2 d) oxygen and oxides

Question 1

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

Answer 1

Nitrogen:
Amount in Air (%): 78.1
Oxygen:
Amount in Air(%): 21.0
Argon:
Amount in Air(%): 0.9
Carbon Dioxide:
Amount in Air(%): 0.004

Question 2

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

Answer 2

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The apparatus originally contains 100cm3 of air. This is pushed forward and backward over the heated copper, which turns black as copper(II) oxide is formed. The volume of gas reduce as the oxygen is used up.

2Cu(s) + O2(g) ==> 2CuO(s)

As the copper reacts, the Bunsen is moved along the tube so that it is always heating fresh copper. Eventually all the oxygen in the air is used up. The volume stops reduc the percentage composition of 21% in air.

http://shawonnotes.ucoz.com/IGCSE_Chemistry/images/clip_image065.jpgA test tube is taken and damp iron wool is placed. This is inverted and placed in a beaker containing water. The tube is now left for weeks. Iron uses oxygen and water to form rust. As long as the wool is damp, the rusting will continue.

The water level rises as oxygen is used. This is marked with a rubber band. At the end of the experiment, we will see all the oxygen is used up. The original water level was 15cm3 and now it is 12 cm3. That means 3 cm3 is used up.

3 / 15 x 100 = 20%.

Question 3

2.18- describe the laboratory preparation of oxygen from hydrogen peroxide, using manganese (IV) oxide as a catalyst

Answer 3

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

2H2O2(aq) ==> 2H2O(l) + O2(g)

Testing for oxygen: Oxygen will relight a glowing splint.

Question 4

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

Answer 4

Burning magnesium

Magnesium burns in air with a bright white flame to give a white, powdery ash of magnesium oxide. The flame is extremely bright in pure oxygen.

2Mg(s) + O2(g) ==> 2MgO(s)

Burning carbon

Carbon burns if it is heated very strongly in air or oxygen to give colourless carbon dioxide gas. The carbon may produce a small yellow-orange flame and perhaps some sparks. It depends on the purity of the carbon.

C(s) +O2(g) ==> CO2(g)

Burning sulfur

Sulfur burns in air with a tiny, almost invisible blue flame. In oxygen it burns much more strongly giving a bright blue flame. Poisonous, colourless sulfur dioxide gas is produced.

S(s) + O2(g) ==> SO2(g)

Acid base character of oxides:

Few metal oxides react or dissolve in water –to form alkaline solutions. Most metal oxides do not. Shaking a solid magnesium oxide with water doesn’t seem to dissolve. alkaline.

MgO(s) + H2O(l) ==> Mg(OH)2(s and aq)

Many non-metals react with water to give acidic solutions except water and carbon dioxide. For example sulfur dioxide reacts with water to give sulfurous acid. Sulfurous acid is fairly acidic.

H2O(l) + SO2(g) ==> H2SO3(aq)

Question 5

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

Answer 5

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Carbon dioxide is most easily made by the reaction between dilute hydrochloric acid and calcium carbonate in the form of marble chips. Carbon dioxide is collected in an inverted jar.

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

Testing carbon dioxide:

Carbon dioxide turns lime water (calcium hydroxide solution) milky. It reacts to give a white precipitate of calcium carbonate.

Ca(OH)2(aq) + CO2(g) ==> CaCO3(s) + H2O(l)

With an excess of carbon dioxide, the precipitate dissolves again to give a colorless solution of calcium hydrogen carbonate.

CaCO3(s) + CO2(g) + H2O(l) ==> Ca(HCO3)2 (aq)

Question 6

2.21- describe the formation of carbon dioxide from the thermal decomposition

Answer 6

When metal carbonates are heated they become carbon dioxide and a metal.
For example:

Copper carbonate > Copper oxide + carbon dioxide CuCO3 > CuO + CO2

Question 7

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

Answer 7

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

Question 8

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

Answer 8

It is used in carbonated drinks because it dissolves in water under pressure. When you open bottle, the pressure falls and the gas bubbles out of solution.

It is also used in fire extinguishers to put out electrical fires, or those caused by burning liquids, where using water could cause problems. The dense gas sinks onto the flames and prevents any more oxygen from reaching them.

Question 9

2.24- understand that carbon dioxide is a greenhouse gas and may contribute to climate change.

Answer 9

Carbon dioxide prevents heat loss from earth. This causes to warm up the atmosphere and may lead to climate change.