1.10 – Rate Differences: Strong vs Weak Acids
Collision Theory · Paper 4 Synthesis · Cambridge IGCSE Chemistry 0620
36
Total Marks
🔬
Section A
Observations
⚛️
Section B
Collision Theory
📈
Section C
Graph Skills
🧪 The Core Concept
Both HCl and ethanoic acid may have the same stated concentration, but they do NOT have the same [H⁺]. HCl is a strong acid (100% dissociation); ethanoic acid is weak (partially dissociates, ~1-4% at 1 mol/dm³). This difference in [H⁺] drives everything.
Section A: Observations
5 questions · 8 marks
Magnesium ribbon is added to separate beakers containing 1.0 mol/dm³ hydrochloric acid (HCl) and 1.0 mol/dm³ ethanoic acid (CH₃COOH) at the same temperature. State TWO differences in the observations between the two reactions.
Calcium carbonate chips are added to 50 cm³ of 0.5 mol/dm³ hydrochloric acid and 50 cm³ of 0.5 mol/dm³ methanoic acid (HCOOH). Describe the difference in the rate of CO₂ gas production between the two experiments.
Zinc is added to 1.0 mol/dm³ sulfuric acid (H₂SO₄) and 1.0 mol/dm³ ethanoic acid at 25°C. State the difference in the time taken for the zinc to fully dissolve.
A student tests the pH of 1.0 mol/dm³ HCl and 1.0 mol/dm³ ethanoic acid using Universal Indicator. Describe the difference in colour observed.
Equal volumes of 1.0 mol/dm³ HCl and 1.0 mol/dm³ ethanoic acid are reacted separately with excess magnesium. After both reactions are complete, state what you would observe about the FINAL VOLUME of hydrogen gas produced.
Section B: The 3-Mark Explanation (Collision Theory)
5 questions · 15 marks
Explain, using collision theory, why 1.0 mol/dm³ HCl reacts faster with magnesium than 1.0 mol/dm³ ethanoic acid of the same concentration.
A student claims: 'Ethanoic acid reacts slowly because its molecules are larger and have higher activation energy.' Evaluate this claim using collision theory.
Explain why diluting HCl from 2.0 mol/dm³ to 1.0 mol/dm³ decreases the rate of reaction with zinc, using collision theory.
A student heats 1.0 mol/dm³ ethanoic acid from 25°C to 60°C before adding magnesium. Explain why the reaction is now faster, using collision theory.
Adding a catalyst to the reaction between ethanoic acid and magnesium increases the rate. Explain how a catalyst achieves this.
Section C: Graph Interpretation
5 questions · 13 marks
A graph shows volume of H₂ produced over time for HCl (steeper gradient) and ethanoic acid (shallower gradient). Both curves level off at the same maximum volume. Explain why both reactions produce the same FINAL volume of hydrogen.
On the graph, the HCl curve has a much steeper initial gradient than the ethanoic acid curve. Explain what the initial gradient represents and why it differs between the two acids.
A student increases the volume of ethanoic acid used from 50 cm³ to 100 cm³ (keeping concentration at 1.0 mol/dm³) while keeping magnesium in excess. Sketch and describe how the graph would change compared to the original ethanoic acid curve.
The graph for HCl levels off earlier than the graph for ethanoic acid. What does this tell you about which reaction is complete first, and why?
A student proposes that if you used 2.0 mol/dm³ ethanoic acid instead of 1.0 mol/dm³ ethanoic acid (same 50 cm³ volume), the final volume of H₂ would be the same as with 1.0 mol/dm³ HCl. Evaluate this proposal.
🏆 The Golden Rule
Strong acid = complete dissociation = high [H⁺] = high collision frequency = fast rate. But the total moles determine total product — not the strength.
Sign In to Keep Going
You've reached the end of the free preview. Sign in — and upgrade to Plus or Pro — to keep going with Chemistry.
Full Access
Unlock all 320 lessons