Gases In The Atmosphere Flashcards
Know the approximate percentages by volume of the four most abundant gases in dry air
Nitrogen - 78.1%
Oxygen - 21%
Argon - 0.9%
Carbon dioxide - 0.04%
Note: there are also other noble gases in the air but they are small amounts
Understand how to determine the percentage by volume of oxygen in air
There are a few different methods for measuring the percentage of oxygen in the air. We react a metal or non metal with oxygen and look at how much the volume of air decreases when the oxygen has all been used up.
To find the percentage of oxygen in the air use:
Percentage of oxygen = Volume of oxygen/ volume of air at start x 100
We want to percentage to be around 21% as that’s the amount of oxygen in the air.
Using copper (metal) to determine the percentage of volume of oxygen in air
- We know that the volume of air at start is 100cm〖^3〗
- The silica tube is heated strongly using a Bunsen Burner.
- You push the left hand side (or right) in which causes the air to pass over the heated copper.
- When pushing the left hand side in, you are pushing the right hand side syringe out.
- The plungers are pushed so that the air keeps passing over the heated copper.
- The pink-brown copper turns to black as copper oxide is formed.
- As the copper reacts (as the syringes are pushed) the Bunsen burner is moved along the tube so that it is always heating fresh copper.
- The volume of gas in the syringes fall as the oxygen reacts with the copper.
- We keep pushing the plungers (one of them) in and out until there is no change in volume.
- The apparatus is then allowed to cool to room temp before taking the final volume of gas.
- So at the start, you have 100cm〖^3〗 of air and you have 80cm〖^3〗 left in the syringe, that means that 20cm〖^3〗 of air is used.
- You can see that 80cm〖^3〗 is left because the gas syringes have markings on.
- So you do the volume of oxygen (20cm〖^3〗) divided by the volume of air at start (100cm〖^3〗) x 100.
- The answer is 20%. 20% of air has been used to react with copper to form copper oxide.
POSSIBLE ERRORS: there is not enough copper to react with oxygen. The apparatus was not cool before measuring the volume. You need to allow it to cool because gases expand when they are heated. So we need to allow it to cool or then the volume would be incorrect.
Using the rusting of iron to determine the percentage by volume of oxygen in air
- First, you put the wet iron filings in the conical flask.
- Record the initial reading on the gas syringe.
- Leave the apparatus in place for about a week until the reading stops changing on the gas syringe.
- Record the final reading.
Volume of air in conical flask – 130
Volume of air in connecting tube – 12
Initial reading on gas syringe – 92
Final reading on gas syringe – 43
The total volume of air inside the apparatus at the beginning of the experiment is 130 + 12 + 92 which equals 234cm^3
The total volume of air is 130 + 12 + 43 = 185cm^3
So, the volume of oxygen used up = 234 – 185 = 49cm^3
The percentage of oxygen in the air is 49/234 x 100 = 21%
POSSIBLE ERRORS: If the answer comes out less than 21% then it could be that the experiment was not left set up for long enough and the iron has not had enough chance to react with all the oxygen. Or not enough iron was added at the beginning.
Using phosphorus
- First, the initial level is marked on the side of the bell jar with a pen or sticker.
- The bung is removed from the bell jar and the phosphorus is touched with a hot metal wire in order to ignite it.
- The bung is quickly put back into the bell jar.
- The phosphorus burns and the bell jar becomes filled with a white smoke (phosphorus oxide) and the level of water rises inside the bell jar.
- When opening the bung, you are letting the air meaning oxygen into the bell jar making it react with the phosphorus to form phosphorus oxide.
- The smoke that it formed eventually clears as the phosphorus oxide dissolves in the water.
- When the level of water stops rising, the final level is marked.
- To find how much the water level has changed, the bell jar is turned upside down, filled with water to each mark in turn and the water is poured into a large measuring cylinder.
- POSSIBLE ERRORS: There is not enough phosphorus to react with all the oxygen.
The combustion of elements in oxygen
Some elements burn in oxygen. These reaction are called combustion reactions.
Elements burn more brightly and rapidly in pure oxygen than in air because air only contains 21% of oxygen where as pure oxygen is 100%. The table at the ends summaries the detail about the next three elements and how they react with oxygen.
Describe the combustion of elements in oxygen, including magnesium, hydrogen and sulfur
Magnesium
Magnesium burns in oxygen with an extremely bright white flame to give a white, powdery ash of magnesium oxide.
The white powder formed is not very soluble in water but a very small amount does dissolve to form an alkaline solution.
This means that the colour of universal indicator would be blue.
Describe the combustion of elements in oxygen, including magnesium, hydrogen and sulfur
Sulfur
Sulfur burns in oxygen with a blue flame.
Poisonous, colourless sulfur dioxide gas is produced.
The sulfur dioxide dissolves in water to form an acidic solution of sulfurous acid.
This means that the colour of universal indicator would be red.
Describe the combustion of elements in oxygen, including magnesium, hydrogen and sulfur
Hydrogen
- Hydrogen burns in oxygen with a pale blue flame. The product is water.
- If you ignite a mixture of hydrogen and oxygen it will explode.
- Because the product is water it is neutral and the universal indicator would turn green.
Summary of describing the combustion of elements in oxygen, including magnesium, hydrogen and sulphur
Elements and their observation of element burning in air + appearance of product from burning + name of oxide + colour of universal indicator after adding water to the product + type of oxide
Sulfur (non-metal) - blue flame - colourless gas - sulphur dioxide - red - acidic
Hydrogen - pale blue flame - colourless gas/liquid - water - green - neutral
Magnesium - bright white flame - white solid/powder - magnesium oxide - blue - basic
The properties of oxides
We can make some ideas about the properties of oxides formed when elements burn in oxygen:
Metal oxides are usually basic oxides, which means that they react with acids to form salts.
Metal oxides are usually insoluble in water
If the oxide reacts with water, it forms an alkaline solution.
Non-metal oxides are usually acidic oxides which reacts with alkalis to form salts.
Non metal oxides are often soluble in water and react with it to form acidic solutions
Carbon dioxide
Carbon dioxide is a colourless gas that is most easily made in the lab by the reaction between dilute hydrochloric acid and calcium carbonate in the form of marble chips.
Carbon dioxide can also be obtained when metal carbonates are heated strongly.
Most carbonate split to give the metal oxide and carbon dioxide when you heat them. This is an example of thermal decomposition, breaking up something by heating it.
For example, carbon carbonate is a green powder which decomposes on heating to produce black copper oxide.
Global warming: the Greenhouse Effect
• Carbon dioxide is produced when fossil fuels burn. For instance, when coal (which is mostly carbon) burns in excess oxygen.
• Petrol is a mixture containing many different hydrocarbons.
• Carbon dioxide is a greenhouse gas. A greenhouse gas are gases such as carbon dioxide which can trap heat radiated from the Earth’s surface.
• The greenhouse effect occurs when high energy UV (ultraviolet radiation) and visible light from the Sun pass through the atmosphere and warm up the surface of the Earth.
• The surface of the Earth radiates infrared radiation.
• This infrared radiation is absorbed by molecules such as CO2 in the atmosphere.
• These then give out this energy again in all directions, heating the atmosphere.
• For a long time, the level of carbon dioxide in the atmosphere has been increasing.
• The reason for this is because the burning of fossil fuels and deforestation.
• Some people think that the increase in carbon dioxide may contribute to climate change.
• If this continues then:
1. Polar ice caps could melt
2. Sea levels could rise
3. There could be more extreme weather including floods and heat waves.
