Summary Topic 2.5 Industrial inorganic chemistry

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2.5a. Conditions for ammonia and nitric acid manufacture
The Haber process that turns nitrogen (from the air and hydrogen (from water or methane) into ammonia. 
3H₂(g) + N₂(g) = 2NH₃(g) DH=-92kJmol^-1
The optimum (best) conditions for this process which give the greatest yield are:

• About 300 atmospheres; high pressure increases yield
• about 450ºC ; high temperature cuts yield but increases rate
• and the use of a catalyst, iron with KOH is used, to increase the rate

Ammonia is changed into nitric acid in the Ostwald process.  This is another equilibrium process which starts with the reaction of ammonia and oxygen.
4NH₃(g) + 5O₂(g) = 4NO(g) + 6H₂O(g)  DH= -900 kJmol^-1
The optimum (best) conditions for this process which give the greatest yield are

• a pressure of 10 atmospheres
• a temperature of about 900^oC
• a catalyst of platinum and rhodium

The nitrogen monoxide is then mixed with oxygen and flows up a column down which water flows.  The result is a solution of 65% nitric acid.
4NO(g) + 3O₂(g) +2H₂O(l) ---> 4HNO₃(aq)
Task 2.5a State and explain the effect on ammonia yield and reaction rate of:
(a) increasing the concentration of hydrogen, (b) decreasing the concentration of nitrogen, (c) increasing the temperature, (d) decreasing the pressure, (e) decreasing the temperature, increasing the pressure, (f) changing the catalyst.

2.5b. Conditions for sulfuric acid manufacture
Sulfuric acid is made in the Contact process.  This takes sulfur dioxide (made by burning sulfur) and reacts it in an equilibrium process with oxygen.
2SO₂(g) + O₂(g) = 2SO₃(g) DH= -192kJmol^-1
The optimum conditions for this process are:

• a temperature of 430^oC
• a pressure of 2 atmospheres
• a catalyst of vanadium pentoxide V₂O₅

Task 2.5b (a) Explain why a higher temperature is not used to speed up this reaction.
(b) Explain the choice of pressure given the position of equilibrium is far to the right.
(c)Write an equation for making sufur dioxide

2.5c. Explanation of choice of conditions for manufacture
Temperature
The reaction below is exothermic, energy given out when ammonia forms, but energy is taken in if ammonia breaks up.
N₂(g)  +   3H₂(g)    ---> 2NH₃(g)  ;  DH = - 92kJ/mol
Reactions resist changes (le Chatelier's principle).
If the temperature goes up the reaction tries to prevent this by taking in energy.
The reaction can take energy by breaking up ammonia. 
So if the temperature increases then ammonia breaks up.
The position of equilibrium moves to the left.
When the temperature falls the position of the equilibrium moves to the right so more ammonia is formed .  A low temperature is best for a high yield.
At low temperature the rate is low so a compromise of about 450^oC is used.
Catalyst
As the rate is low at the optimum temperature a catalyst is used to increase the rate.  The surface catalyst iron is used.  KOH is a promoter used to make the surface of the iron more effective.
Pressure
On the left hand side of the equation there are 4 molecules/4 mol of gas/4 volumes of gas
On the right hand side there are only 2 molecules/2mol of gas/2 volumes of gas.
Pressure is reduced if the position of the equilibrium moves to the right, so an increase in pressure causes a shift to the right so more ammonia is formed.  As high pressure favours a big yield so 200 atmospheres pressure is used.
In the Contact process why is the moderate temperature used instead of a high one?
Why is the pressure higher than 1 atmosphere in the Contact process?

2.5d. The uses of ammonia, nitric acid and sulphuric acid
Plants need nitrogen for growth.  Most plants cannot use nitrogen directly from the air so it must be fixed that is combined in a compound.  Ammonia and nitric acid alone both contain fixed nitrogen but ammonia is too alkaline and nitric acid is too acidic for normal use.  All three substances are used to make nitrogen containing fertilisers e.g.
ammonium nitrate       NH₃(g) + HNO₃(aq) -----> NH₄NO₃(aq) 
ammonium sulphate    H₂SO₄(aq) + 2NH₄OH(aq) ---> (NH₄)₂SO₄(aq) + 2H₂O(l) 

Task 2.5d (i) Vegetable oil reacts with conc sulfuric acid to make a soapless detergent.  Explain how you could make a soapless detergent from corn oil, sufur, air and water.  
(ii) Explain how you can make an explosive from air and water.

2.5e. The electrolytic extraction of aluminium
Al is extracted from the ore bauxite, Al₂O₃.2H₂O.
The ore is purified by dissolving amphoteric Al₂O₃  in hot aqueous sodium hydroxide under pressure.  Basic iron oxide not soluble.  Although silicon (IV) oxide is acidic it is not soluble due to strong bonds in giant structure.
Solid impurities are removed by filtration.
Aluminium hydroxide is precipitated with  'seed' crystals and filtered.
Aluminium hydroxide is heated to produce aluminium oxide.
Present the above steps as a flow diagram.  Show the formation of Al(OH)₄^-(aq) in one step.

Aluminium is manufactured by the electrolysis of pure aluminium oxide dissolved in molten cryolite (Na₃AlF₆) to lower melting point from 2000º to 1000º, which saves money. This mixture is heated and the molten liquid used as the electrolyte. Both electrodes are made of graphite (carbon).  The anode (+ve) is graphite and the cathode (-ve) is a graphite lining to a steel case.


The carbon anode is burned away by oxygen and has to be frequently replaced.
2O^2-(l) -----> O₂(g) + 2e^-
The cathode is steel
Al³⁺(l) + 3e^- -----> Al(l)
Aluminium is expensive despite being common because its manufacture requires large amounts of electricity.
It is important because its low density, high strength, good electrical conductivity and resistance to corrosion make it valuable for aircraft manufacture, overhead power cables and many other uses.
Task 2.5e.  Explain why Aluminium is more expensive than iron despite the fact that aluminium is more abundant than iron.

2.5f. Uses and properties of aluminium

Use	Property
overhead power cables	good electrical conductor, low density
drinks cans	Does not react with water
aircraft parts	high strength and low density
window frame	does not corrode due to oxide layer protection
saucepans	good thermal conductivity

2.5g. The electrolysis of sodium chloride

using a membrane cell

Sodium hydroxide, chlorine and hydrogen are produced from natural deposits of sodium chloride (rocksalt). This is the basis of the chloralkali industry. Electrolysis of brine (salt solution) produces 1 tonne of chlorine at the same time as 1.13 tonnes of sodium hydroxide and 0.028 tonnes of hydrogen. There are many different processes used in the electrolysis of brine. An aqueous solution contains the following ions: Na⁺, Cl^-, H⁺ and OH^-.

Below is a diagram of a membrane cell:

The processes taking place at the electrodes are:

At Cathode 2H⁺(aq) + 2e^- -----> H₂(g)

At Anode 2Cl^-(aq) -----> Cl₂(g) + 2e^-

In the solution in the cathode compartment the equilibrium

H₂O(l) <=> H⁺(aq) + OH^-(aq)

shifts to the right as H⁺ ion is converted to hydrogen gas. There is a build up of hydroxide ion and with the unaffected sodium ions remaining in the solution produces sodium hydroxide.

Saturated brine is pumped into the anode compartment. The brine level in the anode compartment is kept higher than the level in the cathode compartment so that brine seeps through the membrane, but hydroxide ion does not seep in the opposite direction. The mixing of hydroxide ions and chlorine would produce unwanted sodium chlorate(I). The resulting solution leaving the cell, known as cell liquor, contains approximately 12% sodium hydroxide and 15% sodium chloride by mass. Evaporation and crystallisation leave a solution containing 50% sodium hydroxide and less than 1% sodium chloride.
If sodium chlorate (I) is required then the sodium hydroxide solution formed above is treated with gaseous chlorine to obtain sodium chlorate (I).

Cl₂(g) + 2NaOH(aq) -----> NaCl(aq) + NaClO(aq) + H₂O(l)
Task 2.5g (a) What is the purpose of the diaphragm in making chlorine? (b) Why is the liquid level higher in the left than the right. (c) Write the overall balanced equation for the electrolysis of sodium chloride dissolved in water.

2.5h. Uses of chlorine and sodium chlorate (I)
Chlorine: is used

• as a cheap industrial oxidant in the manufacture of bromine because chlorine is a stronger oxidising agent than bromine
• as a bleach because it oxidises coloured dyes to colourless compounds
• as a germicide in the treatment of household water supplies and in swimming baths because it oxidises materials in the cell walls of bacteria hence killing them
• to produce HCl in a reaction with hydrogen
• to manufacture chlorinated hydrocarbon solvents
• to manufacture chloroethene, a precursor for PVC.

• as a bleach in laundries, as a disinfectant and in sewage treatment and as a germicide because it is a strong oxidising agent.
  

Task 2.5h Write balanced chemical equations for making HCl, bromine, Chloroethane and chloroethene giving suitable regents and conditions.