CIE A Level Chemistry (9701) exams from 2022

Revision Notes

5.4.1 Electrolysis: Calculations

Calculations in Electrolysis

  • Faraday’s constant can be used to calculate:
    • The mass of a substance deposited at an electrode
    • The volume of gas liberated at an electrode

Calculating the mass of a substance deposited at an electrode

  • To calculate the mass of a substance deposited at the electrode, you need to be able to:
    • Write the half-equation at the electrode
    • Determine the number of coulombs needed to form one mole of substance at the specific electrode using Faraday’s constant
    • Calculate the charge transferred during electrolysis
    • Use simple proportion and the relative atomic mass of the substance to find its mass

Worked example: Calculating the mass of a substance deposited at an electrode

Principles of Electrochemistry - Worked example - Calculating the mass of a substance deposited at an electrode, downloadable AS & A Level Chemistry revision notes

Answer

The magnesium (Mg2+) ion is a positively charged cation that will move towards the cathode.

  • Step 1: Write the half-equation at the cathode

Mg2+(aq)          +          2e       →         Mg(s)

1 mol                           2 mol               1 mol

  • Step 2: Determine the number of coulombs required to deposit one mole of magnesium at the cathode

For every one mole of electrons, the number of coulombs needed is 96 500 C mol-1

In this case, there are two moles of electrons required

So, the number of coulombs needed is:

F = 2 x 96 500

F = 193 000 C mol-1

  • Step 3: Calculate the charge transferred during the electrolysis

Q = I x t

Q = 2.20 x (60 x 15)

= 1980 C 

  • Step 4: Calculate the mass of magnesium deposited by simple proportion using the relative atomic mass of Mg

Calculating the mass of a substance deposited at an electrode table

Principles of Electrochemistry - Calculating the mass of a substance deposited at an electrode table, downloadable AS & A Level Chemistry revision notes

Therefore, 0.25 g of magnesium is deposited at the cathode

Calculating the volume of gas liberated at an electrode

  • To calculate the volume of gas liberated at an electrode, you need to be able to:
    • Write the half-equation at the electrode
    • Determine the number of coulombs needed to form one mole of substance at the specific electrode using Faraday’s constant
    • Calculate the charge transferred during electrolysis
    • Use simple proportion and the relationship 1 mol of gas occupies 24.0 dm3 at room temperature

Worked example: Calculating the volume of a gas produced at an electrode

Principles of Electrochemistry - Worked example - Calculating the volume of a gas produced at an electrode, downloadable AS & A Level Chemistry revision notes

Answer

The oxygen gas is formed from the oxidation of negatively charged hydroxide (OH) ions at the anode-

  • Step 1: Write the half-equation at the anode

4OH(aq)         →         O2(g)               +          2H2O(l)            +          4e

4 mol                           1 mol                           2 mol                           4 mol

  • Step 2: Determine the number of coulombs required to form one mole of oxygen gas at the anode

For every one mole of electrons, the number of coulombs needed is 96 500 C mol-1

So, for four moles of electrons, the number of coulombs needed is:

F = 4 x 96 500

F = 386 000 C mol-1

  • Step 3: Calculate the charge transferred during the electrolysis

Q = I x t

Q = 0.75 x (60 x 35)

= 1575 C

  • Step 4: Calculate the volume of oxygen liberated by simple proportion using the relationship 1 mol of gas occupies 24.0 dm3 at room temperature

Calculating the volume of a gas liberated at an electrode table

Principles of Electrochemistry - Calculating the volume of a gas produced at an electrode table, downloadable AS & A Level Chemistry revision notes

Therefore, 0.0979 dm3  of oxygen is formed at the anode

Worked example: Calculating the volume of hydrogen gas produced at an electrode

Principles of Electrochemistry - Worked example - Calculating the volume of hydrogen gas produced at an electrode, downloadable AS & A Level Chemistry revision notes

The hydrogen gas is formed from the reduction of positively charged hydrogen (H+) ions at the cathode

  • Step 1: Write the half-equation at the cathode

2H+ (aq)   +     2e       →         H2 (g)              

2 mols             2 mols             1 mol   

  • Step 2: Determine the number of coulombs required to form one mole of hydrogen gas at the cathode

For every one mole of electrons, the number of coulombs needed is:

F = 96 500 C mol-1

F = 1 x 96 500

F = 96 500 C

So, for two moles of electrons, the number of coulombs needed is:

F = 2 x 96 500

F = 193 000 C

  •  Step 3: Calculate the charge transferred during the electrolysis

Q = I x t

Q = 3.25 x (60 x 17.5)

= 3 413 C

  • Step 4: Calculate the volume of hydrogen liberated by simple proportion using the relationship 1 mol of gas occupies 24.0 dm3 at room temperature

Calculating the volume of hydrogen gas produced at an electrode table

Principles of Electrochemistry - Calculating the volume of hydrogen gas produced at an electrode table

Therefore, 0.42 dm3 of hydrogen is formed at the cathode

 

 

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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