Pollutants And Acid Rain

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 4.14 — Formation of oxides of nitrogen in car engines
• 4.15 — How combustion of sulfur impurities forms sulfur dioxide
• 4.16 — How sulfur dioxide and oxides of nitrogen contribute to acid rain

Specification section 4(c) Alkanes and (d) alkenes — also taught in Cracking, Alkanes, Alkenes.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Burning a fuel does not only produce the things the fuel is made of. Two of the most damaging products of a car engine come from somewhere else entirely, and keeping their origins straight is the whole of this lesson.

Sulfur dioxide — the pollutant that comes from the FUEL

1. Crude oil is not pure hydrocarbon. It contains small amounts of sulfur compounds as IMPURITIES.
2. When the fuel made from it is burned, the sulfur in those impurities burns too.
3. Sulfur burning in oxygen gives SULFUR DIOXIDE: S + O₂ → SO₂.
4. The amount produced therefore depends on how much sulfur the fuel contained.
5. This is why removing sulfur from fuel at the refinery works: take the sulfur out and the sulfur dioxide is not formed at all.

Oxides of nitrogen — the pollutant that comes from the AIR

1. Air is about four-fifths nitrogen and one-fifth oxygen, and at ordinary temperatures these two do not react with one another at all.
2. Inside a car engine, the burning fuel raises the temperature to well over a thousand degrees Celsius.
3. At that temperature the particles have enough energy for nitrogen and oxygen to react.
4. They combine to form OXIDES OF NITROGEN: N₂ + O₂ → 2NO, and the nitrogen monoxide is oxidised further in the air to nitrogen dioxide.
5. The nitrogen did NOT come from the petrol. It came from the air drawn into the engine.
6. So purifying the fuel does nothing about this one. The only ways to reduce it are to lower the temperature of combustion, or to treat the exhaust gases afterwards in a CATALYTIC CONVERTER, which converts the oxides back to harmless nitrogen.

Think of it like a bonfire on a windy day

Two different things can end up in the smoke. Some of it is what you put on the fire — damp wood makes one kind of smoke, a plastic wrapper makes another, and you control it by choosing what you burn. But the fire also scorches the branches of the tree above it, and nothing you put on the bonfire changes that: it is the HEAT reaching something that was never part of the fire. Sulfur dioxide is what you burned. Oxides of nitrogen are the scorched branches — the air itself reacting because the fire is hot enough to make it.

Both gases then do the same thing once they are in the atmosphere, and it is not the pollution people expect. They do not simply drift about; they dissolve.

How the two gases become acid rain

1. Sulfur dioxide and oxides of nitrogen rise into the atmosphere.
2. There they DISSOLVE in the water droplets that make up clouds.
3. They are oxidised in the process, forming SULFURIC ACID and NITRIC ACID in solution.
4. The droplets fall as rain that is significantly more acidic than natural rainwater.
5. Because the gases can travel hundreds of miles on the wind before they dissolve, the rain falls somewhere quite different from where the fuel was burned.

That last point is what made acid rain a political problem as well as a chemical one. Sulfur dioxide released in one country's power stations came down as acid rain over another country's forests and lakes, and the country receiving it had no way to stop it. It is a rare case where the chemistry of a reaction determines the shape of an international argument.

The damage itself takes four forms worth remembering. Acid rain lowers the pH of lakes and rivers, which kills fish and the organisms they feed on. It damages the leaves of trees and releases aluminium ions from soil that then poison their roots. It reacts with the calcium carbonate in limestone and marble, eating away buildings and statues. And it corrodes metal structures, particularly ironwork.

One honest qualification the specification expects you to be aware of: ordinary rain is already slightly acidic, at around pH 5.6, because carbon dioxide from the air dissolves in it. Acid rain is rain made considerably more acidic than that by sulfur dioxide and oxides of nitrogen, not rain that is acidic where it would otherwise be neutral.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Where each pollutant comes from — the distinction the marks are on

Sulfur dioxide

Formed when sulfur impurities present in the fuel are burned. SO₂.

Oxides of nitrogen

Formed when nitrogen and oxygen from the AIR react in the high temperature of an engine.

Acid rain

Rain made significantly more acidic than normal by dissolved sulfur dioxide and oxides of nitrogen.

Impurity

A substance present in small amounts alongside the main component, here the sulfur compounds in crude oil.

Catalytic converter

A device fitted in a vehicle's exhaust that converts oxides of nitrogen back to nitrogen. It is unchanged by the reaction.

S + O₂ → SO₂

Conditions: Sulfur impurities in the fuel burning during combustion. The sulfur came from the crude oil.

N₂ + O₂ → 2NO

Conditions: Only at the very high temperature inside an engine. Both elements came from the air, not from the fuel.

The two acid-rain pollutants side by side

From exhaust pipe to damaged lake, in order

1. Sulfur dioxide and oxides of nitrogen are released into the atmosphere.
2. The gases are carried by the wind, often for hundreds of miles.
3. They dissolve in the water droplets of clouds and are oxidised there.
4. Sulfuric acid and nitric acid form in the droplets.
5. The droplets fall as acid rain, lowering the pH of lakes, soils and rivers far from the source.

The four kinds of damage acid rain does

• Lowers the pH of lakes and rivers, killing fish and the organisms they feed on
• Damages the leaves of trees, and releases aluminium ions from soil that harm their roots
• Reacts with the calcium carbonate in limestone and marble, eroding buildings and statues
• Corrodes metal structures, particularly ironwork

Model answer [4 marks]

Explain why removing sulfur from petrol reduces acid rain, but has no effect on the oxides of nitrogen an engine produces. [4]

Sulfur dioxide is formed when sulfur present as an impurity in the fuel is burned, so if the sulfur is removed at the refinery there is nothing left to form it, and less sulfuric acid reaches the clouds. Oxides of nitrogen have a different origin: the nitrogen and the oxygen both come from the air drawn into the engine, and they react only because the temperature of combustion is high enough for them to do so. Since none of that nitrogen came from the petrol, purifying the petrol cannot reduce it; only lowering the combustion temperature or treating the exhaust gases will.

Model answer [3 marks]

Describe how sulfur dioxide released from a power station comes to damage a forest in another country. [3]

The sulfur dioxide is released into the atmosphere and carried by the wind, often for hundreds of miles, before anything happens to it. It then dissolves in the water droplets of clouds and is oxidised, forming sulfuric acid in solution. The droplets fall as acid rain over the forest, where the acid damages the leaves of the trees and releases aluminium ions from the soil that harm their roots — so the damage occurs a long way from where the fuel was burned.

Not this: The nitrogen in oxides of nitrogen comes from nitrogen compounds in the fuel.

This: It comes from the AIR. Air is about four-fifths nitrogen, and the engine simply gets hot enough for it to react with oxygen. This is why removing impurities from fuel does nothing about it, and why exhaust treatment is needed instead.

Mark-losing trap. Sulfur dioxide is from the FUEL; oxides of nitrogen are from the AIR. Every mark on this topic turns on that.

Mark-losing trap. The gases must DISSOLVE in cloud droplets to make acid. They are not themselves acid rain.

Mark-losing trap. Normal rain is already about pH 5.6 from dissolved carbon dioxide. Acid rain is much more acidic than that, not merely acidic.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — Where does the nitrogen come from that forms oxides of nitrogen in a car engine?
2. Grade 7 · Explain [2 marks] — Explain why sulfur dioxide is produced when petrol is burned, even though petrol is made of hydrocarbons.
3. Grade 8 · Describe [3 marks] — Describe how sulfur dioxide in the atmosphere becomes acid rain.
4. Grade 9 · Explain [4 marks] — Select every statement that belongs in a full explanation of why oxides of nitrogen form inside a car engine but not in the open air.
5. 9+ · Evaluate [6 marks] — A country bans high-sulfur fuel and finds sulfur dioxide emissions fall sharply, while its nitrogen dioxide levels barely change. Explain this result and suggest what else would be needed.

The people behind this science

Two ways into the same idea — the one who established what air is made of and that its gases act independently, and the one who showed a pollutant does its damage far from where it was released. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

John Dalton — the one who established what air is made of and that its gases act independently

This page turns on a fact about the atmosphere that had to be established before anyone could reason about engine emissions at all: air is a MIXTURE, roughly four parts nitrogen to one part oxygen, whose gases each behave as though the others were not there. That is Dalton's law of partial pressures, and it came out of fifty-seven years of daily weather observation. He is the right figure to ask why two gases can sit together in the same volume for years without reacting, and what has to change for them to combine.

• “What did fifty-seven years of weather records tell you about air?”
• “How can two gases share a space and take no notice of each other?”
• “What has to happen before nitrogen will react with anything?”
• “Is the air a compound or a mixture, and how would you show it?”
• “Does each gas in a mixture behave as if it were alone?”

Rachel Carson — the one who showed a pollutant does its damage far from where it was released

The hardest thing about acid rain is not its chemistry but its geography: the sulfur dioxide leaves a chimney in one country and the dead lake appears in another, years later, with no visible connection between the two. Carson's whole method in Silent Spring was building exactly that kind of argument — following a chemical from where it was released, through air, water and living things, to a harm nobody had connected to it, and making the case convincingly enough to change law. She is the right figure to ask how you prove a link that nobody can see happening.

• “How do you prove a chemical released here caused damage there?”
• “What makes a delayed or distant harm so hard to argue?”
• “How did you answer people who said the evidence was not conclusive?”
• “Why do pollutants concentrate in some places and not others?”
• “What convinces a government to act on invisible damage?”

Carry on through the course

• Previous lesson: Burning Fuels
• Next lesson: Cracking
• What Organic Chemistry Is — Two facts about carbon generate several million compounds — and three words keep them in order.
• Naming, Drawing And Classifying — A name that tells you the structure, a method that finds every isomer, and three words that cover every organic reaction here.
• Crude Oil And Its Fractions — One mixture, separated by boiling point into six useful materials — and one causal chain that explains every trend down the column.
• Burning Fuels — Two ways a hydrocarbon can burn, and why one of them fills a room with a gas you cannot smell.
• Cracking — Breaking long molecules nobody wants into short ones everybody does — and the reason a refinery has to.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification