Chlorine is manufactured by the electrolysis of brine. Sodium hydroxide is produced at the same time. Three different methods are currently in use. Information on each is given below. Decide which of the three is the “best” method. NB – consider environmental aspects, energy consumption and production rate.
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Mercury cell electrolysis Also known as the Castner process, in which the "primary cell", titanium anodes are placed in a sodium (or potassium) chloride solution flowing over a liquid mercury cathode. When a potential difference is applied and currentflows, chlorine is released at the titanium anode and sodium (or potassium) dissolves in the mercury cathode forming an amalgam. The mercury is then recycled to the primary cell.
Mercury cell electrolysis ADVANTAGES & DISADVANTAGES The mercury process is the least energy-efficient of the three main technologies and there are also concerns about mercury emissions.
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Diaphragm cell electrolysis A diaphragm separates cathode and anode, preventing the chlorine forming at the anode from re-mixing with the sodium hydroxide and the hydrogen formed at the cathode. The brine is continuously fed to the anode compartment and flows through the diaphragm to the cathode compartment, where the sodium hydroxide is produced.
Diaphragm cell electrolysis – Advantages/disadvantages This method produces alkali that is quite dilute (about 12%) but diaphragm cells do not have the problem of mercury discharge into the environment. They also operate at a lower voltage, resulting in an energy savings but large amounts of steam are required if the sodium hydroxide has to be evaporated to the commercial concentration of 50%.
Membrane cell electrolysis The electrolysis cell is divided into two "rooms" by acation permeable membrane acting as an ion exchanger. Saturated sodium (or potassium) chloride solution is passed through the anode compartment, leaving at a lower concentration. Sodium (or potassium) hydroxide solution is circulated through the cathode compartment, exiting at a higher concentration. A portion of the concentrated sodium hydroxide solution leaving the cell is diverted as product, while the remainder is diluted with deionized water and passed through the electrolyzer again.
Membrane cell electrolysis ADVANTAGES & DISADVANTAGES This method is more efficient than the diaphragm cell and produces very pure sodium (or potassium) hydroxide at about 32% concentration, but requires very pure brine.
SODIUM HYDROXIDE IS ESSENTIAL FOR SOAP MAKING SAPONIFICATION
A soap molecule is a sodium or potassium salt of a long-chain fatty acid.
Non polar ends of soap or detergent molecules at the centre of the micelle are attracted to the non-polar molecules in greasy dirt. The polar ends of the molecules are attracted to the water A SOAP MICELLE The formation of soap micelles allows soap to remove greasy dirt.
A detergent molecule – a sulphate group is attached to a long chain alkyl group.
The contribution of the chemical industry to the development of modern society. • heavy chemicals – used as raw materials in the manufacturing industry. Sulphuric acid and ammonia are heavy chemicals. • fine chemicals – used in printing, agriculture, education, research. • fertilisers – increase agricultural output. • plastics.
SULPHURIC ACID • acts as a drying agent – used by chemical engineers to mop up water. • used as an electrolyte in car batteries (lead accumulator batteries). • used in the recovery of metals such as uranium and copper. • used to clean metal surfaces. • used to produce fertilisers.
Because of its importance, sulfuric acid was considered an excellent indicator of a country's industrial well-being. Below we see the amount of sulfuric acid produced in the United States during a period when it experienced enormous economic growth
AMMONIA Ammonia is produced by means of the Haber process.
Use your knowledge of the principles of chemical equilibrium and the graph, to decide on the most cost effective operating conditions for the production of ammonia.
The production of the raw materials for the Haber Process: Hydrogen production – the gasification of coal
Uses of ammonia • as a cleaning agent when dissolved in water. • in the manufacture of fertilizers. • in the manufacture of ammonium carbonate used in the textile industry. • in the manufacture of nitric acid.
He was a German chemist and lived from 1869 to 1934 He won the Nobel prize for Chemistry in 1918 for inventing the Haber Process He invented the glass cathode used in electrolysis, the firedamp whistle used for the protection of miners and the quartz thread manometer used for recording low gas pressures Fritz Haber, together with Carl Bosch, designed the plants that today produce 150 million tonnes of ammonia around the world per year. He was a friend of Albert Einstein
CHALLENGES FOR THE CHEMICAL INDUSTRY: • safety issues regarding the transport of chemicals. • pollution control. • waste disposal. • energy consumption.
The Fertiliser Industry << Fertilisers Fertiliser manufacture Eutrophication
Eutrophication >> <<
Nitrogen must be supplied to the soil in order to fertilise the soil and optimise agricultural production of foodstuffs. • Fertilisers can be in the form of: • RENEWABLE RESOURCES – compost, guano and manure • NON-RENEWABLE RESOURCES – commercial fertilisers
Commercial fertilisersare described in terms of their relative proportions of nitrogen (N), phosphorus (P) and potassium (K). • NPK numbers are printed on fertiliser bags. • eg20:5:10 means the content of the bag is • 20 % nitrogen • 5 % phosphorus • 10 % potassium
THE PRODUCTION OF NITRIC ACID THE OSTWALD PROCESS:
Sources of phosphorus • bonemeal that was acidified to yield phosphoric acid H3PO4 • mined calcium phosphate Ca3(PO4)2 Calcium phosphate is insoluble in water and cannot be absorbed by plants. Calcium phosphate is treated with sulphuric acid to yield water soluble superphosphate, that can be absorbed. Ca3(PO4)2 + H2SO4 → 2CaSO4 + Ca(H2PO4)2 INSOLUBLE!! SOLUBLE!!
Sources of potassium Salts containing potassium are referred to as POTASH when used as fertilisers. Mined potassium salts, such as KNO3 and K2SO4 are used.
Eutrophication Extensive utilisation of fertilisers leads to pollution of surface and ground water and eutrophication of lakes, dams and rivers. Algal bloom on Hartbeespoort Dam due to fertiliser runoff from farm lands. <<
Eutrophication >> <<