This statement is about simple electrolysis calculations. Before you go on, you should find and read the statement in your copy of the syllabus. Electrolysis calculations are no more difficult than any other calculation from an equation. In fact, you may well have done them as a part of whatever course you did before you started doing A level. We will just look at four examples.
You will find a copy of the Data Booklet towards the end of the syllabus. The Data Booklet gives the value F = 9.65 x 10 The first thing to do is to work out how many coulombs of electricity flowed during the electrolysis. Number of coulombs = current in amps x time in seconds Number of coulombs = 0.10 x 10 x 60 = 60 Now look at the equation for the reaction at the cathode: Just as with any other calculation from an equation, write down the essential bits in words: 1 mol of electrons gives 1 mol of silver, Ag. Now put the numbers in. 1 mol of electrons is 1 faraday. 96500 coulombs give 108 g of silver. So, if 96500 coulombs give 108 g of silver, all you have to do is to work out what mass of silver would be produced by 60 coulombs. Mass of silver = 60/96500 x 108 g = 0.067 g | |

Note: If your maths is really bad, so that you aren't happy about simple proportion sums, then think of it like this:
If 96500 coulombs give 108 g, then 1 coulomb would give 108 divided by 96500 g. 60 coulombs would produce 60 times this amount. It doesn't matter in the least how you work this out - all that matters for your chemistry is that you get the answer right! | |

This example shows you how to do the calculation if the product you are interested in is a gas.
The Data Booklet gives the value F = 9.65 x 10 Start by working out how many coulombs of electricity flowed during the electrolysis. Number of coulombs = current in amps x time in seconds Number of coulombs = 1.0 x 15 x 60 = 900 Now look at the equation for the reaction at the cathode: Write down the essential bits in words: 2 mol of electrons give 1 mol of hydrogen, H Now put the numbers in. Two moles of electrons is 2 faradays. 2 x 96500 coulombs give 24 dm So, if 2 x 96500 coulombs give 24 dm Volume of hydrogen = 900/(2 x 96500) x 24 dm Don't quote your answer beyond 2 decimal places. The current and the molar volume are only quoted to that degree of accuracy.
This example shows you what to do if the question is reversed.
The Data Booklet gives the value F = 9.65 x 10 This time you can't start by working out the number of coulombs, because you don't know the time. As with any other calculation, just start from what you know most about. In this case, that's the copper, so start with the electrode equation. Write down the important bits of this in words: 2 mol of electrons give 1 mol of copper, Cu. Now put the numbers in. 1 mol of electrons is 1 faraday. 2 x 96500 coulombs give 63.5 g of copper. You need to work out how many coulombs give 0.635 g of copper. Number of coulombs = 0.635/ 63.5 x 2 x 96500 = 1930 Now what? You know how many coulombs you need, and you know what the current was in amps. You have got all the information you need to work out the time. Number of coulombs = current in amps x time in seconds 1930 = 0.200 x t t = 1930/0.200 = 9650 seconds. Don't waste time trying to convert that into minutes or hours (unless the exam question specifically asks you to).
Another gas example, included because the syllabus mentions the electrolysis of sodium sulphate solution as well as sulphuric acid.
The Data Booklet gives the value F = 9.65 x 10 Start by working out how many coulombs of electricity flowed during the electrolysis. Number of coulombs = current in amps x time in seconds Number of coulombs = 0.50 x 30 x 60 = 900 Now we need to look at the equation for the reaction at the anode. Unfortunately, there are two ways of looking at this, and you may come across either of them. The first one releases oxygen from water molecules: The alternative way releases oxygen from hydroxide ions from the ionisation of the water: Write down the essential bits in words. Both ways of looking at it say the same thing: Releasing 1 mol of oxygen, O Now put the numbers in. Four moles of electrons is 4 faradays. 4 x 96500 coulombs give 24 dm So, if 4 x 96500 coulombs give 24 dm Volume of oxygen = 900/(4 x 96500) x 24 dm Don't quote your answer beyond 2 decimal places. The current and the molar volume are only quoted to that degree of accuracy. To return to the list of learning outcomes in Section 6 To return to the list of all the CIE sections This will take you to the main part of Chemguide.
© Jim Clark 2011 (last modified April 2014) |