|
Chemguide: Support for CIE A level Chemistry Learning outcome 6(l) This statement is about what substances are produced during electrolysis. Before you go on, you should find and read the statement in your copy of the syllabus. The electrodes - a reminder 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 What gets discharged at the electrodes during electrolysis depends on whether the electrolyte is molten or in solution. It has to be one or the other because otherwise the ions can't move. Electrolysis can't happen unless the ions can move to the electrodes. We will start with the simpler case - where the electrolyte is molten. Molten electrolytes At the cathode Positive ions move to the cathode (the negative electrode). When they get there, they pick up electrons from the electrode to form neutral atoms. For example, electrolysing molten lead(II) bromide gives lead at the cathode:
Or electrolysing molten sodium chloride:
Notice that the gain of electrons at the cathode means that reduction is taking place here. At the anode Negative ions are attracted to the positive anode, and are discharged by losing electrons. For example, in the two cases above, you would get chlorine or bromine formed.
The loss of electrons here means that oxidation is taking place at the anode. Aqueous solutions of electrolytes The situation is more complicated here because of the presence of the water. At the cathode Either the metal is deposited or you get hydrogen produced from the water. Which you get depends on the position of the metal in the electrochemical series and, in some cases, on the concentration of the solution.
At the anode As a general rule, if you have a halogen present, you will get the halogen. With all the other common anions (negative ions), you will get oxygen from the water. But concentration does play a part here. For example, if you have a concentrated solution of sodium chloride, you will get mainly chlorine at the anode. With more and more dilute solutions, you will get less chlorine and more oxygen. Very, very dilute solutions will give mainly oxygen. Another complication occurs if the anode isn't inert. You will have come across this as a part of statement 6(b) during the first half of your course. Copper is purified by making the impure copper the anode during the electrolysis of copper(II) sulphate solution. It might be worth you re-reading the page about copper. You don't need the whole page - just the section about the purification. No negative ions are discharged - instead metal atoms in the anode lose electrons and go into solution. This is still oxidation taking place. Now that you know about electrode potentials, it would be useful to add a bit to this. Here are the half-equations and E° values for zinc, copper and silver:
Notice that zinc forms ions more readily than the copper does. The negative E° value shows that the equilibrium lies further to the left than the copper one. That is why zinc ions go into solution as well as copper ions. Why does the copper go into solution as well as the zinc? There isn't much zinc in the anode, but there is a lot of copper. Imagine it like this: once all the zinc atoms near the surface of the anode have ionised, most of what is left on the surface is copper. There is no option other than to turn it into copper ions. What about the silver? That has an even more positive E° value than the copper. There won't be much silver there, and it will be more difficult to persuade it to form ions.
© Jim Clark 2011 (modified August 2013) |
