earths early atmosphere
Composition of the early atmosphere
The early atmosphere probably contained:
a large amount of carbon dioxide
water vapour
small amounts of other gases
As the Earth began to cool, water vapour condensed formed the oceans. This process can be explained by the particle model.
Formation of the modern atmosphere
Over time:
the oxygen levels increased
the carbon dioxide levels decreased
Earth's oxygen levels
Oxygen levels are generally thought to have increased dramatically about 2.3 billion years ago. Photosynthesis by ancient bacteria may have produced oxygen before this time. However, the oxygen reacted with iron and other substances on Earth, so oxygen levels did not rise to begin with. Oxygen levels could only begin to rise when these substances had been oxidised. In addition, early plants and algae began to release oxygen at a faster rate. Oxygen levels then showed a dramatic increase.
Why did carbon dioxide levels decrease?
Carbon dioxide levels decreased because of processes that included:
dissolving in the oceans
use by plants for photosynthesis
formation of fossil fuels as plants died and their carbon compounds became locked up underground
formation of sedimentary rocks from the shells of ancient sea creatures
Types of evidence for the composition of the early atmosphere
Scientists cannot be sure about the composition of the early atmosphere. No measurements can be made, so scientists must analyse indirect evidence from other sources.
Type of Evidence | What it tells scientists |
Analysis of gases from modern volcanoes | Carbon dioxide and water vapour are released by volcanoes today. This means they are likely to have made up most of the early atmosphere, along with other volcanic gases. |
Chemical composition of ancient rocks | Iron sulfide can only exist in rocks that were formed before there was oxygen in the atmosphere. Rocks containing iron oxide can only form if oxygen is present. Dating these rocks gives scientists evidence of when oxygen first built up in the atmosphere. |
Fossils | Life processes can change the composition of the atmosphere. Fossils can tell scientists what living things were around at different times in Earth’s history. Evidence of early plants can tell scientists how oxygen started to be added to the atmosphere long ago. |
Question
Give four reasons why the amount of carbon dioxide in the atmosphere decreased over time.
answers
it dissolved in the oceans
primitive plants used it for photosynthesis
it became trapped underground during the formation of fossil fuels
it became trapped underground during the formation of sedimentary rocks
How human activities affect the atmosphere
A reaction in which oxygen combines with another substance is called an oxidation reaction.
Fuels react rapidly with oxygen, transferring energy to the surroundings by heating and by light. This type of oxidation reaction is known as burning or combustion. The products of combustion then enter the atmosphere.
Hydrocarbon fuels
Hydrocarbons are compounds that contain hydrogen and carbon atoms only.
Hydrocarbon fuels can undergo complete combustion or incomplete combustion, depending on the amount of oxygen available.
Complete combustion
Complete combustion of a hydrocarbon fuel happens when there is a good supply of air or oxygen. Carbon and hydrogen atoms in the fuel react with oxygen:
carbon dioxide and water are produced
energy is given out
In general:
hydrocarbon + oxygen → carbon dioxide + water
Question
Write a balanced chemical equation for the complete combustion of propane (C3H8) with oxygen (O2). Explain why the equation is balanced.
C3H8 + 5O2 → 3CO2 + 4H2O
Reactants: 3 × C, 8 × H, (5 × 2 × O) = 10 × O
Products: 3 × C, (4 × 2 × H) = 8 × H, (3 × 2 × O) + (4 × O) = 10 × O
Incomplete combustion
Incomplete combustion happens when the supply of air or oxygen is poor. Water is still produced, but carbon monoxide and carbon are the other products. Less energy is released than during complete combustion.
Here is one possible equation for the incomplete combustion of propane:
propane + oxygen → carbon monoxide + carbon + water
C3H8 + 3O2 → 2CO + C + 4H2O
Notice that fewer oxygen molecules are needed to balance the equation than are needed for the complete combustion of propane.
Soot
The carbon is released as fine black particles, which we can see in smoky flames, and is deposited as soot. Soot can cause breathing problems and it blackens buildings. It may block boilers and other appliances, or cause a fire.
Carbon monoxide
Carbon monoxide is a toxic gas. It is absorbed in the lungs and binds with the haemoglobin in the red blood cells. This reduces the capacity of the blood to carry oxygen. Carbon monoxide causes drowsiness, and affected people may fall unconscious or even die.
Atmospheric pollutants
Carbon and soot are not the only atmospheric pollutants produced by the combustion of hydrocarbon fuels. Sulfur dioxide and oxides of nitrogen may be produced too.
Scientists monitor the concentration of pollutants in the atmosphere and develop ways to reduce them.
Sulfur dioxide
Many hydrocarbon fuels naturally contain sulfur impurities. When the fuels are burned, the sulfur oxidises to form sulfur dioxide gas:
sulfur + oxygen → sulfur dioxide
S(s) + O2(g) → SO2(g)
Sulfur dioxide dissolves in water in the clouds to form acid rain.
Effects of acid rain
Acid rain damages the natural and built environment. For example, it:
reacts with metals and rocks such as limestone, weakening and damaging buildings and statues
damages the waxy layer on the leaves of trees, making it more difficult for trees to absorb the minerals they need for healthy growth
makes rivers and lakes too acidic for some aquatic life to survive
Oxides of nitrogen
High temperatures are reached when fuels are burned in engines. At these high temperatures, nitrogen and oxygen from the air can react together to produce oxides of nitrogen. For example:
nitrogen + oxygen → nitrogen monoxide
N2(g) + O2(g) → 2NO(g)
Nitrogen oxide gas can be oxidised further in air to produce nitrogen dioxide gas, NO2.
These two oxides of nitrogen are together called NOx. They are atmospheric pollutants. They can react in sunlight with other substances to produce a hazy, harmful smog. Nitrogen dioxide is toxic. It can cause bronchitis and other lung diseases. It also dissolves in water in the clouds, forming an acidic solution that contributes to acid rain.
Question
Write a balanced equation for the reaction between nitrogen monoxide and oxygen, forming nitrogen dioxide.
2NO + O2 → 2NO2
Reducing air pollution
Reducing air pollution from power stations
Power stations generate electricity. By reducing electricity use, everyone can reduce the emission of air pollutants from power stations.
Sulfur dioxide may be removed from the waste gases by a process called gas scrubbing.
Reducing air pollution from cars
The emission of air pollutants from cars may be reduced by people using their cars less by, for example:
walking
taking public transport
car sharing
cycling
Engineers are developing engines for vehicles that are more efficient. This means that the engine uses less fuel and so produces less air pollution.
A catalytic converter, fitted to the exhaust of a car, changes the pollutants carbon monoxide and nitrogen monoxide into less harmful carbon dioxide and nitrogen.
The use of low-sulfur fuels has reduced the amount of sulfur dioxide in car emissions.