Combustion and pollution

In a nutshell

Burning fossil fuels releases pollution to the environment. Pollution refers to harmful substances in the environment, which can harm humans, animals and plants. Pollution causes many issues such as: acid rain, global warming and damage to wildlife.

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Equations

Below are the general equations you need to know: 

​$$hydrocarbon + oxygen \rightarrow carbon ~dioxide + water\\ \ \\​hydrocarbon + oxygen \rightarrow carbon ~monoxide + carbon~dioxide+water​\\ \ \\​hydrocarbon + oxygen \rightarrow carbon ~monoxide + soot +water$$​​

Complete combustion

Definition

During complete combustion, fossil fuels burn completely to produce carbon dioxide and water.  Fossil fuels are made of hydrocarbons. Hydrocarbons are compounds that contain carbon and hydrogen only.

Carbon dioxide in the atmosphere dissolves in oceans, forming carbonic acid (ocean acidification). Carbon dioxide is also a greenhouse gas, which contributes to global warming and climate change.

Complete combustion of a hydrocarbon: 

​$$hydrocarbon + oxygen \rightarrow carbon ~dioxide + water$$

Example

Complete combustion of ethane:

​$$ethane + oxygen \rightarrow carbon~ dioxide + water$$​​

​$$C_2H_6 + 3.5O_2 \rightarrow2CO_2 + 3H_2O$$​​

Greenhouse gas effect

Greenhouse gases (atmospheric gases that absorb infrared radiation) prevent thermal energy from escaping back to space, which causes the Earth's surface to become warmer. 

The greenhouse gas effect is required for life on Earth, as otherwise all heat would be re-emitted back to space and the Earth would be too cold for most species to survive. However, human activity has led to an unusual increase in the greenhouse gas effect, which has led to global warming and climate change. 

Example

Examples of greenhouse gases are: carbon dioxide ($$CO_2$$​), methane ($$CH_4$$​), water vapour ($$H_2O$$​) and nitrous oxide ($$N_2O$$​).

Incomplete combustion

Definition

During incomplete combustion, fossil fuels burn incompletely due to reduced oxygen availability. Incomplete combustion produces carbon monoxide, carbon dioxide, soot and water.

• Carbon monoxide is a poisonous gas, which prevents haemoglobin in red blood cells from carrying oxygen
• Soot is tiny, solid particles (particulate matter) of carbon suspended in the air
• Soot irritates the lungs, causing respiratory issues
• Soot contributes to global dimming
• Global dimming refers to the reduced amount of sunlight reaching the Earth's surface
  

Incomplete combustion of a hydrocarbon:

​$$​hydrocarbon + oxygen \rightarrow carbon ~monoxide + carbon~dioxide+water​$$​​

When there is a lower availability of oxygen, soot is produced too:

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$$​hydrocarbon + oxygen \rightarrow carbon ~monoxide + soot +water​$$​​​

Example

Incomplete combustion of ethane:

​$$ethane + oxygen \rightarrow carbon ~monoxide + carbon~dioxide +water$$​​

​$$C_2H_6 + 3O_2 \rightarrow CO + CO_2+3H_2O​$$

When there is a lower availability of oxygen, soot is produced too:

​$$ethane + oxygen \rightarrow carbon~monoxide + soot + water\\ \ \\C_2H_6 + 2O_2 \rightarrow CO + C+ 3H_2O$$​​

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Acid rain

Sulfur dioxide ($$SO_2$$​) is produced when sulfur ($$S$$​) impurities in fossil fuels are burned in the air with oxygen ($$O_2$$​).

​$$sulfur + oxygen \rightarrow sulfur ~dioxide$$

$$S(s) + O_2(g) \rightarrow​ SO_2(g)$$​ 

Sulfur dioxide is incorporated into the water cycle by reacting with water and oxygen in the air to form sulfuric acid ($$H_2SO_4$$​); this is a source of acid rain.

​$$sulfur ~dioxide + oxygen + water \rightarrow sulfuric~ acid​$$​​

​$$2SO_2 (g) + O_2 (g) + 2H_2O(l) \rightarrow 2H_2SO_4 (aq)$$​​

• Nitric oxide ($$NO$$​) and nitrogen dioxide ($$NO_2$$) are produced when nitrogen impurities in fossil fuels are burned with oxygen at very high temperatures, in a car engine for example.
• These oxides of nitrogen are released to the air, where they react with oxygen and water to produce nitric acid ($$HNO_3$$), causing acid rain.
• Acid rain impacts the environment in many ways, including harm to aquatic wildlife, crop pollution and erosion of buildings made of carbonate.
  
  

Note: $$NO$$​ and $$NO_2$$​ are oxides of nitrogen ($$NO_x$$).

Reducing pollution

Carbon monoxide 

Catalytic converters can be installed in car exhausts to convert toxic carbon monoxide into carbon dioxide. 

Sulfur dioxide 

In power stations, sulfur dioxide an be removed by limestone in a process known as desulfurisation. 

Oxides of nitrogen

Catalytic converters can be installed in car exhausts to convert oxides of nitrogen into nitrogen.

Soot

Pollution caused by soot can be reduced by designing more efficient engines.

Hydrogen fuel

Hydrogen fuel is more environmentally friendly than fossil fuels, as combustion of hydrogen produces water only:

​$$hydrogen + oxygen \rightarrow water$$​​

​$$2H_2(g) + O_2 (g)\rightarrow 2H_2O(l)$$

Petrol vs. hydrogen fuel

Petrol is derived from crude oil so it is an example of a fossil fuel.