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Describe the reactions which take place at the electrodes in the membrane cell and explain why they are redox reactions.

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Introduction

Skills for Chemistry: Open-book Paper 2002 Describe the reactions which take place at the electrodes in the membrane cell and explain why they are redox reactions. A membrane cell uses electricity and various reactants to produce chlorine gas, hydrogen gas and hydroxide ions. A membrane is a tissue which connects biological cells to their organs and other structures. They are permeable to water but impermeable to polar substances. Movement of ions across a membrane occurs by carrier-meditated processesi. In the membrane cell, redox reactions occur at the anode and cathode. At the anode, the following reaction occurs. 2Na+(aq) + 2Cl-(aq) 2Na+(aq) + Cl2(g) + 2e- ii the ionic equation is 2Cl-(aq) Cl2(g) + 2e- Membranes are synthetic polymers and have the ability to transport cations rather than anions. So this allows the Na+ ions to move across it and prevents the Cl- ions from passing through. At the titanium anode, the sodium hydroxide provides the Na+ ions for the reaction as they can pass through the membrane. The brine supplies the Cl- ions so the reaction can take place. The Cl- ions then lose their electrons due to attractive forces with the electrode and join to form a Cl2 molecule. At the cathode, this reaction takes place 2Na+(aq) + 2H2O(l) + 2e- H2(g) + 2Na+(aq) +2OH-(aq) the ionic equation is 2H2O(l) + 2e- H2(g) ...read more.

Middle

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) ...read more.

Conclusion

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 i Taken from Oxford concise science dictionary ii equations taken from article 2 iii From MI.5 sheet from salters advanced chemistry. iv From article 2 v information above and equation from article 1 vi Understanding chemistry for advanced level p294 vii With help from article 2 viii From article 1 ?? ?? ?? ?? Open-book Paper 2002 Fettes College Gordon Kerr 1 ...read more.

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