Chemguide: Support for CIE A level Chemistry


Learning outcome 6.2(a)

This statement deals with the Faraday constant.

Before you go on, you should find and read the statement in your copy of the syllabus.


Coulombs

The coulomb is a measure of the quantity of electricity. If a current of 1 amp flows for 1 second, then 1 coulomb of electricity has passed.

That means that you can work out how much electricity has passed in a given time by multiplying the current in amps by the time in seconds.

Number of coulombs = current in amps x time in seconds

If you are given a time in minutes or hours or days, then you must convert that into seconds before you do anything else.

For example, if a current of 2 amps flows for an hour, then:

Number of coulombs = 2 x 60 x 60 = 7200

(60 minutes in each hour; 60 seconds in each minute.)

That's easy!


The Faraday

Electricity is a flow of electrons. For calculation purposes, we need to know how to relate the number of moles of electrons which flow to the measured quantity of electricity.

The charge that each electron carries is 1.60 x 10-19 coulombs. You can find that figure in the Data Booklet which you will have in an exam. You will find a copy towards the end of the syllabus.

1 mole of electrons contains the Avogadro constant, L, electrons - that is 6.02 x 1023 electrons. That is also in the Data Booklet.

That means the 1 mole of electrons must carry

6.02 x 1023 x 1.60 x 10-19 coulombs

= 96320 coulombs

This value is known as the Faraday constant.

You will see that the syllabus statement quotes the formula F = Le, where F is the Faraday constant, L is the Avogadro constant and e is the charge on an electron (in terms of the number of coulombs it carries). We have just used that without actually stating it - it is basically obvious!


The numbers we are using here are rounded off. The calculation just shows you how to work it out if you have to, but doesn't give the normally-used value. For exam purposes, and in the Data Booklet, the value of the Faraday constant is given as 9.65 x 104 C mol-1.

That is 9.65 x 104 (or 96500) coulombs per mole.

So 96500 coulombs is called 1 faraday. Notice the small "f" when it is used as a unit.

Whenever you have an equation in which you have 1 mole of electrons, that is represented in an electrical circuit by 1 faraday of electricity - in other words, by 96500 coulombs.


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© Jim Clark 2011 (last modified April 2014)