223 words
Suggest why it is necessary to keep the anode and the cathode products apart from each other in the electrolysis of aqueous sodium chloride, and describe how this is done in both the mercury cathode and the membrane cell in the chloro-alkali industry
The products from the membrane cell have to be separated as in a membrane cell when temperature is high, hydrogen and chlorine will explode upon reacting.
This separation is done differently in each cell. In the mercury cathode cell, the products are kept apart by having the products produced in separate containers.
In the membrane cell, the products are separated differently. This is because the products are produced in different halves of the membrane cell. As certain ions can’t pass through the membrane, the products are successfully separated with no mixing or harmful substances used. This means it doesn’t need a large electrical supply to force products apart.
Compare modern methods of extracting chlorine and iodine, relating the methods to the chemistry of these elements.
Most of iodine today is from brine, which contains a high percentage of iodine. Once it has been purified and acidified with sulphuric acid it is reacted with chlorine.
2I-(aq) + Cl2(g) I2(aq) + 2Cl-(aq)
This produces a dilute iodine solution, which has to be concentrated. A stream of air is blown from solution and causes the iodine to vaporise and the iodine-rich air passes into an absorbing tower. Then sulphur dioxide is added to reduce iodine.
SO2(g) + I2(aq) +2H2O(l) 2H+(aq) + 2I-(aq) + H2SO4(aq)
This gives the iodine a much higher concentration than any other process. It is then chlorinated again, so the iodine crystals separate to give a purity of 99.5%.
Another form of iodine extraction is from nitrate ores. The nitrate ion is a very powerful oxidising agent, the iodine in nitrate deposits is in the form of iodate(V) ions IO3-, with 5% of the ore containing sodium iodate. The sodium nitrate ore is leached with water at 40 degrees Celsius and cooled, to create sodium nitrate crystals. A spray of solution is passed in which reduces the iodate(V) ions to iodide ions.
IO3-(aq) + 3SO2(g) + 3H2O(l) I-(aq) + 6H+(aq) + 3SO42-(aq)
This is mixed with some of the original iodate(V) solution to liberate iodine.
5I-(aq) + IO3(aq) + 6H+(aq) 3I2(aq) +3H2O(l)
The iodine separates as a solid so is extracted with kerosene. This is passed into a reactor which heats it to evaporate kerosene ad melt iodine. The iodine is then poured into cold air to give droplets called prill.i
This is the best method of extraction as it has a high purity level and the resources are in plentiful supply. It is also by far the most efficient method available.
409 words
In chlorine extraction. The diaphragm cell is the oldest method and has almost been phased out. This used a porous asbestos barrier to keep products apart.
The mercury cathode cell uses 2 separate containers (see fig 1 and 2 below) to produce chlorine and hydrogen. In the first container, electrolysis takes place. This uses titanium anodes coated in metal oxide. The cathode is a layer of mercury flowing along the base of a container. Brine flows through the cell and the high concentration of chloride ions ensures that they discharge at the anodes, forming chlorine rather than oxygen being produced from the water.
At the mercury cathode, sodium ions discharge in preference to hydrogen ions.
Na+(aq) + e- Na(l)
The sodium dissolves in the mercury forming an amalgam (sodium/mercury mixture) which runs to the second container (decomposer). This is below the electrolyser and water flows into it. Graphite blocks are suspended on the surface of the mercury. It acts like a short-circuited battery. Sodium is oxidised and the ions pass to the solution. The water reacts to form hydrogen gas and hydroxide ions. The ions pass into the solution
2H2O(l) + 2e- H2(g) +2OH-(aq)
The solution leaving the decomposer contains sodium and hydroxide ions. So, the products are chlorine gas and sodium hydroxide solution.
chlorine extraction of late is mostly from membrane cells. This can only happen because of chlorine’s high electronegativity. It forms ionic lattices and can be electrolysed to form chlorine molecules. This is an efficient method and as sodium chloride is plentiful, it won’t run out of resources. This method doesn’t use poisonous substances like the others, so it is the safest method.
286 words
Outline how the methods of extracting iodine changed during the nineteenth and twentieth centuries, suggesting reasons for the changes.
The original extraction of iodine occurred when extra sulphuric acid was added to kelp, which condensing to dark crystals. The kelp had iodine ions absorbed from seaweed and concentrated from biochemical processes. The iodine ions are further concentrated when kelp is burned. The acid acts as an oxidising agent to convert the ions to molecules.
2I- + 3H2SO4(l) I2(g) + 2H2O(l) + SO2(g) + 2HSO4-(s)
8I-(s) + 4H+(aq) + 5H2SO4(l) H2S(g) + 4HSO4-(s) + 4I2(g) + 4H2O(l)
This process was inefficient so more efficient methods were sought after.
Barilla (an alkaline rock) was then found which was a superior source of alkaline ash. This replaced kelp burning entirely, with the discovery of the Leblanc and Solvay processes. It was discovered kelp burnings efficiency could be increased by drying the seaweed and burning in a kiln.
When deposits of sodium nitrate were discovered, along with the evidence it could make plants grow better, mining of these deposits killed kelp burning. This mineral had 640 ppm of iodine and 80% of the world’s iodine came from here.
Newer methods had to be found when deposits were being consumed, so brine from Japan is a new source of iodine.
Represent in appropriate charts the use of chlorine, sodium hydroxide, hydrogen, and iodine in present day industry.
Chlorine
Sodium Hydroxide
Hydrogen
Iodine
200 words
TOTAL WORD COUNT = 1,005
Sources
Article 1- Napoleon’s legacy: riches from the seas
Article 2- The Salt of the Earth
Salters Advanced Chemistry Chemical Ideas book
The Oxford concise science dictionary
MI.5 sheet from Salters Advanced Chemistry
Taken from Oxford concise science dictionary
equations taken from article 2
From MI.5 sheet from salters advanced chemistry.
information above and equation from article 1
Understanding chemistry for advanced level p294
