HYDROGEN, OXYGEN, THE ATMOSPHERE, POLLUTION

2.26    describe the reactions of dilute hydrochloric and dilute sulphuric acids with         magnesium, aluminium, zinc and iron

Metals above hydrogen in the reactivity series will react with acids to form a salt (e.g. magnesium sulfate or zinc chloride) plus hydrogen. The metals are 'displacing' hydrogen. The higher the metal in the series, the more violent the reaction (if you put copper in most acids, you won't see a reaction, as it is below hydrogen in the reactivity series).

Metal + dilute hydrochloric acid è metal chloride + hydrogen

Metal + dilute sulfuric acid è metal sulfate + hydrogen

2.40    describe a simple test for hydrogen gas

If held above hydrogen a lighted splint will make a popping sound.

2.27    describe the combustion of hydrogen

Hydrogen reacts violently with oxygen in the presence of a flame to give water. If there was enough hydrogen this could create an explosion.

2.28    describe the use of anhydrous copper(II) sulphate in the chemical test for water

When water is added to anhydrous copper(II) sulphate (which is white) it turns blue.

2.29    describe a physical test to show whether water is pure

Heat the water and if it boils at exactly 100 °^C  then it is pure.

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) è 2H₂O(l) + O₂(g)

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

2.40    describe a simple test for oxygen gas

A glowing splint held over the unknown gas will relight if it is oxygen.

2.19    describe the reactions of oxygen in air with 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) è 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) è CO₂ (g)

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

S (s) + O₂ (g) è 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. 

2.3      explain the classification of elements as metals or non-metals on the basis of their      electrical conductivity and the acid-base character of their oxides

Metals are generally conductive. Non metals (excluding graphite) are not conductive. If an element is conductive with a basic oxide it’s a metal. If an element is not conductive and the oxide is acidic then it’s a non metal.

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

2.17    describe 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 the air

One way to find out what percentage of the air is oxygen is to pass 100cm³ of air from side to side over copper being heated by a Bunsen burner. All the oxygen will eventually react with the copper, so the volume of air will decrease. Keep passing the air back and forth until the volume stops decreasing and then measure the volume. There should be around 79cm³ left which means 21cm³ of the original 100cm³ was oxygen. The copper should turn black during this experiment as copper oxide is forming.

2.25    recall that sulphur dioxide is a pollutant gas which contributes 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.

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 and becomes sulphuric acid (H₂SO₄).

Problems caused by acid rain:

•    Acid rain corrodes limestone and metals.

•    Acid rain changes the pH of water which kills aquatic life.

•    Acid rain kills plants.

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

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 è acid rain

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

Problems caused by acid rain:

•    Acid rain corrodes limestone and metals.

•    Acid rain changes the pH of water which kills aquatic life.

•    Acid rain kills plants.

5.11    recall that, in car engines, the temperature reached is high enough to allow nitrogen    and oxygen from air to react, forming nitrogen oxides

Self explanatory. Lightning can also cause nitrogen to react with oxygen.