2.3 Gases in the Atmosphere

REVISION NOTES

IGCSE Edexcel Chemistry

2.3 Gases in the Atmosphere

2.3.1 Know the approximate percentages by volume of the four most abundant gases in dry air

PROPORTION OF GASES

The proportion of gases in the air has been relatively constant in the past 200 million years
- 78% nitrogen (N₂)
- 21% oxygen (O₂)
- 1% other gases (e.g. carbon dioxide CO₂)

2.3.2 Understand how to determine the percentage by volume of oxygen in air using experiments involving the reactions of metals (e.g. iron) and nonmetals
(e.g. phosphorus) with air

The percentage of oxygen in the air can be found by:

Reacting a metal (e.g. iron) in a fixed volume of air
Reacting non-metal (e.g. phosphorus) in a fixed volume of air
By calculating the volume of total air and the volume of combusted oxygen, the percentage of oxygen in the air can be found

Details of experiment will be discussed in 2.3.6

2.3.3 Describe the combustion of elements in oxygen, including magnesium, hydrogen and sulphur

COMBUSTION OF ELEMENTS

Combustion is the process of burning
All combustions involve chemical changes
- When elements react with oxygen, oxides are produced
- As there are new compounds formed, combustion is considered a chemical change
All combustions release heat
- They are thus exothermic reactions

2.3.4 Describe the formation of carbon dioxide from the thermal decomposition of metal carbonates, including copper(II) carbonate

THERMAL DECOMPOSITION

When a substance breaks down due to heat
The compounds would decompose to several products

Thermal decomposition of metal carbonate

Metal carbonate → metal oxide + carbon dioxide

Example: copper (II) carbonate

CuCO3 (s) → CuO (s) + CO2 (g)

Copper (II) carbonate → Copper (II) oxide + Carbon dioxide

Copper (II) carbonate is a green powder
Copper (II) oxide produced is black
Carbon dioxide produced can be tested by passing the gas through lime water and turning it milky

2.3.5 Know that carbon dioxide is a greenhouse gas and that increasing amounts in the atmosphere may contribute to climate change

ENHANCED GREENHOUSE EFFECT

The Sun emits shortwave radiation to the Earth
When the radiation hits the Earth’s surface, it is absorbed and re-emitted as infrared (IR) radiation
However, the IR radiation is mostly trapped in the Earth’s atmosphere due to
greenhouse gases
Enhanced greenhouse effect: increasing levels of greenhouse gases trap extra heat energy in the Earth’s atmosphere

GREENHOUSE GASES

Include carbon dioxide, methane and water vapour
Sources of carbon dioxide:
- Combustion of wood and fossil fuels
- Respiration of plants and animals
- Thermal decomposition of carbonate rocks
- Effect of acids on carbonate

2.3.6 Practical: determine the approximate percentage by volume of oxygen in air using a metal or a non-metal

REACTION 1 COMBUSTION OF PHOSPHORUS

Place a small amount of phosphorus in an evaporating dish
Place the evaporating dish in a trough of water
Burn the phosphorus in a bell jar

Measurement and calculations

Initially the water levels inside and outside the jar should be the same

As the phosphorus burns, oxygen inside the bell jar is used up
The water level inside the bell jar rises
By measuring the water levels before and after the combustion, percentage of oxygen can be calculated

Advantage and disadvantage

Advantage: phosphorus is very suitable for this experiment as it burns readily until all oxygen available in the bell jar is used up
Disadvantage: phosphorus is toxic, so it must be taken good care to handle it safely

REACTION 2 OXIDATION OF IRON