Global warming and the greenhouse effect are topics that everyone feels they know well enough (because you’ve done them to death at GCSE), but you do need to be able to explain what is going on in terms of the chemistry …
Complete combustion of a hydrocarbon fuel produces carbon dioxide and water vapour, however the exhaust gases from the combustion of fossil fuels contain large amounts of carbon monoxide, unburnt hydrocarbons, carbon particulates and nitrogen oxides.
Why is this?
Carbon monoxide, unburnt hydrocarbons and carbon particulates are the result of the incomplete combustion of a fuel. It takes almost 13dm3 of air to completely combust just 1g of petrol, working on the basis that air is 21% oxygen by volume, so you can understand why even the most efficient internal combustion engines struggle to burn fuel completely.
Nitrogen oxides are formed when nitrogen in the air burns at the extremely high temperatures of a petrol engine. They are considered to be both primary pollutants (contributing to acid rain) and secondary pollutants (contributing to photochemical smog).
All of these gases are present in the troposphere where they are contributing to global warming, though it is important to recognise that the products of burning fossil fuels are not the only source of greenhouse gases on Earth.
You should start by watching this very clever little demonstration from Iain Stewart which shows what happens when the carbon dioxide levels in the atmosphere rise.
Understanding the chemistry behind the greenhouse effect
All hot bodies (by which we mean anything above absolute zero) emit electromagnetic radiation – the hotter the body, the higher the frequency and energy of that radiation.
The sun is an extremely hot body, radiating infrared, visible and ultraviolet light. It is mainly visible and uv radiation that passes through our atmosphere (a little is absorbed by gaseous molecules, the ozone layer etc) and is absorbed by the earth.
The earth is much cooler than the sun. It emits lower frequency, longer wavelength infrared radiation, mostly in the mid-IR range (3-50μm).
Remember that c = 𝛌𝛎 (where c is the speed of light) so frequency and wavelength have an inverse relationship.
Many of the gases in our atmosphere strongly absorb IR radiation in the range that the earth emits. When a molecule absorbs IR radiation, its vibrational energy increases – the bonds in the molecule vibrate more strongly and the molecule collides with neighbouring molecules. Vibrational energy is converted to kinetic energy and passed from one molecule to another, leading to warming of the atmosphere.
Now the vibrational energy levels in molecules are quantised (fixed like the rungs on a ladder) so a specific frequency of IR radiation must be absorbed for a molecule to move from a lower vibrational level to a higher one.
ΔE for one molecule, say carbon dioxide, will different to ΔE for another molecule, say water. So, all the different molecules in the troposphere are absorbing different specific frequencies of the IR radiation emitted by earth which means this radiation is not being lost to space – it is contributing to the greenhouse effect.
The ‘global warming potential’ of a gas depends on its abundance, how strongly it absorbs IR radiation, which frequencies it absorbs and its lifetime / stability in the atmosphere.
Practice questions
- CHCl3 is described as a ‘greenhouse gas’. Explain how these molecules are involved in the processes of energy transfer that start with UV radiation from the sun and result in warming of the troposphere.
- Describe the evidence for the relationship between the increased concentration of greenhouse gases and global warming.
- (a) The exhaust gases of car engines are a major source of carbon monoxide in the atmosphere. Explain why.
(b) Give two reasons why carbon monoxide is classes as a polluting gas.
- Scientists have found that the concentration of carbon dioxide in dry, unpolluted tropospheric air has increased from 300ppm in 1900 to 380ppm today.
(i) Calculate the percentage increase in carbon dioxide in the air over this time.
(ii) A sample of air was found to contain 1.20 x 10-5% carbon monoxide by volume. If the concentration of carbon dioxide in the same sample is taken to be 380ppm, how much more abundant is carbon dioxide than carbon monoxide in the sample?
- Hydrogen fuelled cars have been developed that have an internal combustion engine similar to that of petrol fuelled cars. No carbon dioxide is produced but nitrogen oxides are present in the exhaust gases.
(i) How are nitrogen oxides formed in a car engine?
(ii) In what two ways are nitrogen oxides considered pollutants?
(iii) Write an equation to show how carbon monoxide and nitrogen(II) oxide are removed from the exhaust gases of a petrol engine by a catalytic converter.
Answers
- The earth absorbs UV radiation emitted by the sun and and radiates infrared radiation. CHCl3 molecules absorb specific frequencies of the IR radiation which causes their bonds to vibrate more strongly. This vibrational energy is transferred to other molecules as kinetic energy which warms the troposphere / becomes thermal energy .
- There is a correlation between the models of increasing gas concentration and increasing temperature of the troposphere.
- (a) Incomplete combustion of hydrocarbon fuels / petrol leads to carbon monoxide being present in the exhaust gases.
(b) CO is toxic / poisonous and it is a primary pollutant contributing to the formation of photochemical smog.
- (i) (80/300) x 100 = 26.7%
(ii) 380ppm = 3.8 x 10-2% ; (3.8 x 10-2) / (1.2 x 10-5) = 3.17 x 103 times more CO2
- (a) Nitrogen from the air burns in oxygen in the very high temperatures of an engine
(b) Nitrogen oxides form acid rain and react in the presence of sunlight / uv light to form secondary pollutants that make up photochemical smog (breathing difficulties etc)
(c) NO(g) + CO(g) ⇾ CO2(g) + N2(g)