Chemistry - Gasses In the Atmosphere

What are the main gases in the air and their percentages?

• nitrogen (78%)

• oxygen (21%)

• argon (0.9%)

• carbon dioxide (0.04%)

• water vapour

• noble gasses

What are the three main methods to find the percentage of oxygen in air?

The Copper method, Iron method, and Phosphorus method.

Explain how the copper method works for finding oxygen percentage.

The plunger on one of the gas syringes is pushed all the way in and the other moved out to 100 cm'. We now know that the apparatus contains 100 cm3 air.

The silica tube is heated strongly (roaring Bunsen flame).

The plunger in the left-hand gas syringe is pushed in. This causes the air to pass over the heated copper. This pushes out the plunger on the right-hand gas syringe.

The plungers are pushed in sequence so that the air in the system keeps passing over the heated copper. The pink-brown copper turns black as copper(Il) oxide is formed.

As the copper reacts, the Bunsen burner is moved along the tube so that it is always heating fresh copper.

The volume of gas in the syringes falls as the oxygen is consumed.

We keep pushing the plungers in and out until there is no change in volume.

• The apparatus is then allowed to cool to room temperature again before taking the final volume of gas (because gases expand as they are heated).

Explain how the iron method works for finding oxygen percentage.

• Iron wool is placed in a measuring cylinder that is full of air

• End of measuring cylinder is placed a trough of water

• Record the starting position of the water on the scale of the measuring cylinder - this is the starting volume of air

• Over time, the level of the water in the measuring cylinder will rise
  This is because the iron reacts with the oxygen in air to make
  iron oxide

• The water rises to fill the space the oxygen took up
  Leave the measuring cylinder for around a week or until the
  water level stops changing

• Record the finishing position of the water — this is the final volume of air

Explain how the phosphorous method works for finding oxygen percentage.

• The apparatus shown in Figure 13.4 is set up with the piece of phosphorus on an evaporating basin, which is floating in the water.

• The initial level of water is marked on the side of the bell jar with a waterproof pen or a 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, the bell jar becomes filled with a white smoke (phosphorus oxide) and the level of water rises inside the bell jar.

The smoke eventually clears as the phosphorus oxide dissolves in the water.

• When the level of water inside the bell jar 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.

It is important that there is still some phosphorus left on the evaporating basin at the end of the experiment. We have used an excess of phosphorus so that there is more than enough to react with all the oxygen. If there was no phosphorus left, then we would probably get a lower value for the percentage of oxygen in the air because not all the oxygen might have been used up.

Combustion of magnesium — observations and equation.

Observation: Intense white flame, white powder (magnesium oxide).
Equation: 2Mg + O₂ → 2MgO

Combustion of sulfur — observations and equation.

Observation: Blue flame, colourless poisonous gas (sulfur dioxide).
Equation: S + O₂ → SO₂

Combustion of hydrogen — observations and equation.

Observation: Pale blue flame, produces water, explosive.
Equation: 2H₂ + O₂ → 2H₂O

What is thermal decomposition?

Breaking down a compound using heat.

Equation for thermal decomposition of copper carbonate.

CuCO₃ → CuO + CO₂

Equation for thermal decomposition of calcium carbonate.

CaCO₃ → CaO + CO₂

Define a greenhouse gas.

A gas that absorbs and re-emits infrared radiation, trapping heat in Earth’s atmosphere

What is the greenhouse effect?

The natural warming of the Earth caused by greenhouse gases trapping heat that would otherwise escape into space.

Main sources of excess carbon dioxide.

• Burning fossil fuels

• Deforestation

• Burning biomass fuels

• Cement production (decomposition of limestone)

Main greenhouse gases.

Carbon dioxide (CO₂), methane (CH₄), and water vapour (H₂O).

Effects of global warming.

• Rising sea levels (melting ice caps)

• Extreme weather (floods, droughts, heatwaves)

• Habitat loss and species extinction

• Disruption of agriculture and food supply

Steps of the greenhouse effect process.

• Sun emits UV radiation.

• Earth absorbs and re-emits it as IR radiation.

• Greenhouse gases trap some IR radiation.

• Excess CO₂ and CH₄ increase trapped heat → climate change.