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.

Question

Give four reasons why the amount of carbon dioxide in the atmosphere decreased over time.

answers

1. it dissolved in the oceans

2. primitive plants used it for photosynthesis

3. it became trapped underground during the formation of fossil fuels

4. 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 NO_x. 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.