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Chemguide: Support for CIE A level Chemistry Learning outcome 6(b) This statement deals with three industrial applications of electrolysis. Before you go on, you should find and read the statement in your copy of the syllabus. This is quite strange, because they are asking for applications of electrolysis before they ask for any basic electrolysis. You are having to rely on what you may remember from chemistry courses you have taken in the past. So before we do this, we need a quick revision of some basic electrolysis work. Some basic electrolysis Electrolysis Electrolysis is a chemical change caused by passing an electric current through a compound which is either molten or in solution. Electrolytes An electrolyte is a substance which undergoes electrolysis. Electrolytes have to contain ions which are capable of movement - that is why they are either molten or in solution. The current is carried in the electrolyte by the movement of the ions. Electrodes The positive electrode is called the anode; the negative one the cathode. Remember PANC - positive anode, negative cathode. Because positive ions are attracted to the cathode, they are known as cations. Negative ions, attracted to the anode, are known as anions. Discharge of ions at the electrodes At the cathode Positive ions are attracted to the negative cathode. When they get there, they pick up electrons from the cathode, and turn into neutral atoms or molecules. You will get a metal formed if metal ions are discharged, or hydrogen gas if hydrogen ions are discharged. The electrode equations could be, for example:
or:
Notice that in both cases, the ion is gaining electrons. A gain of electrons is reduction. Cathode reactions are always examples of reduction. If there is more than one kind of positive ion present, how do you decide which gets discharged?
At the anode Negative ions are attracted to the positive anode. When they get there, they give up electrons to the anode. The examples you will come across produce either oxygen or a halogen. For example:
In all of these examples, the ion loses electrons, and is therefore oxidised. Anode reactions are examples of oxidation. | |
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Note: If you have just come from learning how to write electron-half-equations, you may wonder why I have written this equation with minus 2e- on the left-hand side rather than the more usual plus 2e- on the right-hand side. The two versions are, of course, exactly equivalent to each other. Just like any other equation, if you moved the electrons to the other side, you would just reverse the sign. We often write equations like this for anode reactions, because it stresses the fact that things lose electrons at the anode. You will find both versions used - either is acceptable. | |
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If there are more than one kind of negative ion present, how do you decide which gets discharged? You are only likely to come across the problem here for aqueous solutions. As a general rule, if there is a halide ion present, you will tend to get the halogen. Otherwise you get oxygen coming from the water. The only halogen you have to worry about for this statement is chlorine. Concentrated solutions of a chloride produce chlorine, but more dilute ones give oxygen as well. There are two ways of thinking about this. For neutral solutions such as sodium chloride solution, CIE prefer you to think of this in terms of a reaction involving the water itself:
But some teachers will explain that the oxygen comes from hydroxide ions from the water.
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Note: The truth is probably more complicated than either of these, and varies with the pH of the solution. Use the version CIE prefers. | |
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Statement 6(b)(i) This is about the electrolysis of brine. Read the page about manufacturing chlorine in the Inorganic Chemistry section of Chemguide. You only need to go as far as the end of the section about the diaphragm cell. You do not need to know about the membrane cell. In addition to what is on that page, in the teacher support material, CIE say they want the overall equation for the cell.
Statement 6(b)(ii) This is about the extraction of aluminium. Read the page about aluminium. There is more on this page than you actually need for this statement. You do not need to know how the bauxite is treated to produce pure aluminium oxide (the Bayer Process). You do need to know about the electrolysis process, including the electrode equations, the use of the cryolite, the need to replace the carbon anodes, and some environmental problems associated with the process. The teacher support material mentions two environmental problems. One is the formation of carbon monoxide from the reaction of the carbon anodes with oxygen. Carbon monoxide is poisonous. They also mention hydrogen fluoride, which is both poisonous and corrosive. I don't really understand where this comes from! There is a loss of fluorine-containing compounds during the electrolysis process which we ignore in the usual equations, but there is no obvious source of hydrogen in the electrolyte which might produce HF (unless perhaps there is a reaction between some of the fluorine-containing compounds and water vapour in the air). Hydrogen fluoride is used during the manufacture of cryolite, but it is hard to see why significant amounts of this would be lost. Don't worry about this - sometimes you just have to tell examiners what they want to hear. You can safely ignore the other environmental stuff and the uses of aluminium. Statement 6(b)(iii) This is about the purification of copper. Read the page about copper. There is much more on this page than you actually need for this statement. All you need to read is the section (about 2/3 of the way down the page) headed "Purification of copper". Read down as far as the green box.
© Jim Clark 2011 (last modified August 2013) |
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