Chemistry • Year 11 • Module 3 • Lesson 12
Factors Affecting Reaction Rate
Apply collision theory to real rate data, cause-and-effect reasoning, and experimental contexts including the cold-start problem and particle-size experiments.
1. Interpret rate data — marble chips and hydrochloric acid
A student reacts excess marble chips (CaCO3) with 50 cm3 of 1.0 mol L−1 HCl in a conical flask placed on a balance. Carbon dioxide gas escapes, so mass decreases over time. The experiment is repeated three times, changing one variable each time. The graph below shows mass lost (g) against time for the four conditions. 8 marks
Figure 1.1. Illustrative data for CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g). Mass loss from balance used to measure CO2 evolved.
1.1 Identify which curve (A, B, C or D) shows the fastest initial rate of reaction. Justify your choice using approximate data from the graph. 2 marks
1.2 Curves A and B reach the same final mass of CO2 lost. Explain why the final mass is identical even though curve B reacts faster. 2 marks
1.3 Explain, using collision theory, why curve D is steeper than curve A in the first 60 seconds. Your answer must reference the Maxwell-Boltzmann distribution. 2 marks
1.4 Compare curves A and C. Both use medium chips, but curve C uses 2.0 mol L−1 HCl. Explain the steeper gradient in C using collision theory, and state whether doubling concentration changed the activation energy. 2 marks
2. Cause-and-effect chain — the cold-start problem
A new car is started on a cold morning. The catalytic converter is physically present from the moment the engine starts, but the car still emits high levels of CO for the first 30–90 seconds. Trace the causal chain that explains this. Fill in each empty effect box. 5 marks
Cause 1: Car is started; exhaust gases (CO, NOx) enter the catalytic converter immediately.
Effect 1 (1 mark): The platinum surface is at approximately room temperature (<50°C) — far below the ~300–400°C needed for catalytic activity. Therefore…
Effect 2 (1 mark): CO and O2 molecules cannot _________________ onto the platinum active sites.
Effect 3 (1 mark): The catalytic cycle (adsorption → surface reaction → desorption) _________________.
Effect 4 (1 mark): CO passes through the converter _________________ and is emitted as a toxic gas.
Overall outcome (1 mark): During the cold-start period, a vehicle emits significantly more _________________ than when the converter is at operating temperature — even though the catalyst is physically intact and undamaged.
3. Compare temperature and catalyst effects
Both increasing temperature and adding a catalyst increase reaction rate. Complete the comparison table. 6 marks (1 per cell in rows 1–3 and row 6; 1 per named example in row 5)
| Feature | Increasing temperature | Adding a catalyst |
|---|---|---|
| What changes on a Maxwell-Boltzmann diagram? | ||
| Does the activation energy (Ea) change? | ||
| Does ΔH of the reaction change? | ||
| Is the catalyst consumed? | N/A | |
| Named real-world example of this factor in use | ||
| Why does it increase the proportion of effective collisions? |
4. Predict and justify — catalyst poisoning in Australia
Until 1985, Australian petrol contained tetraethyllead as an octane booster. Vehicles fitted with catalytic converters were prohibited from using leaded petrol. 4 marks
4.1 Predict and explain what would happen to the efficiency of a catalytic converter if lead compounds coated the platinum surface. Use the terms active sites and adsorption in your answer. 3 marks
4.2 Unlike the cold-start problem, catalyst poisoning by lead is permanent. Explain why reheating the converter to operating temperature would not restore efficiency. 1 mark
Q1.1 — Fastest initial rate
Curve D (45°C) shows the steepest gradient in the first 60 seconds — it accumulates the most CO2 loss in the shortest time. Accept curve B (powder) if a gradient-based justification from approximate graph values is provided.
Q1.2 — Final mass is the same for A and B
The final mass of CO2 depends on the amount of limiting reactant (HCl at 1.0 mol L−1 in 50 cm3) — the same in both A and B [1]. Surface area only affects the rate at which the reaction occurs, not the total amount of product formed when the limiting reactant is fully consumed [1].
Q1.3 — Curve D vs A; Maxwell-Boltzmann
At 45°C the Maxwell-Boltzmann distribution shifts right — the peak is lower and broader, and the high-energy tail extends further [1]. A significantly larger proportion of HCl and CaCO3 particles now exceed the activation energy Ea, so the frequency of effective collisions is higher and the rate is faster in the first 60 s [1].
Q1.4 — Curves A vs C; Ea
Doubling [HCl] from 1.0 to 2.0 mol L−1 increases the number of H+ ions per unit volume, so collision frequency between H+ and CaCO3 surface particles increases, producing a steeper gradient in C [1]. Concentration does not change Ea — the activation energy is a fixed property of the reaction determined by bond energies, not by particle counts [1].
Q2 — Cold-start causal chain
Effect 1: At room temperature, CO and O2 molecules do not have enough energy to interact effectively with the Pt surface.
Effect 2: adsorb (CO and O2 cannot adsorb onto the Pt active sites).
Effect 3: cannot proceed / does not occur.
Effect 4: unreacted (CO passes through unreacted).
Overall outcome: toxic emissions / CO and NOx (the vehicle emits significantly more toxic gases during cold-start).
Q3 — Temperature vs catalyst comparison
| Feature | Increasing temperature | Adding a catalyst |
|---|---|---|
| M-B diagram change | Curve shifts right; peak lower and broader; same total area | Ea line shifts left; curve unchanged |
| Ea change? | No | Yes — lower Ea via alternative pathway |
| ΔH change? | No | No |
| Catalyst consumed? | N/A | No — regenerated at end of each catalytic cycle |
| Real-world example | Fever accelerating metabolic reactions (10°C rule, Card 1 Insight); Haber process operated at ~450°C to give a commercially viable rate (Incitec Pivot, Brisbane) | Pt/Pd in catalytic converter oxidising CO to CO2; iron catalyst in Haber process (Incitec Pivot, Brisbane); copper deposited on zinc in Zn + HCl (Worked Example 1) |
| Why more effective collisions? | More particles have energy ≥ Ea (distribution shifted right) | More particles already exceed the new, lower Ea at the same temperature (threshold lowered) |
Q4.1 — Lead catalyst poisoning
Lead compounds physically coat the active sites on the Pt surface [1]. Active sites are the specific locations where adsorption of reactant molecules (CO, O2) occurs. When lead occupies these sites, CO and O2 cannot adsorb [1], so the surface reaction step of the heterogeneous catalytic cycle cannot proceed and CO is not oxidised to CO2 — converter efficiency falls to near zero [1].
Q4.2 — Heating does not fix poisoning
Cold-start failure is kinetic — low temperature prevents adsorption, but the surface is undamaged and adsorption resumes once operating temperature is reached. Lead poisoning is a chemical/physical blockage: lead compounds bind strongly to the Pt surface and are not removed or decomposed by reheating to operating temperature, so active sites remain blocked [1].