Gases in the Atmosphere

Question 1

What is the composition of gases in earths atmosphere in percentages?

Answer 1

• 78% - Nitrogen
• 21% - Oxygen
• 0.9% - Argon
• 0.04% - Carbon Dioxide
• 0.06% - Negligible other gases

Question 2

How can you find the percentage of oxygen in the air?

(Using phosphorus)

Answer 2

• Set up a bell jar containing water sitting on a trough of water with an evaporating dish sitting on the water in the bell jar
• Make sure the bell jar contains a fixed, known volume of air
• Burn phosphorus on the dish in oxygen until all the oxygen is used up, and measure how much the water level increases
• Substract the final, smaller volume of air (as more volume is used up by water) from the starting volume of air, then divide the difference by the starting volume and times by 100 to find the percentage of oxygen

• Important: In a similar method, iron filings in an inverted buirette left for 3-4 days can also be used for this experiment although you only react it with the oxygen instead of burning it
• However, phosphorus is suitable as it burns readily with oyxgen though is toxic

Question 3

What happens when magnesium is combusted in oxygen?

Answer 3

• An intense white flame is observed and a white powder (magnesium oxide) is produced
• Magnesium is oxidised to produce magnesium oxide

All combustions are exothermic, and can be considered oxidation reactions

Question 4

What happens when hydrogen is combusted in oxygen?

Answer 4

Hydrogen is oxidised to produce water (which is a liquid at room temperature)

Question 5

What happens when sulfur is combusted in oxygen?

Answer 5

• A blue flame is observed, and a colourless, poisonous gas is produced (sulfur dioxide)
• Sulfur is oxidised to produce sulfur dioxide

Question 6

What is the general equation for the thermal decomposition for metal carbonates?

Thermal decomposition describes the breakdown of a substance due to heat action

Answer 6

Metal carbonate –> Metal oxide + Carbon dioxide

Question 7

What is the symbol equation with state symbols for the thermal decomposition of copper carbonate?

Answer 7

CuCO₃(s) –> CuO(s) + CO₂(g)

Copper carbonate is a green powder which slowly darkens as copper oxide is produced

Question 8

What is the greenhouse effect?

Answer 8

• Shortwave radiation from the sun passes through the atmosphere and hits the earths surface
• Some is absorbed, and some is re-emitted as infrared radiation
• However some of this re-emmited radiation is trapped by greenhouse gases in the atmosphere such as carbon dioxide, methane and water vapour which absorb and store this energy, trapping the heat

Question 9

What is the enhanced greenhouse effect?

Answer 9

• Increased levels of greenhouse gases caused by human activity (mostly carbon dioxide) in the atmopshere mean more infrared radiation (so heat energy) is being trapped inside of the earths atmosphere
• As a result, the earth is becoming hotter as levels of these greenhouse gases increase and more heat is trapped

The normal greenhouse effect is responsible for keeping the earth at a habitable temperautre, however the enhanced greenhouse effect is responsible for global warming