The catalytic converter in a car uses platinum and palladium to convert toxic exhaust gases into harmless ones — without a catalyst, those reactions would require temperatures so high the car would melt.
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A brand-new car’s catalytic converter works within about 30 seconds of starting the engine. But in the first 30 seconds before it warms up, the car emits far more toxic gases — carbon monoxide, nitrogen oxides, and unburned hydrocarbons — than it does once the catalyst is active.
Here is the puzzle: the catalyst is physically present in the exhaust system from the moment you start the car. Why does it not work immediately? And once it starts working, what exactly is it doing to the exhaust gases — and why does it never need to be refuelled or replaced with a new chemical supply? Write your predictions before reading on.
Type your predictions below — you will revisit them at the end of the lesson.
Write your predictions in your book. You will revisit them at the end.
Not all particles in a sample move at the same speed — they have a spread of kinetic energies, and only the particles at the high-energy tail of this distribution can undergo effective collisions.
The Maxwell-Boltzmann distribution is a graph showing the number of particles plotted against their kinetic energy for a sample at a given temperature. Key features:
The activation energy (Eₐ) is marked as a vertical line on the energy axis. Only particles to the right of this line have sufficient energy for an effective collision.
Effect of increasing temperature from T₁ to T₂ (T₂ > T₁):
Concentration and surface area both affect reaction rate through the same mechanism: they change how frequently reactant particles encounter each other, without changing the energy requirements for an effective collision.
Concentration effect: Increasing the concentration of a dissolved reactant means more solute particles are present per unit volume. With more particles per unit volume, the average distance between reactant particles decreases and they collide more frequently. Since the proportion of collisions that are effective (those exceeding Eₐ) is unchanged, the increased collision frequency directly increases effective collisions per second → reaction rate increases.
Surface area effect: For reactions involving a solid reactant (heterogeneous reactions), only the particles on the surface of the solid are available to collide with particles in solution or in the gas phase. Increasing surface area — by grinding into smaller particles — exposes more solid particles to collisions with the other reactant. Collision frequency at the reaction interface increases; proportion of effective collisions is unchanged.
| Variable | Mechanism | Effect on Collision Frequency | Effect on Proportion Exceeding Eₐ |
|---|---|---|---|
| Increase concentration | More particles per unit volume | Increases | No change |
| Decrease concentration | Fewer particles per unit volume | Decreases | No change |
| Increase surface area | More solid surface exposed | Increases (at interface) | No change |
| Decrease surface area | Less solid surface exposed | Decreases (at interface) | No change |
A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the overall reaction. Catalysts work by providing an alternative reaction pathway with a lower activation energy than the uncatalysed pathway. With a lower Eₐ, a greater proportion of the Maxwell-Boltzmann distribution exceeds the activation energy at the same temperature → more effective collisions per second → reaction rate increases.
Critically: a catalyst does not change the enthalpy change (ΔH) of the reaction — the reactants and products are identical, so the energy difference between them is unchanged. The catalyst only lowers the height of the energy barrier, not the starting or finishing energy levels.
Catalysts are classified by their physical state relative to the reactants:
Adding a catalyst to an energy distribution diagram shifts the activation energy line to the left — more of the existing particle distribution is now above the threshold, without changing the distribution itself.
The effect of a catalyst is distinct from the effect of temperature:
In both cases, more particles can undergo effective collisions — but the mechanism is different.
On a reaction progress (energy) diagram, both the uncatalysed and catalysed pathways have the same reactant and product energy levels. Only the transition state peak height is lower in the catalysed pathway. Therefore ΔH is identical in both pathways.
A car’s internal combustion engine produces three major toxic exhaust components:
A catalytic converter contains a ceramic honeycomb structure coated with platinum (Pt) and palladium (Pd) — heterogeneous catalysts. Three key reactions are catalysed on the surface:
Mechanism of heterogeneous catalysis: Exhaust gas molecules adsorb onto the platinum surface (adsorption), react on the surface (surface reaction), then desorb as products (desorption). The platinum surface is regenerated at the end of each catalytic cycle — it is not consumed.
The cold-start problem: The converter does not work at startup because the platinum catalyst requires a minimum operating temperature (approximately 300–400°C) before it can adsorb and activate exhaust gas molecules effectively. In the first 30–90 seconds, the converter is too cold — this cold-start period is when most vehicle emissions occur.
Problem: A student investigates the rate of the reaction between zinc powder and dilute hydrochloric acid. They conduct four experiments, changing one variable at a time from the control condition (zinc powder, 1.0 mol/L HCl, 25°C, no catalyst).
For each experiment, predict whether the rate increases, decreases, or stays the same, and explain using collision theory.
Granules have a smaller surface area than powder. Fewer zinc surface particles are exposed to HCl molecules. Collision frequency at the zinc surface decreases. Fewer effective collisions per second.
Rate decreases.
Higher concentration means more H⁺ ions per unit volume. Collision frequency between H⁺ and zinc particles increases. Proportion of effective collisions is unchanged (Eₐ is unchanged by concentration). More effective collisions per second.
Rate increases.
Higher temperature increases the average kinetic energy of all particles. The Maxwell-Boltzmann distribution shifts right — a greater proportion of particles exceed Eₐ. More effective collisions per second even at the same collision frequency. A 20°C increase near room temperature approximately doubles the rate.
Rate increases significantly.
Copper deposited on the zinc surface provides an alternative reaction pathway with a lower activation energy. A greater proportion of collisions now exceed the (lower) Eₐ at 25°C. More effective collisions per second. The catalyst is not consumed — copper is regenerated.
Rate increases.
For each change below, predict the effect on the rate of the reaction: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). Justify each answer using collision theory in 1–2 sentences.
Type your predictions below — one per change.
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Problem: A reaction has an activation energy of 60 kJ/mol. (a) Describe the fraction of particles that can undergo effective collisions at temperature T₁. (b) Describe how the diagram changes when temperature is increased to T₂. (c) Describe how the diagram changes when a catalyst is added at temperature T₁ instead. (d) Explain why the catalyst increases reaction rate without changing ΔH.
At T₁, the Eₐ line is at 60 kJ/mol on the x-axis. The fraction of particles with energy ≥ Eₐ is the area under the Maxwell-Boltzmann curve to the right of this line. For most reactions at room temperature, this is a small fraction of the total particle population.
At T₂ (T₂ > T₁): the distribution curve shifts to the right and becomes lower and broader. The total area is unchanged (same number of particles). The peak moves to higher energy. The area to the right of the Eₐ line (60 kJ/mol) is now significantly larger — a greater proportion of particles exceed Eₐ. The Eₐ line itself does not move — it is a fixed property of the reaction.
With catalyst at T₁: the Maxwell-Boltzmann distribution curve is unchanged (same temperature, same particle energies). A new Eₐ line is drawn to the left of the original at a lower energy value (e.g. 40 kJ/mol). The area to the right of this new (lower) Eₐ line is larger than the area to the right of the original Eₐ. More particles now exceed the activation energy threshold at the same temperature.
The catalyst provides an alternative reaction pathway — a different sequence of bond-breaking and bond-forming steps — that reaches the same products from the same reactants but via a lower-energy transition state. Because the reactants and products are chemically identical in both the catalysed and uncatalysed pathways, the energy difference between them (ΔH) is unchanged. The catalyst only lowers the barrier height, not the starting or finishing energy levels.
Without drawing, describe in words what you would see on a Maxwell-Boltzmann energy distribution diagram if you showed:
Type your descriptions below.
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Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
Wrong: Increasing temperature increases reaction rate because molecules collide more frequently.
Right: While collision frequency does increase slightly with temperature, the main reason reaction rate increases is that a much larger proportion of molecules exceed the activation energy. The Maxwell-Boltzmann distribution shifts — more molecules have sufficient energy for effective collisions.
5 random questions from a replayable lesson bank — feedback shown immediately
Q8. A reaction vessel contains a gaseous reaction at temperature T₁. Explain, using a Maxwell-Boltzmann energy distribution diagram, how increasing the temperature to T₂ increases the rate of the reaction. In your answer, describe: (a) the change in the distribution curve; (b) the change in the proportion of particles that can react; (c) the resulting change in effective collision frequency. (5 marks)
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Q9. Compare homogeneous and heterogeneous catalysts. In your response: (a) define each type with a named example; (b) explain the mechanism by which a heterogeneous catalyst operates; (c) explain why a heterogeneous catalyst is generally preferred in large-scale industrial processes. (5 marks)
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Q10. A car’s catalytic converter is in perfect working order. (a) Write a balanced equation for the catalytic oxidation of carbon monoxide in the converter. (b) Explain, using your knowledge of heterogeneous catalysis, why the converter does not reduce CO emissions in the first 30 seconds after the car is started. (c) Predict and explain the effect on CO emissions if the catalytic converter is damaged and its platinum surface is coated with lead compounds. (5 marks)
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Q1 C: Increasing temperature shifts the distribution to higher energies (right shift), lowers and broadens the peak, but the total area under the curve is constant — the same number of particles exist at both temperatures. Option B incorrectly states area increases. Option A describes a decrease in temperature. Option D incorrectly states temperature changes Eₐ — activation energy is a fixed property of the reaction, not affected by temperature.
Q2 B: A heterogeneous catalyst is in a different phase from the reactants (e.g. solid Pt catalysing gas-phase reactions). It operates by adsorbing reactant molecules onto its surface, facilitating the reaction, then releasing products. Option A is wrong — catalysts are not consumed. Option C is wrong — catalysts do not change ΔH. Option D is wrong — catalysts lower Eₐ, not raise it.
Q3 C: Heterogeneous catalysis requires reactant molecules to adsorb onto the catalyst surface and be activated. Below approximately 300–400°C, the platinum surface cannot effectively adsorb and activate CO and O₂ molecules — the catalytic cycle cannot begin. Option A is factually wrong — Pt is always present. Option B describes permanent poisoning, which does not occur from CO alone at low temperatures. Option D is incorrect — CO and O₂ enter the exhaust regardless of temperature.
Q4 C: Surface area determines the number of solid particles exposed to the other reactant. Powder has a greater surface area than granules of the same mass → more zinc atoms are accessible to H⁺ ions → higher collision frequency at the interface → higher rate. The zinc atoms in both forms have the same kinetic energy and the same activation energy threshold (options A and D are incorrect). Concentration (option B) refers to dissolved species, not solid amounts.
Q5 B: Catalysts lower the activation energy by providing an alternative pathway with a lower-energy transition state. They do not add thermal energy to the system. The Maxwell-Boltzmann distribution (particle energies) is unchanged; only the threshold (Eₐ) is lowered. This is mechanistically distinct from the temperature effect, which raises particle energies without changing Eₐ.
Q6 C: Temperature and catalyst effects both increase the proportion of effective collisions, but by different mechanisms. Temperature shifts the energy distribution rightward (more particles at high energy, same Eₐ). A catalyst lowers Eₐ (same distribution, lower threshold). Option A is wrong because catalysts do not shift the curve. Option B is wrong because temperature does not lower Eₐ. Option D is wrong — temperature does not change ΔH.
Q7 B: Adding a heterogeneous catalyst lowers Eₐ (more effective collisions at same temperature, ΔH unchanged) and increasing the partial pressure of gas-phase reactants increases concentration (higher collision frequency). Together these both increase rate without changing temperature or ΔH. Option A is partly wrong — decreasing pressure reduces concentration and rate. Option C violates the constraint of not changing temperature. Option D reduces rate.
Q8: (a) The Maxwell-Boltzmann curve shifts to higher kinetic energy values (rightward shift). The peak of the curve decreases in height and becomes broader. The total area under the curve is unchanged — the same number of particles exists at both temperatures. (b) The activation energy Eₐ line (fixed at the same energy value) now intersects the curve further to the left on the T₂ curve. The area to the right of Eₐ is significantly larger for T₂ — a greater proportion of particles now have kinetic energy greater than or equal to Eₐ and can undergo effective collisions. (c) With a greater proportion of particles exceeding Eₐ per unit time, the frequency of effective collisions increases. More product is formed per second → reaction rate increases.
Q9: (a) Homogeneous catalyst: in the same phase as the reactants. Example: H⁺(aq) ions catalysing ester hydrolysis in aqueous solution — catalyst and reactants all in the aqueous phase. Heterogeneous catalyst: in a different phase from the reactants. Example: solid platinum (Pt) catalysing gas-phase reactions in a catalytic converter. (b) Mechanism of heterogeneous catalysis: reactant molecules (gas or liquid phase) adsorb onto the solid catalyst surface, becoming bound at active sites. The adsorbed molecules react on the surface (surface reaction), then desorb as product molecules. The catalyst surface is regenerated and not consumed. (c) Industrial preference: heterogeneous catalysts are physically separate from the reactant and product phases (solid catalyst in a gas or liquid stream), making them easy to recover, reuse, and replace without contaminating the product. Homogeneous catalysts require a separation step (e.g. distillation or extraction) to remove them from the product mixture.
Q10: (a) 2CO(g) + O₂(g) → 2CO₂(g). (b) At startup, the platinum surface is below its operating temperature (~300–400°C). Heterogeneous catalysis requires reactant gas molecules to adsorb onto the platinum surface and be activated at active sites. At low temperatures, CO and O₂ molecules do not have sufficient energy to adsorb effectively onto the platinum surface — the adsorption step cannot proceed, so the catalytic cycle does not begin. CO passes through the converter unreacted, increasing emissions. Once the converter reaches operating temperature, adsorption proceeds and catalysis begins. (c) Lead compounds (catalyst poisons) physically coat the platinum surface and block the active sites — the specific sites on the platinum surface where adsorption and reaction occur. With active sites blocked, CO and O₂ molecules cannot adsorb onto the surface and the catalytic cycle cannot proceed. CO emissions will increase significantly, as the converter can no longer oxidise CO to CO₂. The effect is analogous to the cold-start problem but permanent — heating the converter will not regenerate the poisoned surface.
Answer questions on Factors Affecting Reaction Rate before your opponents cross the line. Fast answers = faster car. Pool: lessons 1–12.
Earlier you were asked: A brand-new car’s catalytic converter works within about 30 seconds of starting the engine. But in the first 30 seconds before it warms up, the car emits far more toxic gases than it does once the catalyst is active. Why does it not work immediately? And what exactly is it doing to the exhaust gases — and why does it never need to be refuelled?
The key insight: a catalyst speeds up a reaction by providing an alternative reaction pathway with a lower activation energy. But the catalyst itself must be in the correct physical and chemical state to do this. A catalytic converter only works once it reaches its operating temperature (~300–400°C). This is because heterogeneous catalysis requires reactant molecules to adsorb onto the platinum surface. At low temperatures, CO and O₂ molecules do not stick to the surface strongly enough for the catalytic reaction to occur. Once hot, the Pt surface activates the reactants, the activation energy is lowered, and the exhaust gases are converted to less harmful products. The catalyst is not consumed in the reaction — it is regenerated at the end of each catalytic cycle — which is why it never needs refuelling.
Now revisit your initial response. What did you get right? What has changed in your thinking?
Look back at your initial response in your book. Annotate it with what you now understand differently.