2.1 - Gases in the Earths Atmosphere

The composition of air varies from place to place because….

air is a mixture of gases

What is a mixture?

A mixture is made up of two or more simple substances mixed together, that can be separated by physical means.

Name the gases that are found in clean dry air (with percentage volume)

The main gases in clean dry air are:

• Nitrogen (78%)

• Oxygen (21%)

• Carbon Dioxide (0.03%)

• Noble gases (approx. 1%) - including Aragon, neon, helium, krypton, xenon

What gas is found as part of humidity?

water vapour

What are the impurities and pollutants found in the air?

• Sulfur Dioxide (SO₂)

• Oxides of Nitrogen (NO and NO₂)

• Carbon Monoxide (CO)

• Ozone (O₃)

What is the percentage volume of water vapour and the impurities and pollutants found in the air. Explain

They are variables: amount changes from location to location

What is the most abundant of the noble gases (+ percentage volume)

Aragon (0.9%)

Explain the procedure to find the percentage of oxygen in air

1. Place an excess of red-brown copper turnings in a hard glass tub connected to two other gas syringes, one on each side

2. Check that the apparatus is airtight by pushing the air from one syringe to another for several times (precaution)

3. Pass 100cm3 of air over the heated copper turnings by pushing the plumber to and from until the volume in the gas syringe does not change

4. Leave the syringes to cool to room temperature before taking any readings since air expands on heating and a higher value would be obtained if read at a high temperature.

What is another way to check for leaks before starting the experiment

Apply soap to the joints and if bubbles form there is a leak

How does this reaction work

The oxygen in the air reacts with the hot copper, forming black copper oxide. This removes the oxygen from the air, so the volume decreases. CuO is a solid therefore fore not take up space as a gas.

Give an approximate value of the gas expected to remain in the syringe tube and name the main gas that is present at the end of the experiment

79cm3

Nitrogen

What is the name and colour of the solid left in the glass tube at the end of the experiment

Copper(II) Oxide - Black

What is a word equation to represent the reaction of copper with oxygen

Copper + Oxygen → Copper (II) Oxide

What is a balanced chemical equation to represent the reaction of copper with oxygen

2Cu (s) + O₂ (g) → 2CuO (s)

Explain why the following precautions were taken during this experiment

• Copper was in excess

• Air was pushed from one syringe to the other

• Apparatus was allowed to cool before taking final volume readings

• To ensure all the oxygen reacts

• To ensure the apparatus was airtight

• Air increases in volume when heated so a higher value would be obtained if read at a high temperature

Would you expect the copper to have increased or decreased in mass. Why?

Decreased: the pure copper gets used up during the reaction with oxygen, therefore its mass decreases. However, total mass of the solid increases

Nitrogen is the most _______ gas in air

abundant

Name 6 of Nitrogen’s properties

• It is a colourless and odourless gas

• It is a neutral gas (pH 7)

• It is slightly soluble in water

• It has a similar density to air (air = 1.29g/l - Nitrogen = 1.25g/l)

• It is very unreactive compared with O₂

• Nitrogen combines with oxygen at high temperatures to form oxides (occurs naturally in the air due to lightening during thunderstorms, as well as inside running car engines and power station furnaces)

What are some situations that use nitrogen and why?

• It is used as a refrigerant/used to quick freeze food in food factories because it is very cold (boils at -196C). Also used for storing sperm, ova, and other cells for medical research and reproductive technologies. It is also used in hospitals to store tissue samples

• It is flushed through food packing to remove oxygen and keep food fresh for a longer period of time due to its lack of reactivity

• It is used to produce ammonia when combined with hydrogen under special conditions

What are 5 properties of oxygen?

• It is a colourless and odourless gas

• It is a neutral gas

• It is slightly soluble in water - in fact supports aquatic life

• It is slightly denser than air (O2=1.43 g/l - air =1.29g/l)

• It is a reactive element, combining vigorously with many metals (eg sodium, magnesium, copper) and non-metals (eg carbon, sulfur) Supports combustion

Name 6 situations when oxygen is required

• Required during combustion reactions

• As an aid to breathing where the natural supply of oxygen is insufficient.

• In the oxyacetylene torch (used for welding and cutting very thick plates)

• For making steel

• In sewage treatment (if not for this treatment many rivers and lakes would be spoiled by sewage)

List 5 properties of Carbon Dioxide

• It is a colourless and odourless gas

• It is an acidic gas because it is slightly soluble in water, forming carbonic acid (H₂CO₃)

• It is denser than air (CO_{2 =} 1.96g/l - air = 1.29g/l)

• Things will not burn in it (does not support combustion)

• It is not poisonous but does not support respiration.

Name 4 situations where Carbon Dioxide is used

• It is taken up by plants during the process of respiration

• It is used in the production of sparkling mineral water, fizzy drinks and beers.

• It is used in fire extinguishers

• It is used as a coolant

What are the noble gases?

• Helium

• Neon

• Argon

• Krypton

• Xenon

• Radon

Name 5 of the properties of noble gases

• they are colourless and odourless gases

• they are monoatomic (exist as single atoms eg Helium - He)

• they are very unreactive because their outer electron shell is full

• they become denser on going down the group (helium and neon are less dense than air, but argon, krypton and xenon are denser than air)

When are the following noble gases used:

• Helium

• Argon

• Neon

• Krypton and Xenon

• Helium - used in weather balloons and in mixtures with oxygen for deep sea diving tanks

• Argon - was used to fill filament light bulbs because the hot filament wont react with it

• Neon - used extensively in advertising signs, in lasers, and produces a bright red colour

• Krypton and xenon - used in lighthouse lamps and lasers