AP Environmental Science

## Learning Outcomes

- Understand the difference between complete and incomplete combustion of fossil fuels
- Identify the major combustion products of coal, oil, and natural gas
- Explain how sulfur dioxide and nitrogen oxides lead to acid deposition
- Distinguish primary pollutants from secondary pollutants such as ground-level ozone and acids
- Compare the main pollution-control technologies used for fossil fuel emissions
- Apply combustion chemistry and emissions data to AP Environmental Science questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand fossil fuel combustion products, with a focus on the following syllabus points:

- Know that complete combustion of hydrocarbons produces mainly carbon dioxide and water
- Identify incomplete combustion as a source of carbon monoxide and soot or particulate matter
- Explain how sulfur in coal and oil produces sulfur dioxide during combustion
- Understand that nitrogen oxides form during high-temperature combustion and contribute to both acid deposition and photochemical smog
- Distinguish primary pollutants such as CO, SO2, NOx, and PM from secondary pollutants such as ozone, sulfuric acid, and nitric acid
- Know that acid deposition includes both wet deposition and dry deposition
- Identify fly ash, bottom ash, boiler slag, and flue-gas desulfurization material as coal combustion residuals
- Compare scrubbers, electrostatic precipitators, and baghouse filters as emission-control methods

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What products are expected from complete combustion of methane?
2. Why does incomplete combustion produce carbon monoxide instead of only carbon dioxide?
3. How do sulfur dioxide and nitrogen oxides contribute to acid deposition?
4. What is the difference between a primary pollutant and a secondary pollutant?
5. Which device removes sulfur dioxide from smokestack gases, and which devices remove particulates?

## Introduction

Burning fossil fuels gives humans a large share of the world's energy, but the useful energy released is only part of the story. The AP Environmental Science course expects you to connect the chemistry of combustion to real environmental effects: climate change, smog, acid deposition, and human health damage.

This topic often appears in questions that move from simple recall to cause-and-effect chains. You may be asked to identify a pollutant, explain how it forms, describe its impact, and suggest a control method. That means you need more than a list of emissions. You need to understand why different fuels produce different pollutants, and how those pollutants behave after release.

> **Key Term: combustion**
> The process of burning a fuel in oxygen, releasing energy and producing new chemical substances.

### Complete and incomplete combustion

The starting point is the standard combustion reaction. Fossil fuels are made mostly of **hydrocarbons**, compounds containing carbon and hydrogen. When a hydrocarbon burns with enough oxygen, the main products are carbon dioxide, water, and energy.

> **Key Term: complete combustion**
> Combustion in which a fuel burns with sufficient oxygen, producing mainly carbon dioxide (CO2) and water (H2O).

A useful APES example is methane, the main component of natural gas:

$$
\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}
$$

Another example is octane, a major gasoline component:

$$
\mathrm{2C_8H_{18} + 25O_2 \rightarrow 16CO_2 + 18H_2O}
$$

These equations matter because they show that **CO2 is an unavoidable product of hydrocarbon combustion**. Even when a fossil fuel burns “cleanly,” it still releases carbon dioxide, which contributes to the greenhouse effect.

Incomplete combustion happens when oxygen supply is limited or mixing is poor. Then the fuel does not fully oxidize. Instead of producing only CO2, some carbon ends up as carbon monoxide or elemental carbon particles.

> **Key Term: incomplete combustion**
> Combustion in which oxygen is limited, producing carbon monoxide (CO) and often soot or particulate matter as well as some CO2 and H2O.

Why does this happen? Carbon in fuel “wants” to combine with oxygen, but if too little oxygen is available, it may only combine with one oxygen atom instead of two. That creates carbon monoxide, CO, rather than carbon dioxide, CO2. If oxidation is even less complete, some carbon remains as soot.

This distinction is a common exam trap. Students often memorize “burning fossil fuels makes carbon dioxide” and then forget that APES also tests the less complete pathway.

> **Test Tip:** If a question mentions limited oxygen, enclosed combustion, poor engine tuning, or smoky flames, think incomplete combustion and look for **CO** and **particulates**, not just CO2.

### Major combustion products and why fuel type matters

The main combustion products emphasized in AP Environmental Science are carbon dioxide, carbon monoxide, sulfur dioxide, nitrogen oxides, and particulate matter. Water vapor is also a normal combustion product, but exam questions usually focus on the pollutants with environmental or health impacts.

> **Key Term: carbon dioxide (CO2)**
> A greenhouse gas produced during complete combustion of carbon-based fuels.

CO2 is the main gas linking fossil fuel use to global warming. Coal, oil, and natural gas all release it, but not equally. On an energy basis, coal releases the most CO2, petroleum is intermediate, and natural gas releases the least. A useful comparison is:

- **Coal:** 211.63 lb CO2 per million Btu
- **Petroleum products:** about 159–168 lb CO2 per million Btu
- **Natural gas:** 116.65 lb CO2 per million Btu

This ranking matters because APES questions often ask which fuel has the lowest carbon emissions per unit of energy. The answer is usually natural gas, not because it is nonpolluting, but because it contains more hydrogen relative to carbon and burns more efficiently.

> **Key Term: carbon monoxide (CO)**
> A colorless, odorless gas produced by incomplete combustion that reduces the blood's oxygen-carrying capacity.

CO is especially dangerous because it binds strongly to hemoglobin. This reduces the ability of blood to transport oxygen. In environmental questions, CO is mainly associated with vehicle exhaust and poorly ventilated combustion.

> **Key Term: sulfur dioxide (SO2)**
> A primary pollutant produced mainly by burning sulfur-containing coal and oil.

Coal often contains much more sulfur than natural gas. That is why coal combustion is strongly linked to sulfur dioxide emissions, industrial smog, and acid deposition. Natural gas generally produces much less SO2 because methane contains no sulfur in its basic formula.

> **Key Term: nitrogen oxides (NOx)**
> A group of nitrogen-oxygen gases, mainly NO and NO2, formed during high-temperature combustion.

A common mistake is thinking NOx comes only from nitrogen in the fuel. In fact, much NOx forms because high temperatures allow nitrogen gas and oxygen gas from the air to react. That means any very hot combustion process, including car engines and power plants, can form NOx.

> **Key Term: particulate matter (PM)**
> Tiny solid particles and liquid droplets, including soot, smoke, ash, and dust, that can damage respiratory and cardiovascular health.

Particulates can be directly emitted, such as soot from incomplete combustion and fly ash from coal plants. Some particulates also form secondarily in the atmosphere from sulfur and nitrogen compounds. That is why PM can be both a primary and a secondary pollutant depending on the context.

Coal generally produces more particulates, ash, sulfur emissions, and CO2 than natural gas. This is why APES often treats coal as the clearest example of a “dirty” fossil fuel.

### From emissions to environmental effects

Knowing the pollutants is not enough. You must understand what happens next in the atmosphere.

First, classify them correctly.

> **Key Term: primary pollutant**
> A pollutant emitted directly from a source into the atmosphere.

Primary pollutants in this topic include:

- carbon monoxide
- sulfur dioxide
- nitrogen oxides
- much particulate matter
- some volatile organic compounds, when present
  > **Key Term: secondary pollutant**
  > A pollutant formed in the atmosphere by reactions involving primary pollutants.

Secondary pollutants in this topic include:

- ground-level ozone
- sulfuric acid
- nitric acid
- some sulfate and nitrate particulates
- photochemical smog

This distinction is a high-frequency APES question. Ozone is not emitted directly by cars or smokestacks. It forms later.

> **Key Term: ground-level ozone (O3)**
> A secondary pollutant formed when NOx and VOCs react in the presence of sunlight.

The basic APES chain is:

fossil fuel combustion → NOx released  
NOx + VOCs + sunlight → ozone and photochemical smog

That means NOx contributes to two different problems students often mix up:

1. **photochemical smog and ozone formation**
2. **acid deposition**

For ozone, sunlight matters. For acid deposition, atmospheric oxidation and water matter.

> **Key Term: acid deposition**
> The transfer of acidic substances from the atmosphere to Earth as wet deposition (rain, snow, fog) or dry deposition (acidic gases and particles).

Sulfur dioxide leads to sulfuric acid through this pathway:

$$
\mathrm{S + O_2 \rightarrow SO_2}
$$

$$
\mathrm{2SO_2 + O_2 \rightarrow 2SO_3}
$$

$$
\mathrm{SO_3 + H_2O \rightarrow H_2SO_4}
$$

Nitrogen oxides lead to nitric acid through this pathway:

$$
\mathrm{N_2 + O_2 \rightarrow 2NO}
$$

$$
\mathrm{2NO + O_2 \rightarrow 2NO_2}
$$

$$
\mathrm{4NO_2 + O_2 + 2H_2O \rightarrow 4HNO_3}
$$

These reaction chains explain why coal combustion is a classic acid deposition question. Sulfur in the fuel becomes SO2, then sulfuric acid. The environmental impacts include lower lake pH, mobilization of toxic metals like aluminum, forest damage, and weathering of limestone buildings.

Particulate matter causes a different set of effects. Fine particles, especially **PM2.5**, can enter deep into the lungs and bloodstream. That leads to respiratory illness, cardiovascular damage, and reduced visibility. Particulates are not the main direct cause of acid formation, which is another point students confuse.

> **Test Tip:** If a question asks about reduced visibility or lung damage, think **particulates**. If it asks about lower pH in rain, lakes, or soils, think **SO2 and NOx**.

### Coal combustion residues and pollution control

APES also expects you to separate gases from solid combustion residues.

> **Key Term: fly ash**
> Fine ash particles carried upward with flue gas during coal combustion.

Fly ash is not a gas. It is a solid particulate residue. Because it rises with the flue gas, it must be captured by pollution-control equipment.

Other coal combustion residuals include:

- **bottom ash**: heavier ash that settles at the bottom of the boiler
- **boiler slag**: molten bottom ash that cools into hard pellets
- **flue-gas desulfurization material**: residue produced by sulfur-removal systems

This is a useful distinction because AP questions may ask which material is airborne and which remains in the furnace.

Now compare the major control technologies.

> **Key Term: scrubber**
> A pollution-control device, also called flue-gas desulfurization, that removes acidic gases such as sulfur dioxide from smokestack emissions.

Scrubbers usually use alkaline materials such as limestone. The sulfur dioxide reacts and is removed before it enters the atmosphere.

> **Key Term: electrostatic precipitator**
> A device that removes fine particulate matter from flue gas by using an electric charge.
>
> **Key Term: baghouse filter**
> A fabric-filter system that removes particulates from emissions.

The main distinction is simple:

- **scrubbers remove gases**, especially SO2
- **electrostatic precipitators and baghouse filters remove particulates**, such as fly ash

Students often swap these roles. That costs easy marks.

Coal plants may also reduce NOx by altering combustion conditions or by using selective catalytic reduction, though the APES core idea is mainly that high-temperature combustion forms NOx and that controls are needed.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Complete combustion of fossil fuels produces mainly carbon dioxide, water, and energy
- Incomplete combustion produces carbon monoxide and often soot or particulate matter
- Carbon dioxide is a major greenhouse gas and is emitted by all fossil fuels during combustion
- Coal generally emits more CO2, SO2, particulates, and ash per unit energy than natural gas
- Sulfur dioxide forms mainly from burning sulfur-containing coal and oil
- Nitrogen oxides form during high-temperature combustion when nitrogen and oxygen in air react
- SO2 and NOx are primary pollutants that can lead to acid deposition
- Ground-level ozone is a secondary pollutant formed from NOx, VOCs, and sunlight
- Acid deposition includes both wet deposition and dry deposition
- Fly ash is a particulate coal combustion residue carried with flue gas
- Scrubbers remove sulfur dioxide, while electrostatic precipitators and baghouse filters remove particulates
- Particulate matter is strongly linked to respiratory and cardiovascular harm and reduced visibility

## Key Terms and Concepts

- combustion
- complete combustion
- incomplete combustion
- carbon dioxide (CO2)
- carbon monoxide (CO)
- sulfur dioxide (SO2)
- nitrogen oxides (NOx)
- particulate matter (PM)
- primary pollutant
- secondary pollutant
- ground-level ozone (O3)
- acid deposition
- fly ash
- scrubber
- electrostatic precipitator
- baghouse filter


Fossil Fuels - Fossil fuel combustion products