Polluting the Atmosphere | Chemistry for Grade 10 PDF Download

The greenhouse effect

Without greenhouse gases in its atmosphere, the Earth would be about 18°C colder on average than it is now. That would make it too cold to support life as we know it.
Greenhouse gases present in the atmosphere include:

• water vapour
• carbon dioxide
• methane

These greenhouse gases:

1. absorb heat radiated from the Earth
2. then release energy in all directions, which keeps the Earth warm

The diagram gives more details about this process, called the greenhouse effect.
How the greenhouse effect works

1. Electromagnetic radiation at most wavelengths passes through the Earth's atmosphere.
2. The Earth absorbs most of the radiation and warms up.
3. The Earth radiates energy as infrared radiation.
4. Some of the infrared radiation goes into space.
5. Some of the infrared radiation is absorbed by greenhouse gases in the atmosphere.
6. The lower atmosphere warms up.

Example: Name three greenhouse gases.

Carbon dioxide, methane and water vapour.

Human activities and the greenhouse effect

Human activities are increasing the amount of some greenhouse gases in the atmosphere. For example:

• farming cattle releases methane
• farming rice in paddy fields releases methane
• burning fossil fuels in vehicles and power stations releases carbon dioxide
• deforestation releases carbon dioxide and reduces the absorption of carbon dioxide through photosynthesis
  Methane is released by rice paddy fields

Are humans causing global warming?

• Not everyone in the world agrees that humans are causing global warming by causing the release of greenhouse gases into the atmosphere. Some people believe that the current and ongoing rise in global temperatures is being caused by natural factors and cycles of climate change.
• However, the vast majority of scientists do believe that humans are responsible for the increase in greenhouse gases and therefore global warming. This is because the majority of evidence in peer-reviewed journals supports the theory that human activities are causing an increase in greenhouse gases and this is causing global warming.
• However, climate science is complicated and it is difficult to predict and explain what will happen to global temperatures in the future, so scientists cannot be certain about global warming.
• The media (eg websites, TV news, newspapers) may sometimes present opinions and articles which are simplified, inaccurate, based on only some of the evidence or biased. It is important for new evidence to be shared with as many people as possible, so that other scientists can check the results and interpretation, and repeat the experiments for themselves.

When evaluating the quality of evidence on an issue like global warming, you should consider:

• who did the research and whether they are trustworthy, skilled and experienced
• who funded the research, because that might cause some bias
• what methods were used to collect and analyse the data, because that might cause uncertainties in the evidence base
• which organisation is reporting or publishing the evidence

Global climate change

Climate versus weather

Climate is different from weather because climate refers to the average temperature and cycles of weather over long periods of time - decades at least. You might talk about the weather being windy last week, or hotter last year than the year before. But unless you compare data for many years you cannot make a judgment about whether the climate is changing.

Climate change
The Earth’s climate has been constantly changing since the Earth was formed 4.6 billion years ago. This includes several ice ages and periods of much warmer global temperatures. Until 200 years ago, these changes were all caused by natural changes such as volcanic eruptions and changes in the energy that reaches the Earth from the Sun.

Global warming
Global warming is not the same as climate change. Global warming is usually used to describe the warming of the climate in the past 200 years, which the vast majority of scientists are almost certain has been caused by human activities.
The Earth’s average temperature has gradually increased over time

The correlation between carbon dioxide and global warming
One of the commonly used pieces of evidence that humans are causing global warming is that there is a strong correlation between the increase in global carbon dioxide levels caused by human activities and the increase in global temperatures over the same timescale. Compare the following graph to the one above.
The concentration of carbon dioxide in the atmosphere passed 0.04% in 2016

The effects of global warming

The effects of global warming include:

• glaciers and polar ice melting
• sea levels rising
• patterns of rainfall changing, producing floods or droughts
• habitats changing

We are already experiencing some of these effects. The consequences of global warming will affect billions of people, all around the world.

Example: Suggest two reasons why climate change leads to rising sea levels.

Water from melting glaciers and polar ice enters the oceans. As the Earth’s temperature increases, seawater warms up and expands.

The carbon footprint and its reduction

The carbon footprint of a product is the total amount of carbon dioxide (and other greenhouse gases) which are emitted over the full life cycle of a product, service or event.
For example, the carbon footprint of a car would include the carbon dioxide emitted by:

• machinery whilst the metal is extracted from the Earth's crust and processed
• power stations which generate the electricity used to manufacture the car
• the car itself when it is driven for many years
• the machinery which is used to dispose of or recycle the car

The carbon footprint of a person during one year would consist of the total amount of carbon dioxide released into the atmosphere by all of the activities they take part in, and the manufacture, use and disposal of all the products and resources they use. For example, a typical young person might:

• use electricity at home, probably generated from fossil fuels, for lighting their room and using electronic devices
• use a gas-powered boiler at home (which produces carbon dioxide) for heating and hot water
• travel to school by bus or car (which probably runs on petrol or diesel)
• eat beef or rice which have been farmed using methods that release methane
• travel abroad once per year on an aeroplane (which produces carbon dioxide)

Example: Suggest and explain three ways that someone living in the UK could reduce their carbon footprint.

1. Fit photovoltaic solar panels onto the roof of their house, which will reduce the amount of electricity they use which has been generated from fossil fuels.
2. Commute to work by bicycle instead of driving.
3. Improve their home insulation (eg loft insulation or double glazing) which will reduce the amount of gas burned in the boiler.

It can sometimes be difficult for individuals to make all the changes that they would like to do to reduce their carbon footprint. For example, it might be too expensive to fit photovoltaic cells on the roof of your house, and you might work too far from your house to be able to cycle.

Combustion of hydrocarbon fuels

• Most fuels contain carbon and/or hydrogen. When hydrogen atoms are present in a fuel, they are oxidised to water. Water is not a pollutant.
• When there are lots of oxygen atoms present during combustion, the carbon atoms are completely oxidised to carbon dioxide. When there are fewer oxygen atoms present, carbon monoxide or carbon may be produced during combustion.

Complete combustion

Complete combustion of a hydrocarbon fuel occurs when there is a good supply of oxygen. It releases the maximum amount of energy and produces carbon dioxide and water.
To construct a symbol equation for the complete combustion of a hydrocarbon fuel, remember that the fuel reacts with O₂ and the only products are CO₂ and H₂O. For example, using methane (CH₄) as the fuel:
methane + oxygen → carbon dioxide + water
CH₄ + O₂ → CO₂ + H₂O
When this equation is balanced, you get:
CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete combustion

Incomplete combustion of a hydrocarbon fuel occurs when there is a poor supply of oxygen. Less energy is released. Water is still produced from the hydrogen atoms. Instead of carbon dioxide, you might get carbon monoxide or particulate carbon, known commonly as soot, or a mixture of both.
To construct a symbol equation for the incomplete combustion of a fuel, you need to read the question to see whether carbon monoxide (CO) or carbon (C) is produced. For example, using ethane (C2H6) as a fuel, and producing CO as one of the products:
ethane + oxygen → carbon monoxide + water
C₂H₆ + O₂ → CO + H₂O
When this equation is balanced, you get:
2C₂H₆ + 5O₂ → 4CO + 6H₂O

Problems with incomplete combustion

• Carbon monoxide is a poisonous gas which binds to haemoglobin in your red blood cells, preventing them from carrying oxygen to the cells in your body. Carbon monoxide is colourless and has no smell, so it is very difficult to tell if you are breathing it in. It can be detected by electronic detectors which are often fitted near to boilers.
• Particulate carbon can cause health problems for humans because it irritates the lining of the lungs, can make asthma worse, and perhaps even cause cancer. Particulate carbon can also cause global dimming, which may reduce rainfall.

Atmospheric pollutants

When fuels are burned, a number of atmospheric pollutants are produced.
Here are some of the pollutants from burning fuels, and where they come from:

Sulfur dioxide as a pollutant

Sulfur dioxide is caused when sulfur atoms which are present in some fossil fuels are oxidised:

sulfur + oxygen → sulfur dioxide
S + O₂ → SO₂
Sulfur dioxide is then further oxidised in the atmosphere to sulfur trioxide, SO3. This gas dissolves in rainwater to make acid rain, which is a dilute solution of sulfuric acid, H2SO4.
The two reactions are:
sulfur dioxide + oxygen → sulfur trioxide
2SO₂ + O₂ → 2SO₃
And then:
sulfur trioxide + water → sulfuric acid
SO₃ + H₂O → H₂SO₄
Acid rain harms and kills plants and animals, especially those that live in aquatic environments. It can also damage man-made objects like statues and buildings.

NO_x gases as pollutants

• Nitrogen is not present in fuels, but the high temperatures and pressures inside a car engine can cause the nitrogen and oxygen in the air to react together to make oxides of nitrogen.
• There are several compounds formed when nitrogen bonds with oxygen, but the two which are made inside engines are NO and NO₂. These two pollutants are grouped together with the general formula NO_x.
• NO_x gases can cause acid rain, and they also react in the atmosphere with other pollutants to make photochemical smog. Smog can have major health effects, causing asthma attacks and even death.

Example: Diesel sometimes contains sulfur impurities. List the pollutants which could be produced when it is burned incompletely inside a car engine.

Carbon monoxide, particulate carbon, sulfur dioxide and nitrogen oxides.