CIE A Level Chemistry (9701) exams from 2022

Revision Notes

5.3.2 Faraday's Law & Avogadro

Faraday's Law

  • The amount of substance that is formed at an electrode during electrolysis is proportional to:
    • The amount of time where a constant current to passes
    • The amount of electricity, in coulombs, that passes through the electrolyte (strength of electric current)
    • The relationship between the current and time is:

Q = I x t

Q = charge (coulombs, C)

I = current (amperes, A)

t = time, (seconds, s)

  • The amount or the quantity of electricity can also be expressed by the faraday (F) unit
    • One faraday is the amount of electric charge carried by 1 mole of electrons or 1 mole of singly charged ions
    • 1 faraday is 96 500 C mol-1
  • Thus, the relationship between the Faraday constant and the Avogadro constant (L) is:

F = L x e

F = Faraday’s constant (96 500 C mol-1)

L = Avogadro’s constant (6.022 x 1023 mol-1)

e = charge on an electron

Worked example: Determining the amount of electricity required

Principles of Electrochemistry - Worked example - Determining the amount of electricity required, downloadable AS & A Level Chemistry revision notes

Answer

One Faraday is the amount of charge (96 500 C) carried by 1 mole of electrons

Answer 1
As there is one mole of electrons, one faraday of electricity (96 500 C) is needed to deposit one mole of sodium.

Answer 2
Now, there are two moles of electrons, therefore, two faradays of electricity (2 x 96 500 C) are required to deposit one mole of magnesium.

Answer 3
Two moles
of electrons are released, so it requires two faradays of electricity (2 x 96 500 C) to form one mole of fluorine gas.

Answer 4
Four moles
of electrons are released, therefore it requires four faradays of electricity (4 x 96 500 C) to form one mole of oxygen gas.

Determining Avogadro's Constant by Electrolysis

  • The Avogadro’s constant (L) is the number of entities in one mole
    • L = 6.02 x 1023 mol-1
    • For example, four moles of water contains 2.41 x 1024 (6.02 x 1023 x 4) molecules of H2O
  • The value of L (6.02 x 1023 mol-1) can be experimentally determined by electrolysis using the following equation:

Determining Avogadros Constant by Electrolysis equation 1

Finding L experimentally

  • The charge on one mole of electrons is found by using a simple electrolysis experiment using copper electrodes

Principles of Electrochemistry - Apparatus Electrolysis, downloadable AS & A Level Chemistry revision notes

Apparatus set-up for finding the value of L experimentally

  • Method
    • The pure copper anode and pure copper cathode are weighed
    • A variable resistor is kept at a constant current of about 0.17 A
    • An electric current is then passed through for a certain time interval (e.g. 40 minutes)
    • The anode and cathode are then removed, washed with distilled water, dried with propanone, and then reweighed
  • Results
    • The cathode has increased in mass as copper is deposited
    • The anode has decreased in mass as the copper goes into solution as copper ions
    • Often, it is the decreased mass of the anode which is used in the calculation, as the solid copper formed at the cathode does not always stick to the cathode properly
    • Let’s say the amount of copper deposited in this experiment was 0.13 g
  • Calculation:
    • The amount of charge passed can be calculated as follows:

Q = I x t

= 0.17 x (60 x 40)

= 408 C

  • To deposit 0.13 g of copper (2.0 x 10-3 mol), 408 C of electricity was needed
  • The amount of electricity needed to deposit 1 mole of copper can therefore be calculated using simple proportion using the relative atomic mass of Cu

Calculating the amount of charge required to deposit one mole of copper table

Principles of Electrochemistry - Calculating the amount of charge required to deposit one mole of copper table, downloadable AS & A Level Chemistry revision notes

  • Therefore, 199 292 C of electricity is needed to deposit 1 mole of Cu
  • The half-equation shows that 2 mol of electrons are needed to deposit one mol of copper:

Cu2+(aq) +  2e →   Cu(s)

  • So, the charge on 1 mol of electrons is:

= 99 646 C

  • Given that the charge on one electron is 1.60 x 10-19 C, then L equals:

= 6.23 x 1023 mol-1

  • The experimentally determined value for L of 6.23 x 1023 mol-1 is very close to the theoretical value of 6.02 x 1023 mol-1

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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