Composition of air
Air is a mixture of gasses of different boiling points. This means that the gasses in air can be seperated if the air is liquified.
|Carbon dioxide, water vapour, rare gases||1%|
Fractional distillation of liquid air
- Air is pumped into the plant, and filtered to remove dust particles.
- It is then cooled to -78°C. Water vapour and carbon dioxide are
removed as solids (since these would block the pipes).
- Now the air is repeatedly compressed, cooled and allow to expand rapidly several times until the temperature drops to – 200°C.
- At -200°C, the air is liquid, except for rare gases (neon and helium). These gases are removed.
- The liquid air is pumped into the fractionating column where it is
slowly warmed up. Nitrogen which has a boiling point of -196°C boils off and is collected from the top of the fractionating column. Oxygen which has a boiling point of -183°C is collected as a liquid from the bottom of the column.
Liquid oxygen is blue in colour
Uses of oxygen
- Planes carry oxygen supplies for passengers to breathe at high altitudes
- Divers carry oxygen tanks to breathe under water
- Astronauts carry oxygen to breathe in the vacuum of space
- In hospitals, patients with breathing problems are given oxygen
through an oxygen mask, or in an oxygen tent
- In steel works, oxygen is used in converting the impure iron from
the blast furnace into steels
- A mixture of oxygen and the gas acetylene (C2H2) is used as the fuel in oxy-acetylene torches for cutting and welding metals
Uses of nitrogen
- Liquid nitrogen is used to quick freeze food in food factories
- Liquid nitrogen is used in hospitals to store tissue samples
- Since nitrogen is unreactive it is flushed through food packaging to remove oxygen and keep the food fresh. (Oxygen helps decay.)
Electrolysis is the decomposition of an ionic compound using electricity. The compound being broken down is called the electrolyte. An electrolyte is a compound that conducts electricity either in solution or molten state. The conduction of the electricity is caused by the movement of ions. The electric current enters and leaves the electrolyte through electrodes, which are usually made of unreactive metals such as platinum or of the non-metal carbon (graphite).
These are said to be inert electrodes because they do not react with the products of electrolysis. Positively charged ions move away from the anode (the positive electrode) to the cathode (the negative electrode). Negatively charged ions move in the opposite direction.
Electrolysis of molten compounds containing two elements
Electrolysis of all molten ionic compounds of two elements follow the same general pattern:
The molten ionic compound is broken down to its elements, giving the metal at the cathode, and the non-metal at the anode.
Lets look at the electrolysis of molten lead bromide as an example.
First the compound is heated until it melts. Then the electric current is switched on. The positive lead ions (Pb2+) move to the cathode (-) and the negative bromide ions (Br2-)
move to the anode (+).
At the cathode
The lead ions Pb2+ each receive two electrons and become lead atoms.
The half-equation is:
Pb2+ (l) + 2e– → Pb (l)
Molten lead collects on the cathode and eventually drops off it and solidify.
At the anode
The bromide ions each give up an electron, and become atoms. Two bromide atoms Br pair up to form bromine molecules.
The half-equation is:
2Br– (l) → Br2 (g) + 2e–
The bromine gas bubbles off at the anode.
Solid lead is produced at the cathode and brown bromine gas is produced at the anode.
Electrolysis of water
Pure water is a poor conductor of electricity. Ionic impurities in water increase its conductivity. Since water is a compound of hydrogen and oxygen only, electrolysis of water produces these two elements as expected.
Electrolysis of water
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Lets look at the electrolysis of acidified water.
Drops of sulphuric acid are added to water to increase its conductivity. When the electric current is switched on, the positive hydrogen ions (H+) move to the cathode (-) and the negative hydroxyl ions (OH–) move to the anode (+).
At the cathode
The Hydrogen ions H+ receive one electron each and become hydrogen atoms. The hydrogen atoms pair up to form hydrogen molecules H2
The half-equation is:
2H+ (aq) + 2e– → H2 (g)
Hydrogen gas bubbles off the cathode.
At the anode
The hydroxyl ions each give up an electron, and produce water and oxygen molecules as follows:
4OH– (aq) → 2H2O (l) + 2O2 (g)
The oxygen gas bubbles off at the anode.
Hydrogen gas is produced at the cathode and oxygen gas is produced at the anode. The hydrogen produced is twice as much as the oxygen.
Overall equation of the electrolysis of water.
Uses of hydrogen
- To make ammonia (haber proccess)
- To ‘harden’ vegetable oils to make margarine
- As a fuel in hydrogen fuel cells
Electroplating is the process of using electricity to coat one metal with another.
A metal is electroplated
- to make it look better
- to prevent corrosion.
The metal to be plated is connected to the cathode in the electrolytic cell. The plating metal is made the anode, e.g. in copper plating copper is the anode. The atoms in the copper anode loses electrons an become copper ions Cu2+. The copper ions travel through the copper (ii) sulphate electrolyte to the cathode which is the object to be plated. At the cathode, the copper ions gain electrons and become copper atoms which the coat the cathode.
There many different metals that can be used to coat objects e.g
- Steel car bumpers are coated with chromium
- Steel food cans are coated with tin
- And cheap metal jewellery and medals are often coated with silver
|Copper||Copper (ii) sulphate||Cu → Cu2+ + 2e–||Decorative purposes and corrosion resistance|
|Chromium||Chromic acid||Cr → Cr3+ + 3e–||Decorative purposes and corrosion resistance|
|Nickel||Nickel (ii) sulphate||Ni → Ni2+ + 2e–||Corrosion resistance|
|Tin||Tin (ii) sulphate||Sn → Sn2+ + 2e–||Corrosion resistance especially on food containers|
Ammonia is manufactured from hydrogen and nitrogen by the haber proces.
Lets look at the haber process in detail
The reactants are nitrogen and hydrogen. The nitrogen is obtained
from air, and the hydrogen from the electrolysis of water.
- The two gases are mixed, and cleaned to remove
- The mixture is compressed until the pressure reaches 200 atmospheres.
- The compressed gases are passed into the converter which contains hot iron at 450°C. The iron acts as a catalyst for the reaction.
- The mixture is cooled until the ammonia condenses to a liquid.
The unreacted nitrogen and hydrogen are recycled to the converter for another chance to react again.
- The ammonia is run into tanks, and stored as a liquid under pressure.
A catalyst is a substance that increases the rate of forward reaction without itself being used up in the reaction.
Nitrogen + hydrogen ⇌ ammonia
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
Since the reaction is exothemic and reversable, it needs low temperatures to proceed forward. A catalyst is therefore needed to speed up the forward reaction at low temperatures. Only 15% of the mixture leaving the converter is ammonia.
Uses of ammonia
- Making fertilisers such as ammonium nitrate
- Making nitric acid
- Making nylon
- As a refrigerant
- Manufacture of dyes
Sulphuric acid is manufactured from sulphur dioxide by the contact process
Lets look at the contact process in detail
- Sulphur dioxide is produced either by the oxidation of sulphur or
a sulphide ore such as iron sulphide
- Sulphur dioxide is then mixed with oxygen and passed into the converter where the temperature is raised to 450°C. Vanadium (v) oxide is used as a catalyst
- The sulphur trioxide is not directly dissolved in water because the heat of reaction would produce a mist of sulphuric acid that is difficult to condense. Instead, the sulphur trioxide is dissolved in concentrated sulphuric acid to produce a thick fuming liquid called oleum.
- The oleum is then diluted with water to produce more concentrated sulphuric acid.
Sulphur + oxygen → sulphur dioxide
S(s) + O2(g) → SO2
Iron sulphide + oxygen → iron oxide + sulphur dioxide
4FeS(s) + 7O2(g) → 2Fe2O3 + 4SO2
The reaction is reverseble and exothermic, so it needs low temperatures to proceed forward. However any reaction is slow at low temperatures, so a catalyst is used to speed the reaction at a moderate temperature of 450°C. The pressure is maintained at 1 atmosphere. Higher pressure would have been desirable for the forward reaction but due to the corrosive nature of the products and reactants, a high pressure is costly to maintain. Sulphur trioxide is produced.The yield is about 95%.
Sulphur dioxide + oxygen ⇌ sulphur trioxide
2SO2 (g) + O2 (g) ⇌ 2SO3 (g)
sulphuric acid + sulphur trioxide → oleum
H2SO4 + SO3 → H2S2O7
Oleum + water → sulphuric acid
H2S2O7 + H2O → 2H2SO4
Uses of sulphuric acid
- fertilisers such as ammonium sulfate.
- paints, pigments, and dyestuffs.
- fibres and plastics.
- soaps and detergents.
It is also the acid used in car batteries.