The shape of the lesson
By the end of the lesson, learners can
- Making and removing NO x: Run the engine model in the simulator at different temperatures, then fit a catalytic converter and watch the exhaust composition change.
- Smog and eutrophication: Nitrogen compounds cause two further problems: one in city air, one in rivers and lakes.
- Engine: N2 + O2 → 2NO.
- Converter: 2CO + 2NO → 2CO2 + N2.
- NO2 catalyses SO2 → SO3.
- Eutrophication: algae bloom, decay, oxygen loss.
One picture every idea returns to
At the very high temperatures inside an engine (or in lightning), nitrogen and oxygen react: N2 + O2 → 2NO. In air NO is oxidised to NO2. Catalytic converters remove NO and CO together: 2CO + 2NO → 2CO2 + N2. In the atmosphere NO2 acts as a catalyst for the oxidation of SO2 to SO3, and NO2 also forms nitric acid, so both add to acid rain. NO and unburnt hydrocarbons in sunlight form photochemical smog containing peroxyacetyl nitrate (PAN). Nitrate fertilisers washed into water cause eutrophication.
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, Born in the engine.. Engines run hot enough to make nitrogen and oxygen react. 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: Making and removing NO x | Run the engine model in the simulator at different temperatures, then fit a catalytic converter and watch the exhaust composition change. Project tab 3 of the lab (Nitrogen and sulfur lab, nitrogen oxides) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Smog and eutrophication | Nitrogen compounds cause two further problems: one in city air, one in rivers and lakes. | 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, 12 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 Smog and eutrophication, 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 |
|---|---|
| In the oxidation of SO 2, NO 2 is a catalyst: it is used in the first step and regenerated in the second, so it is not used up overall. | A catalytic converter removes CO, NO and hydrocarbons; it produces CO 2, it does not remove it. |
Nitrogen oxides and pollution: worksheet
6 questions, 12 marks. Written in the style of the papers; no past paper question is reproduced.
Explain why NO forms in a car engine but not in the atmosphere at ordinary temperatures.
Write equations to show how NO2 catalyses the oxidation of SO2 in the atmosphere.
Use oxidation numbers to show that the reaction 2CO + 2NO → 2CO2 + N2 is a redox reaction.
Describe how the use of nitrate fertilisers can lead to the death of fish in a lake.
Name the substances that react to form photochemical smog and one harmful product.
Write an equation for the formation of nitric acid from NO2, oxygen and water.
Answer key and marking notes
- The N≡N bond is very strong, so the reaction N2 + O2 → 2NO has a very high activation energy.
- Only the very high temperatures in the engine (or lightning) provide enough energy.
- SO2 + NO2 → SO3 + NO.
- NO + ½O2 → NO2 (NO2 regenerated).
- C: +2 in CO to +4 in CO2 (oxidised).
- N: +2 in NO to 0 in N2 (reduced).
- Nitrates are washed (leached) from fields into the lake.
- Algae grow rapidly and block light, so plants below die.
- Bacteria decomposing the dead material use up dissolved oxygen, so fish suffocate.
- Nitrogen oxides and unburnt hydrocarbons, in sunlight.
- Products include peroxyacetyl nitrate (PAN) and ozone.
4NO2 + O2 + 2H2O → 4HNO3.
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.
How is nitrogen monoxide, NO, formed in a car engine?
- Anitrogen and oxygen from the air react at the very high temperature of the engine: N2 + O2 → 2NO
- Bnitrogen compounds in the petrol burn: the fuel is the main source of the nitrogen
- Cnitrogen reacts with carbon dioxide in the exhaust: N2 + 2CO2 → 2NO + 2CO
- Dnitrogen reacts readily with oxygen at room temperature, so NO forms as soon as air enters the engine
A catalytic converter removes two pollutants from car exhaust in one reaction. Which equation is correct?
- A2CO + 2NO → 2CO2 + N2
- B2CO2 + 2NO → 2CO + N2 + 2O2
- C2CO + O2 → 2CO2, which removes CO2 from the exhaust
- D2NO + O2 → 2NO2
The diagram shows how NO2 in the atmosphere takes part in the oxidation of SO2. What is the role of NO2?
- Ait is a catalyst: NO2 + SO2 → NO + SO3, then NO + ½O2 → NO2, so it is regenerated
- Bit is a reactant that is used up, because it is converted into NO
- Cit is a reducing agent that turns SO3 back into SO2
- Dit is a product of the reaction between SO2 and oxygen
Nitrogen dioxide contributes to acid rain. Which equation shows how it forms nitric acid in moist air?
- A4NO2 + O2 + 2H2O → 4HNO3
- BNO2 + H2O → HNO3 + H2
- C2NO2 + H2O → 2HNO3
- DNO2 + CO2 + H2O → HNO3 + H2CO3
Peroxyacetyl nitrate (PAN) is a harmful component of photochemical smog. Which three things are needed for it to form?
- Aoxides of nitrogen, unburnt hydrocarbons and sunlight
- Bsulfur dioxide, carbon dioxide and rain
- Ccarbon monoxide, nitrogen gas and darkness
- Dammonia, nitrates from fertilisers and water
For the step NO2 + SO2 → NO + SO3, which statement about oxidation numbers is correct?
- AN goes from +4 to +2 (reduced) and S goes from +4 to +6 (oxidised)
- BN goes from +2 to +4 (oxidised) and S goes from +6 to +4 (reduced)
- CN goes from +4 to +2 (reduced) and S goes from +2 to +3 (oxidised)
- Dno oxidation numbers change, because oxygen is only transferred
Nitrate fertilisers wash off farmland into a lake. Which sequence describes the eutrophication that follows?
- Anitrates promote rapid growth of algae; the algae block light, so plants below die; bacteria decomposing the dead matter use up dissolved oxygen, so fish die
- Bnitrates poison the fish directly, then the algae grow on the dead fish
- Cnitrates make the water strongly acidic, which dissolves the algae and kills the fish
- Dnitrates react with dissolved oxygen to form NO2, removing the oxygen the fish need
2.30 g of NO2 dissolves in rainwater with excess oxygen: 4NO2 + O2 + 2H2O → 4HNO3. What mass of nitric acid forms? (Mr NO2 = 46.0, HNO3 = 63.0)
- A3.15 g
- B0.788 g
- C1.58 g
- D12.6 g
Quiz key
Air drawn into the engine is about 78% N2. The temperature of the burning fuel is high enough to supply the large activation energy needed to break the N≡N bond, so N2 + O2 → 2NO; lightning causes the same reaction naturally. Petrol contains almost no nitrogen, so the nitrogen comes from the air, not the fuel. CO2 is not the oxidising agent; the oxygen comes from air. N2 is very unreactive and does not combine with oxygen at room temperature; that is why air is a stable mixture.
On the hot platinum/rhodium surface, CO is oxidised (C +2 to +4) and NO is reduced (N +2 to 0): 2CO + 2NO → 2CO2 + N2. Both products are naturally present in air. A catalytic converter does not remove CO2; it produces it, so 'removes CO2' is the classic error. Oxidising CO with O2 does happen in some converters, but it makes CO2 rather than removing it, and it does not remove NO. Converting NO into NO2 would make the pollution worse, not remove it.
In step 1, NO2 oxidises SO2 to SO3 and is itself reduced to NO. In step 2, NO is oxidised by oxygen in the air back to NO2. Adding the steps gives SO2 + ½O2 → SO3: NO2 appears on both sides and is regenerated, so it acts as a catalyst. Saying it is used up is the classic error; it is only temporarily converted to NO. NO2 is the oxidising agent, not a reducing agent, and it oxidises SO2 rather than reducing SO3. It is not formed from SO2 and O2, which contain no nitrogen.
With oxygen and water, NO2 (N +4) is oxidised to nitric acid (N +5): 4NO2 + O2 + 2H2O → 4HNO3. (Without oxygen it disproportionates: 2NO2 + H2O → HNO2 + HNO3.) Hydrogen gas is not formed, and that equation is unbalanced in hydrogen (2 H on the left, 3 on the right). 2NO2 + H2O → 2HNO3 is unbalanced (5 O on the left, 6 on the right) and has nothing to accept the electrons lost by N. CO2 is not the cause of acid rain and does not react with NO2.
Photochemical smog forms in sunny, traffic-heavy cities. Sunlight drives reactions between nitrogen oxides (NO, NO2) and unburnt hydrocarbons from vehicle exhaust, producing PAN, which irritates the eyes and lungs and damages plants. Sulfur dioxide, carbon dioxide and rain relate to acid rain and the greenhouse effect, not PAN. Unreactive N2 is not the nitrogen source, and the reactions need light, not darkness. Ammonia and fertiliser nitrates are linked to eutrophication of water, not smog.
In NO2, N = +4 (2 × −2 = −4); in NO, N = +2, so nitrogen is reduced. In SO2, S = +4; in SO3, S = +6, so sulfur is oxidised. The two-unit changes balance. The second option reverses both changes. The third option halves the oxidation numbers of sulfur, as if each oxygen were −1. Oxygen transfer from N to S is exactly what changes the oxidation numbers, so this is a redox reaction.
Nitrates are plant nutrients, so they cause an algal bloom. The algae cover the surface and block light, so plants below cannot photosynthesise and die. Aerobic bacteria decompose the dead material and use up the dissolved oxygen, so fish and other aquatic animals suffocate. Fish are killed by lack of oxygen, not by direct nitrate poisoning, and algal growth comes first. Nitrate salts such as NH4NO3 give near-neutral solutions; eutrophication is not an acidity effect. Nitrate ions do not react with dissolved oxygen; N is already in its highest state (+5).
n(NO2) = 2.30 / 46.0 = 0.0500 mol. The ratio NO2 : HNO3 is 4 : 4 = 1 : 1, so n(HNO3) = 0.0500 mol and the mass = 0.0500 × 63.0 = 3.15 g. 0.788 g divides by 4, misreading the 4 as applying to NO2 only. 1.58 g uses the oxygen-free equation 2NO2 + H2O → HNO2 + HNO3, where only half the N becomes HNO3; here oxygen is in excess. 12.6 g multiplies by 4 instead of using the 1 : 1 ratio.
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.