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Module 3 · L12 of 12 35 min ⚡ +50 XP in Learn · +25 to complete

Factors Affecting Reaction Rate

In September 2015, the US EPA issued a notice of violation to Volkswagen: 11 million diesel cars worldwide contained software that detected emissions testing conditions and switched the engine to a special mode — one that held the catalytic converter at its optimal temperature for maximum NOₓ conversion. On real roads, the ECU defaulted to economy mode: cooler catalyst, slower reaction, 40 times the permitted nitrogen oxide emissions.

Today's hook — VW Dieselgate, September 2015: the US EPA found that 11 million VW diesel cars used a "defeat device" — software that recognised test conditions and switched to a mode holding the catalytic converter at peak operating temperature. On real roads: cooler catalyst, slower catalytic reactions, 40× permitted NOₓ. The scandal cost VW €30 billion. The mechanism? Catalytic reaction rate is temperature-dependent, and VW exploited that.

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Worksheets

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Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.

Build Foundations & guided practice Apply Application practice Master Mastery challenge Exam Exam-style questions

Recall — your gut answer first

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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.

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What you'll master

Know

Key facts

The five factors affecting reaction rate: temperature, concentration (solutions), pressure (gases), surface area (solids), and catalysts
How each factor is explained using collision theory — collision frequency, proportion exceeding Eₐ, or both
The adsorption-surface reaction-desorption mechanism of heterogeneous catalysis

Understand

Concepts

Why a catalyst is the only factor that changes the proportion of particles exceeding Eₐ WITHOUT changing temperature — it lowers Eₐ, not the energy distribution
How to show a catalyst on a Maxwell-Boltzmann diagram (move Eₐ line left, curve unchanged) and on a reaction progress diagram (lower the peak, reactant/product levels unchanged)
Why the catalytic converter requires a warm-up period — the Pt surface must reach ~300–400°C to adsorb reactant molecules effectively

Can do

Skills

Explain any rate factor using full collision theory language: effective collision frequency, proportion of particles exceeding Eₐ, activation energy
Draw a Maxwell-Boltzmann distribution correctly showing the effect of a catalyst (Eₐ line moves left) vs. temperature increase (curve shifts right)
Explain the cold-start problem and catalytic converter poisoning using the adsorption mechanism

Key terms

Concentration effect

Increasing reactant concentration increases collision frequency → faster reaction rate; more particles per unit volume.

Temperature effect

Increasing temperature raises both collision frequency and average kinetic energy; more particles exceed Ea → exponentially faster rate.

Surface area effect

Dividing a solid into smaller pieces increases surface area exposed to reactants → more effective collisions per unit time.

Catalyst

Provides an alternative reaction pathway with lower activation energy; increases rate without being consumed; does not change ΔH or equilibrium position.

Pressure effect on gases

Increasing pressure of a gas reaction increases concentration of gas-phase reactants → higher collision frequency.

Cross-lesson links: In L11 you built the collision theory model — effective collisions require sufficient energy (≥ E_a) and correct orientation. In L12 you apply that model to four controllable variables: temperature, concentration, surface area, and catalysts. Each factor has a precise particle-level explanation that HSC examiners test frequently in 4–6 mark questions.

Four factors that affect reaction rate

core concept

Pour dilute hydrochloric acid onto zinc granules at room temperature: steady fizzing, hydrogen gas released at a measurable rate. Now run four parallel experiments — replace granules with fine zinc powder, raise the acid temperature to 60°C, increase acid concentration from 1.0 to 3.0 mol/L, and add a copper catalyst. Each change speeds up the reaction. The question is why: what is happening at the particle level in each case?

From L11, reactions occur only when particles undergo effective collisions — sufficient energy (≥ E_a) and correct orientation. There are only two ways to increase effective collision frequency:

Increase how often particles collide — more particles per unit volume, or more surface exposed
Increase the proportion of collisions that are effective — raise particle energy or lower the E_a threshold

Factor	Particle-level change	Collision frequency	Proportion exceeding Eₐ
↑ Temperature	M-B curve shifts right; average KE increases	Slight increase	Large increase
↑ Concentration (↑ pressure for gases)	More particles per unit volume	Increases	No change
↑ Surface area	More solid surface exposed	Increases at interface	No change
Add catalyst	Alternative pathway with lower E_a	No change	Large increase

Four rate factors, via collision theory: Temperature — increases collision frequency AND proportion exceeding Eₐ (the most powerful factor). Concentration/Pressure — increase collision frequency only. Surface area — increases frequency at the interface only. Catalyst — lowers Eₐ, increasing proportion exceeding it.

Pause — copy the highlighted table into your book before moving on.

Mini-task: For each of the four rate factors (temperature, concentration, surface area, catalyst), write one sentence explaining HOW it increases reaction rate using collision theory language. Do not just say "increases collisions" — name the specific mechanism.

1. Temperature and the Maxwell-Boltzmann Distribution

core concept

1. Temperature and the Maxwell-Boltzmann Distribution

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:

Starts at zero (no particles have zero energy)
Rises to a peak (the most probable energy)
Falls as a long tail toward higher energies (a small number of particles have very high energies)

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):

The peak of the distribution shifts to the right (higher average energy)
The peak height decreases (same number of particles, now spread over a wider energy range)
The tail extends further to the right
The area to the right of Eₐ increases significantly — a much larger proportion of particles now exceed the activation energy
The total area under the curve remains the same — the number of particles has not changed

HSC Must-Do: When asked to describe the effect of temperature using an energy distribution diagram, address three things: (1) the curve shifts to higher energies; (2) the proportion of particles exceeding Eₐ increases; (3) effective collision frequency and reaction rate increase. All three are required for full marks.

Common Error: Many students draw the T₂ curve higher than T₁. This is wrong. At higher temperature, the peak is lower and broader, shifted to the right. The area under both curves must be equal — a higher peak at T₂ would imply more particles exist at higher temperature, which is incorrect.

Insight: This is why the “10°C rule” works — for many reactions near room temperature, raising the temperature by 10°C roughly doubles the fraction of particles in the high-energy tail beyond Eₐ. The exact factor depends on Eₐ, but this approximation holds well for biological systems — which is why fever accelerates metabolic reactions.

We just saw the overview table linking all four rate factors to collision theory. That raises a question: what does the energy distribution of particles actually look like, and exactly how does temperature change that picture? This card answers it → the Maxwell-Boltzmann curve shifts right and flattens at higher temperature, dramatically increasing the fraction of particles that exceed Eₐ.

At higher temperature (T₂ > T₁), the Maxwell-Boltzmann curve shifts right and becomes lower and broader — total area is unchanged (same number of particles). A much greater proportion of particles exceed Eₐ, so effective collision frequency and rate increase. Drawing error: the T₂ peak must be lower, never higher, than T₁.

Add the highlighted rule to your notes before the check below.

Quick check: When temperature increases from T₁ to T₂, what happens to the peak of the Maxwell-Boltzmann distribution curve?

2. Concentration and Surface Area

core concept

2. Concentration and Surface Area

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

HSC Must-Do: Surface area only applies to heterogeneous reactions — those involving a solid reactant. For reactions entirely in solution (homogeneous), surface area is not a relevant variable. Always identify whether the reaction is heterogeneous before invoking surface area in your answer.

Common Error: Students say increasing concentration increases the energy of the particles. It does not — concentration has no effect on the kinetic energy distribution of the particles. It only affects how frequently they collide. Energy effects are the domain of temperature and catalysts.

We just saw that temperature shifts the Maxwell-Boltzmann curve, increasing the proportion of particles exceeding Eₐ. That raises a question: how do concentration and surface area fit into this picture — do they change particle energies too? This card answers it → no: both factors increase only collision frequency, leaving the energy distribution and Eₐ completely unchanged.

Concentration and surface area increase reaction rate by increasing collision frequency only — neither changes the proportion of particles exceeding Eₐ. Concentration: more particles per unit volume → more frequent collisions. For gases, increasing pressure = increasing concentration. Surface area: applies only to heterogeneous (solid) reactions; more surface exposed → more frequent collisions at the interface.

Pause — write the highlighted point into your book.

True or false: Increasing the concentration of a dissolved reactant increases the collision frequency between reactant particles but does not change the proportion of particles with energy ≥ Eₐ.

3. Catalysts — Homogeneous and Heterogeneous

core concept

3. Catalysts — Homogeneous and Heterogeneous

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:

Homogeneous Catalyst

Same phase as reactants

Forms intermediate in solution

H⁺(aq) in ester hydrolysis

Must be separated from products

Pharmaceutical synthesis

Heterogeneous Catalyst

Different phase from reactants

Surface adsorption and reaction

Pt(s) in catalytic converter

Physically separate — easily recovered

Catalytic converters, Haber process

HSC Must-Do: When explaining how a catalyst works, always state three things: (1) it provides an alternative reaction pathway; (2) with a lower activation energy; (3) it is not consumed in the overall reaction. All three are required for full marks.

Common Error: Students say catalysts “give energy to the particles” or “heat up the reaction.” Catalysts do not add energy — they lower the energy barrier. The particles do not need as much energy to react — the threshold is lowered, not the particle energies raised. This is fundamentally different from the temperature effect.

We just saw that concentration and surface area raise rate purely through collision frequency, leaving particle energies unchanged. That raises a question: is there a way to increase the proportion of particles that succeed at an effective collision without raising the temperature? This card answers it → yes: a catalyst lowers Eₐ via an alternative pathway, so more particles exceed the (lower) threshold at the same temperature.

A catalyst provides an alternative reaction pathway with a lower Eₐ — without being consumed. Lower Eₐ → greater proportion of the Maxwell-Boltzmann distribution exceeds threshold at the same T → more effective collisions/s → faster rate. ΔH is unchanged (same reactants and products). Homogeneous: same phase as reactants; heterogeneous: different phase (e.g. solid Pt, gas reactants).

Pause — copy the highlighted definition into your book before moving on.

Mini-task: A catalytic converter uses platinum (solid) to convert CO gas and O₂ gas into CO₂. (a) Is this an example of a homogeneous or heterogeneous catalyst? (b) Name the three stages of the catalytic mechanism. (c) Explain why the catalyst is not consumed in the reaction.

4. Energy Distribution Diagrams with Catalysts

core concept

4. Energy Distribution Diagrams with Catalysts

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:

Temperature changes the distribution of particle energies (shifts the curve right)
Catalyst changes the activation energy threshold (shifts Eₐ to the left)

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.

HSC Must-Do: HSC questions frequently ask you to show the effect of a catalyst on both a Maxwell-Boltzmann distribution diagram AND a reaction progress (energy) diagram. On the Maxwell-Boltzmann move the Eₐ line left (curve unchanged). On the reaction progress lower the peak height (reactant and product levels unchanged).

Common Error: Students shift the Maxwell-Boltzmann curve when showing a catalyst effect — drawing the curve moved to the right as if temperature had increased. This is wrong. The catalyst lowers Eₐ (moves the threshold line), not the curve. Only temperature shifts the curve.

We just saw that a catalyst lowers Eₐ via an alternative pathway without changing ΔH. That raises a question: how exactly do you represent this on a Maxwell-Boltzmann distribution and on a reaction progress diagram — and how is this different from showing a temperature increase? This card answers it → on M-B: move the Eₐ line left (curve unchanged); on reaction progress: lower the peak height (reactant and product levels unchanged).

Catalyst on Maxwell-Boltzmann diagram: draw the same curve, move Eₐ line to the LEFT — larger area now to the right. Catalyst on reaction progress diagram: lower the transition-state peak height only — reactant and product energy levels are identical (ΔH unchanged). Temperature: shifts the curve right. Catalyst: shifts the Eₐ line left. Never confuse these two.

Add the highlighted rule to your notes before the check below.

Quick check: On a Maxwell-Boltzmann diagram, how is the effect of adding a catalyst correctly shown?

5. Catalytic Converters — Heterogeneous Catalysis in Action

core concept

5. Catalytic Converters — Heterogeneous Catalysis in Action

A car’s internal combustion engine produces three major toxic exhaust components:

Carbon monoxide (CO) — from incomplete combustion
Unburned hydrocarbons (CₓHₕ) — from incomplete combustion
Nitrogen oxides (NOₓ) — formed when N₂ and O₂ from air react at the high temperatures inside the engine

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:

Balanced Equation

2CO(g) + O₂(g) → 2CO₂(g)

CₓHₕ(g) + O₂(g) → CO₂(g) + H₂O(g)

2NO(g) → N₂(g) + O₂(g)

Toxic → Harmless

CO → CO₂

CₓHₕ → CO₂, H₂O

NO → N₂

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.

HSC Must-Do: Ensure you can balance 2NO → N₂ + O₂ correctly. Left: 2N, 2O. Right: 2N, 2O. ✓ Balanced as written. Be ready to explain the cold-start problem — adsorption requires the catalyst surface to reach operating temperature, not for the catalyst to be “produced” or “activated chemically.”

Common Error: Students say the catalyst is consumed in the catalytic converter. It is not — platinum is regenerated at the end of each cycle. Catalytic converter failure is caused by poisoning (lead, sulfur, or phosphorus compounds coating the platinum surface and blocking active sites), not by the platinum being consumed.

We just saw how to represent a catalyst’s effect on both types of diagrams. That raises a question: where is heterogeneous catalysis actually used to solve a large-scale emissions problem, and how does the cold-start failure tie back to the adsorption mechanism? This card answers it → the catalytic converter uses Pt/Pd on a ceramic honeycomb to convert CO, CₓHₕ, and NOₓ to harmless gases — but only once the surface exceeds ~300–400°C.

Catalytic converter (heterogeneous, Pt/Pd): 2CO + O₂ → 2CO₂; CₓHₕ + O₂ → CO₂ + H₂O; 2NO → N₂ + O₂. Mechanism: adsorption → surface reaction → desorption; Pt regenerated each cycle. Cold-start failure: Pt surface below ~300–400°C cannot adsorb exhaust molecules → no catalysis → toxic emissions pass through untreated in first 30–90 s.

Pause — write the highlighted equations and cold-start explanation into your book.

True or false: A catalytic converter fails to reduce emissions during cold-start because the platinum surface requires a temperature of approximately 300–400°C to adsorb and activate exhaust gas molecules.

Worked examples · reveal as you go

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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).

Experiment 1: zinc granules (instead of powder)
Experiment 2: 2.0 mol/L HCl
Experiment 3: temperature increased to 45°C
Experiment 4: a drop of copper(II) sulfate solution added (acts as a catalyst by depositing copper on the zinc surface)

For each experiment, predict whether the rate increases, decreases, or stays the same, and explain using collision theory.

Exp 1: zinc granules

Granules have a smaller surface area than powder. Fewer zinc surface particles are exposed to HCl molecules. Collision frequency at the zinc surface decreases. Rate decreases.

Exp 2: 2.0 mol/L HCl

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). Rate increases.

Exp 3: 45°C

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.

Exp 4: copper catalyst

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.

Worked example +5 XP on full reveal

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.

Part (a): Fraction at T₁

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.

Part (b): Effect of raising T to T₂

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.

Part (c): Effect of adding a catalyst at T₁

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.

Part (d): Why ΔH is unchanged

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.

Sort the steps +5 XP

Put these statements in order to explain how a catalyst increases reaction rate using the Maxwell-Boltzmann energy distribution.

The catalyst provides an alternative reaction pathway with a lower activation energy (Eₐ).

More effective collisions occur per unit time → reaction rate increases.

The Maxwell-Boltzmann distribution curve is unchanged (temperature is constant).

The Eₐ threshold line moves to the left — a greater proportion of the existing particle population now exceeds the lower Eₐ.

Formula reference · this lesson

core formula

📐

Key Concepts — This Lesson

$\text{Rate} \propto \text{effective collision frequency}$

All four factors affect rate by changing either collision frequency or the proportion of collisions that are effective

$T\uparrow \;\Rightarrow\; \text{M-B curve shifts right} \;\Rightarrow\; \text{more particles exceed }E_a \;\Rightarrow\; \text{rate}\uparrow$

Approximation: rate roughly doubles per 10°C increase near room temperature (reaction-dependent)

Catalyst: $E_a\downarrow$ via alternative pathway $\Rightarrow$ more particles exceed $E_a$ at same $T$ $\Rightarrow$ rate$\uparrow$ $\Rightarrow$ $\Delta H$ unchanged

Catalyst is not consumed — regenerated at end of each catalytic cycle

Common errors · the 3 traps that cost marks

Common misconception

Increasing temperature increases reaction rate because molecules collide more frequently.

Fix: 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.

A catalyst shifts the Maxwell-Boltzmann distribution curve to higher energies

A student draws a Maxwell-Boltzmann diagram with the curve shifted to the right to show the effect of a catalyst — like drawing the effect of increased temperature.

Fix: A catalyst does NOT shift the Maxwell-Boltzmann curve — that is what temperature does. A catalyst lowers the activation energy (Eₐ), moving the Eₐ threshold line to the left on the diagram. The particle energy distribution curve itself remains unchanged at constant temperature. In an exam diagram: draw the same curve, but move the Eₐ vertical line to a lower energy value.

The catalytic converter works from the moment the engine starts

A student writes that the catalytic converter removes CO and NOₓ as soon as the car is started.

Fix: The catalytic converter does NOT function until the platinum catalyst reaches its operating temperature (~300–400°C). In the first 30–90 seconds (the cold-start period), CO, unburned hydrocarbons, and NOₓ pass through untreated. The platinum surface cannot adsorb and activate reactant molecules at low temperatures. Most vehicle emissions per trip occur during cold-start.

Work mode · how are you completing this lesson?

Quick-fire practice · 5 reps +2 XP per reveal

(5 marks) 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) 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) 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.

Describe the three steps of the mechanism by which a heterogeneous catalyst operates.

A homogeneous acid catalyst (H⁺) is added to a reaction. Explain (a) why the rate increases and (b) why the yield of product is unchanged.

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Revisit your thinking

You've now seen both sides of the same phenomenon: the cold-start problem, and VW Dieselgate (September 2015) — where VW exploited temperature-dependent catalyst performance to pass emissions tests while emitting 40× permitted NOₓ on real roads. Using activation energy (E_a), the Maxwell-Boltzmann distribution, and collision theory, explain in four sentences: (1) why the cold catalytic converter is ineffective below ~200°C; (2) why the hot catalyst converts NOₓ efficiently; and (3) why VW's "defeat device" held the catalyst at peak temperature during testing but not on real roads.

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.

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Interactive Tool — Rates of Reaction Open fullscreen ↗

The Reaction Rates tool shows that adding a catalyst increases reaction rate without changing temperature because…

Multiple choice

+5 XP per correct · +25 XP all-correct

Pick your answer, then rate your confidence — that tells the system what to drill next.

Fill the blanks +5 XP

Complete this model answer about a catalytic converter:

A catalytic converter uses and palladium as catalysts. Exhaust gases onto the catalyst surface, react, then as less harmful products. The converter does not work during cold-start because the catalyst surface must reach approximately before it can activate reactant molecules.

Short answer

ApplyBand 3

Q1. 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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ApplyBand 3

Q2. 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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ApplyBand 3

Q3. 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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Five timed questions on factors affecting reaction rate. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).

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Science Jump · Factors Affecting Reaction Rate

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