The diagram shows an aluminium oxide electrolysis cell. The negative electrodes cathodes and the positive electrodes anodes are made of graphite, a form of carbon. During electrolysis:. The oxygen reacts with the carbon in the electrodes, forming carbon dioxide which bubbles off.
Carbon is therefore lost from the positive electrodes, so they must be replaced frequently. This adds to the cost of the process. Extracting metals using electrolysis Metals are found in ores combined with other elements. Electrolysis is a chemical process where electricity is passed through an electrolyte, causing it to decompose into its constituent elements. The picture below shows an opencut mine in Kazakhstan where bauxite, the most abundant of all the aluminum ores, is mined.
Bauxite can be thought of as impure aluminum oxide that gets its red-brown color, clearly seen in the photograph, from impurities of iron III oxide. Aluminum smelting plants are enormous and use a tremendous amount of space. The picture below shows one such plant in Russia. They consist of multiple banks of cells and casting facilities as well as anode production areas. But before the extraction process can begin, the bauxite ore must first be purified into pure alumina, A l O 2 3.
The purification of alumina from bauxite is known as the Bayer process and revolves around a reaction between the aluminum oxide and sodium hydroxide. The alumina will be decomposed using electrolysis, a costly industrial process with the plants using vast amounts of electricity. An aluminum smelting plant uses around MW of power each year , so let us remind ourselves why it is necessary to use electrolysis for the extraction of aluminum.
Aluminum is a very active metal that is more reactive than reducing agents such as carbon and hydrogen. For example, it is not possible to reduce alumina using carbon as we can with other metal ores such as hematite. Hematite is an iron ore containing iron III oxide, which can be reduced using carbon in the blast furnace to form iron metal. So for aluminum and other reactive metals such as sodium, we must use the process of electrolysis.
For electrolysis to happen on such a large industrial scale, multiple electrolytic cells are required. Each cell consists of a steel shell lined with heat resistant bricks followed by a layer of carbon that acts as the negative cathode for the electrolysis process. Molten cryolite, N a A l F 3 6 , fills the cells and serves multiple purposes, which include helping the alumina conduct electricity and effectively lowering the melting point of the alumina.
The alumina dissolving into the cryolite creates the electrolyte mixture for this reaction. Which of the following is not a reason why cryolite is part of a molten electrolyte in the extraction of aluminum? The question is asking us which statement is not a reason why cryolite is added to each of the cells during the electrolysis process. Answer A suggests that cryolite reduces the temperature at which the alumina melts. This reduction would have cost implications as less energy is used so this is likely a good reason for having cryolite as part of the electrolyte mixture, meaning that A is not the correct answer.
We know that cryolite increases the conductivity of the electrolyte as stated in answer B and so answer B is not the correct answer either. The reasons behind this are quite complex but essentially cryolite increases the mobility of the ions in the electrolyte. Answer D suggests that cryolite increases the amount of electricity used; however, the amount of electricity would not be affected by a substance that is not involved in either of the reactions at the two electrodes, so this is likely the correct answer, but we will also check answer E.
Aluminum has a very high melting point and strong bonding between atoms, so it doesn't readily dissolve in water. Instead, molten cryolite Na 3 AIF 6 is used. The pure aluminum is attracted to the cathode, which is a lining of graphite. The oxygen is attracted to the anode, and bubbles through the solution. At the anode , oxidation takes place as electrons are lost:. At the anode also, the oxygen formed will react with the anode which is made of carbon to form carbon dioxide.
This means the anodes must be frequently replaced. This process uses a lot of electricity and is expensive. Therefore, aluminum is much more expensive than other metals that are easier to extract like iron , but its desirable characteristics mean that it is still widely used.
Was this helpful? Yes No I need help Uses Aluminum is the most widely used metal after iron. It is mostly used in an alloy with another metal, this means it is mixed with another metal to produce another compound that has a certain desirable characteristic - like stainless steel.
Yes No I need help Some common uses of aluminum include making cars, trains, and bicycles. Because it's reasonably strong, but not too heavy, your aluminum bicycle won't break and won't be too difficult to ride. Some packaging like foil and cans are also made from aluminum. This is especially important in recycling since some soft drink cans are made of steel rather than aluminum - but they can be sorted using magnets. Cooking utensils are often made from aluminum because it is very good at conducting heat, and will warm the food evenly.
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