Oxygen and Oxides

Note: This has just been updated as of 5/08, I expanded and changed bits on Spec 2.25, right at the bottom of this post. :) 


2.16 recall the gases present in air and their approximate percentage by volume

This is for unpolluted, dry air. Normally air contains a little water vapour too!

2.17 describe how experiments involving the reactions of elements such as copper, iron and phosphorus with air can be used to determine the percentage by volume of oxygen in air

An experiment was carried out to find the percentage of air that is oxygen.  100 cm³ of air was passed from side to side over copper that was being heated with a Bunsen.  All the oxygen in the air will react with the copper.  No air could get in or out of the system while it was passed to and fro between the syringes.  As it was passed to and fro, the volume of air went down.  It was passed until the volume stopped decreasing, and a few minutes later the volume of remaining air was recorded.  There was 79 cm³ left. This shows that 21cm3 of the original 100cm3 of air was oxygen, because it was the oxygen that reacted with the copper to form black copper oxide. During this experiment, you should see the copper go black as it forms copper (II) oxide.

copper + oxygen à copper (II) oxide

2Cu (s) + O₂ (g) à 2CuO (s)

http://www.gcsescience.com/w10-air-oxygen.htm

2.18 describe the laboratory preparation of oxygen from hydrogen peroxide

Oxygen is most easily made in the lab from hydrogen peroxide solution using manganese (IV) oxide as a catalyst. The reaction is known as the catalytic decomposition (splitting up using a catalyst) of hydrogen peroxide. 

2H₂O₂ (aq) à 2H₂O(l) + O₂(g)

The oxygen produced is collected in an inverted glass cylinder by the downward displacement of water in a trough.

2.19 describe the reactions with oxygen in air of magnesium, carbon and sulphur, and the acid-base character of the oxides produced

When a substance burns in air, it reacts with oxygen gas and is said to be oxidised. 

2Mg (s) + O₂ (g) à 2MgO (s)

Magnesium burns with a bright white flame to give a white, powdery ash of magnesium oxide. The product is basic.

C (s) + O₂ (g) à CO₂ (g)

Carbon burns with a yellow flame, to give colourless carbon dioxide. The product is acidic.

S (s) + O₂ (g) à SO₂ (g)

Sulphur burns with a bright blue flame to give colourless sulphur dioxide, the product is acidic. Remember, sulphur dioxide gas is poisonous, and forms acid rain, so it's acidic! 

Any metal forms basic oxides, any non-metal forms acidic oxides. 

2.20 describe the laboratory preparation of carbon dioxide from calcium carbonate and dilute hydrochloric acid

The marble chips are basically calcium carbonate. Remember acids react with carbonates to give a salt, carbon dioxide and water. 

Acid + metal carbonate à salt + carbon dioxide + water

 CaCO₃ (s) + 2HCl (aq) à CaCl₂ (aq) + CO₂ (g) + H₂O (l)

It displaces the air as it is denser. 

2.21 describe the formation of carbon dioxide from the thermal decomposition of metal carbonates such as copper (II) carbonate

Most carbonates decompose (split up) when heated to from the metal oxide and carbon dioxide. 

In the case of copper (II) carbonate, this is a green powder which decomposes to give copper (II) oxide, which is a black powder and carbon dioxide when heated. 

CuCO₃ (s) à CuO (s) + CO₂ (g)

2.22 recall the properties of carbon dioxide, limited to its solubility and density

Carbon dioxide is a colourless, odourless gas, denser than air and slightly soluble in water. 

2.23 explain the use of carbon dioxide in carbonating drinks and in fire extinguishers, in terms of its solubility and density

It is used in carbonate (fizzy) drinks because it dissolves in water under pressure and when you open the bottle/can, the pressure falls and the gas bubbles out of the solution.

It is used in fire extinguishers because it is a dense gas, denser than oxygen and can displace it, basically pushing it out of the way so that no more oxygen reaches the fire. Thereby putting it out. 

2.24 recall the reactions of carbon dioxide and sulphur dioxide with water to produce acidic solutions

Rain is naturally slightly acidic because of dissolved carbon dioxide. Sulphur dioxide makes it even more acidic.

2.25 recall that sulphur dioxide and nitrogen oxides are pollutant gases which contribute to acid rain, and describe the problems caused by acid rain

Acid rain is formed when acidic air pollutants such as sulphur dioxide and nitrogen dioxide dissolve in rainwater. The sulphur dioxide and nitrogen oxides mainly come from power stations and factories burning fossil fuels, or from motor vehicles. The acid rain produces many problems. 


Sulphur dioxide dissolves in water in the atmosphere to form sulphurous acid (H₂SO₃). In the presence of oxygen in the air, this acid is slowly oxidised to sulphuric acid (H₂SO₄).

Oxides of nitrogen also contribute to acid rain. In the presence of oxygen and water, nitrogen dioxide is converted to nitric acid.

nitrogen dioxide + water + oxygen à acid rain

4NO₂ (g) + 2H₂O (l) + O₂ (g) à 4HNO₃ (aq)

The pH value of unpolluted rainwater is usually slightly below 7. This is because carbon dioxide in the air dissolves in rainwater to form carbonic acid, which is a weak acid.

CO₂ (g) + H₂O (l) à H₂CO₃ (aq)

However, acid rain is much more acidic than rain that only contains carbonic acid. Acid rain has a pH value of 4 or less. Which is slightly less acidic than vinegar, which is at around pH 3. 


So, what are the effects of acid rain?

• Acid rain reacts with metals and with carbonates in marble and limestone (calcium carbonate). When this happens, metal bridges and stone buildings are damaged, even statues if they're made of limestone. 
• Acid rain can reduce the pH of natural water bodies from between 6.5 and 8.5 to below 4, which will kill fish and other aquatic life. The water is then too acidic to support life.
• Acid rain also leaches important nutrients from the soil and destroys plants. Without these nutrients, plant growth is stunted. In some cases, acid rain dissolves aluminium hydroxide (Al(OH)₃) in the soil to produce Al³⁺  ions, which are toxic to plants. Forests throughout most of Central and Eastern Europe have been destroyed in this manner by acid rain. The plants are literally 'sick' and dying. 

acid rain eroding a statue