The shape of the lesson
By the end of the lesson, learners can
- Homogeneous catalysts change oxidation state: Compare uncatalysed and catalysed routes in the simulator and look for the extra dip that marks an intermediate.
- Heterogeneous catalysts work on a surface: Follow the three stages for the iron catalyst in the Haber process and for a catalytic converter.
- Homogeneous: same phase; works via an intermediate.
- Fe2+/Fe3+: two steps between oppositely charged ions.
- Heterogeneous: adsorption, reaction, desorption.
- Adsorption neither too strong nor too weak.
One picture every idea returns to
A catalyst increases the rate by providing an alternative route with a lower activation energy, and is regenerated at the end. A homogeneous catalyst is in the same phase as the reactants and works by forming an intermediate, so its energy profile has two humps with a dip between them. Transition-metal ions are good homogeneous catalysts because they change oxidation state easily: Fe2+ or Fe3+ catalyses S2O82− + 2I−, and NO2 catalyses the oxidation of SO2 in the air. A heterogeneous catalyst is in a different phase, usually a solid with gaseous reactants, and works by adsorption, reaction on the surface and desorption. Adsorption must be neither too strong nor too weak. Enzymes are protein catalysts with highly specific active sites.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, A different route with a lower barrier.. A catalyst does not just lower a hump on a diagram: it opens up a new mechanism. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Homogeneous catalysts change oxidation state | Compare uncatalysed and catalysed routes in the simulator and look for the extra dip that marks an intermediate. Project tab 6 of the lab (Rate equations and reaction kinetics lab, catalysts) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Heterogeneous catalysts work on a surface | Follow the three stages for the iron catalyst in the Haber process and for a catalytic converter. Project tab 6 of the lab (Rate equations and reaction kinetics lab, catalysts) and let a student drive it while the class predicts. | Quiz-Quiz-Trade |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Heterogeneous catalysts work on a surface, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Quiz-Quiz-Trade
Each student holds a card with a question on the front and the answer on the back. They pair up, quiz each other, coach if needed, trade cards and find a new partner. Three rounds covers the whole set.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| A catalyst lowers E a by providing a different mechanism; it does not give molecules more energy and does not change Δ H or the position of equilibrium. | Homogeneous means the same phase, not the same substance. In heterogeneous catalysis, stronger adsorption is not always better: products must be able to desorb. The catalysed profile has two humps with an intermediate in the dip. |
Homogeneous and heterogeneous catalysts: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Write two equations to show how Fe2+ ions catalyse the reaction between S2O82− and I−.
Explain why the Fe-catalysed route is faster than the direct reaction.
Sketch and label the energy profile for an exothermic reaction with and without a homogeneous catalyst.
Describe how the catalyst in a catalytic converter removes CO and NO from exhaust gases.
Silver adsorbs gases weakly and tungsten adsorbs them very strongly. Explain why neither is a good catalyst.
Write equations showing how NO2 catalyses the oxidation of SO2 in the atmosphere.
Answer key and marking notes
- S2O82− + 2Fe2+ → 2SO42− + 2Fe3+.
- 2Fe3+ + 2I− → 2Fe2+ + I2.
- Fe2+ is regenerated; the steps add up to S2O82− + 2I− → 2SO42− + I2.
- The direct reaction needs two negative ions to collide, so repulsion gives a high activation energy.
- Each catalysed step is between oppositely charged ions, so each has a lower activation energy.
- Uncatalysed: one large hump from reactants down to lower-energy products.
- Catalysed: two smaller humps, with the intermediate in the dip between them.
- Reactant and product levels are the same for both, so ΔH is unchanged.
- Adsorption: CO and NO bond to Pt/Rh active sites, weakening their bonds and holding them close together.
- Reaction: 2CO + 2NO → 2CO2 + N2 on the surface.
- Desorption: CO2 and N2 leave, freeing the sites for more molecules.
- Silver: too few reactant molecules are held long enough, so bonds are not weakened.
- Tungsten: products bind so strongly that they do not desorb, so active sites stay blocked.
- SO2 + NO2 → SO3 + NO.
- NO + ½O2 → NO2 (NO2 regenerated).
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which is an example of heterogeneous catalysis?
- Asolid iron catalysing the reaction of gaseous N2 and H2 to make NH3
- BFe2+(aq) catalysing the reaction of S2O82−(aq) with I−(aq)
- CNO2(g) catalysing the oxidation of SO2(g) in the air
- DH+(aq) catalysing the iodination of propanone in aqueous solution
How does a catalyst affect a reversible reaction?
- Ait provides a route with a lower activation energy and leaves ΔH and the position of equilibrium unchanged
- Bit lowers ΔH so that the reaction becomes more exothermic
- Cit gives the reactant particles more kinetic energy
- Dit shifts the position of equilibrium towards the products
Fe2+ ions catalyse S2O82− + 2I− → 2SO42− + I2. Which equation is the first step of the catalysed route?
- AS2O82− + 2Fe2+ → 2SO42− + 2Fe3+
- B2Fe2+ + I2 → 2Fe3+ + 2I−
- CS2O82− + 2Fe3+ → 2SO42− + 2Fe4+
- D2Fe2+ + 2I− → 2Fe + I2
In what order do the stages of heterogeneous catalysis happen?
- Aadsorption of reactants, reaction on the surface, desorption of products
- Bdesorption of reactants, reaction, adsorption of products
- Creaction in the gas phase, adsorption of products, desorption
- Dadsorption of products, reaction, desorption of reactants
Which reaction removes two pollutants at once on the platinum and rhodium surface of a catalytic converter?
- A2CO + 2NO → 2CO2 + N2
- B2CO + O2 → 2CO2
- CN2 + O2 → 2NO
- D2SO2 + O2 → 2SO3
Use these E° values: S2O82−/SO42− +2.01 V; Fe3+/Fe2+ +0.77 V; I2/I− +0.54 V; V3+/V2+ −0.26 V; Cr3+/Cr2+ −0.41 V. Which ion could catalyse the reaction between S2O82− and I−?
- AFe2+, because its E° lies between those of the two reacting couples, so both steps are feasible
- BV2+, because it is a powerful reducing agent
- CCr2+, because the most negative E° makes the fastest catalyst
- DZn2+, because it is a d-block ion
Nickel is a good catalyst for hydrogenating alkenes, but tungsten and silver are poor ones. What is the best explanation?
- Atungsten adsorbs too strongly so products do not desorb, and silver adsorbs too weakly so bonds are not weakened; nickel is in between
- Btungsten and silver are not transition elements
- Ctungsten adsorbs too weakly and silver adsorbs too strongly
- Dstronger adsorption always gives a faster reaction, and nickel adsorbs most strongly
In the atmosphere, NO2 catalyses the oxidation of SO2: SO2 + NO2 → SO3 + NO, then 2NO + O2 → 2NO2. Which statement is correct?
- ANO2 is a homogeneous catalyst and NO is an intermediate
- BNO is the catalyst and NO2 is an intermediate
- CNO2 is a heterogeneous catalyst because it is a gas
- DNO2 is used up overall, so it is a reactant
Quiz key
In the Haber process the catalyst is solid and the reactants are gases, so it is heterogeneous: the gases adsorb onto the iron surface. The other three are homogeneous: Fe2+ and H+ are aqueous like their reactants, and NO2 is a gas like SO2 and O2.
The catalysed route has a lower Ea for both forward and reverse reactions, so equilibrium is reached faster but its position is unchanged. Reactant and product energies are the same, so ΔH is unchanged. A catalyst does not heat the particles; instead, more of them already have enough energy for the lower barrier. Equal speeding up of forward and reverse reactions means no shift in equilibrium.
Step 1: peroxodisulfate oxidises Fe2+ to Fe3+. Step 2: 2Fe3+ + 2I− → 2Fe2+ + I2, regenerating the catalyst. Fe2+ + I2 is the reverse of step 2 and does not happen. Fe4+ is not formed. Reducing Fe2+ to Fe metal with iodide is not feasible and would not regenerate the catalyst.
Reactants first adsorb onto active sites, which weakens their bonds and holds them close together; they react on the surface; then the products desorb, freeing the sites. Desorption cannot come first, as nothing is yet attached. The reaction happens on the surface, not in the gas phase. Products are formed on the surface, not adsorbed from outside.
Carbon monoxide is oxidised to CO2 and nitrogen monoxide is reduced to N2, both harmless; the gases adsorb onto the metal, react and desorb. CO + O2 does happen in the converter but removes only one pollutant. N2 + O2 is how NO forms in the engine, not how it is removed. SO2 oxidation is the Contact process, and SO3 is not wanted in exhaust.
Fe3+ (+0.77 V) is more positive than I2/I− (+0.54 V), so it oxidises iodide; Fe2+ is oxidised by S2O82− (+2.01 V). Both steps are feasible and the iron is recycled. V2+ and Cr2+ would be oxidised by S2O82−, but V3+ and Cr3+ (−0.26 V and −0.41 V) are too weak to oxidise I−, so the cycle stops after one step. Zn2+ has only one oxidation state, so it cannot shuttle electrons.
Good heterogeneous catalysts adsorb reactants strongly enough to weaken their bonds and hold them close, but weakly enough for the products to desorb and free the active sites. Tungsten binds so strongly that sites stay blocked; silver binds so weakly that few molecules are activated. Both tungsten and silver are transition elements. Swapping the two strengths reverses the facts. Stronger is not always better, because products must leave.
NO2 is used in step 1 and regenerated in step 2 (doubling step 1 and adding gives 2SO2 + O2 → 2SO3), so it is the catalyst, and it is in the same gas phase as the reactants: homogeneous. NO is made and then used up: an intermediate. Being a gas makes it homogeneous here, not heterogeneous. NO2 cancels from the overall equation, so it is not consumed.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.