Topic 24 - Electrochemistry
24.1 Electrolysis
● Electrolysis is the breaking down of a compound into its elements using an electric
current
Why do we use electrolysis:
1. Extract metals from their metal ores when the metals cannot be extracted by heating
their ores with carbon
2. Purify metals
3. Produce non-metals
Electrolysis is carried out in an electrolytic cell
● An electrolyte: This is the compound that is broken down during electrolysis
○ A molten ionic compound
○ A concentrated aqueous solution of ions
● Electrodes: These are metal or graphite rods conduct electricity to the electrolyte and
away from the electrolyte
○ Positive electrode: Anode
○ Negative electrode: Cathode
● The power supply: is direct current
Electrolysis of Molten Ionic Compounds
● An ionic compound consists of both a cation and anion
○ Cation: Positive charge
○ Anion: Negative charge
● Since opposite charges attract
○ Cation moves to the anode
○ Anion moves to the cathode
, ANODE CATHODE
Its charge Negative (-ve) Positive (+ve)
It attracts Cations (+ve) Anions (-ve)
Type of reaction Oxidation Reduction
Electron involvement Loses electron Gains electrons
Electrolysis of aqueous solutions
● In an aqueous solution, beside the ions of the solution itself, we have Hydrogen and
hydroxide ions (H+ and OH–)
● Water is a weak electrolyte
● The result of the electrolysis depends on
○ The electrode potential
■ The ion with higher electrode potential is deposited at the electrode
○ The concentration of ions
■ Ions with a higher concentration are more likely to be discharged
■ If the solution is dilute: oxygen is deposited at the Anode
○ Reactivity compared to hydrogen
■ If it is more reactive, hydrogen is deposited
■ If it is less reactive, it is deposited
, 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 passes
○ The amount of charge that passes through the electrolyte (strength of electric
current)
○ Q = Charge in Coulombs / C
○ I = Current in amperes / A
○ t = Time in seconds / s
● The amount or the quantity of electricity can also be expressed by the faraday's unit
● One faraday is the amount of electric charge carried by 1 mole of electrons
○ 1 faraday = 96’500 C.mol-1
○ F = Faraday’s Constant / 96’500 C.mol-1
○ L = Avogadro's Constant / 6.022x1023
○ e = Charge on the electron / 1.6x10-19
Determining Avogadro's constant by electrolysis
● Moles of electrons = Charge / Faraday's constant
24.1 Electrolysis
● Electrolysis is the breaking down of a compound into its elements using an electric
current
Why do we use electrolysis:
1. Extract metals from their metal ores when the metals cannot be extracted by heating
their ores with carbon
2. Purify metals
3. Produce non-metals
Electrolysis is carried out in an electrolytic cell
● An electrolyte: This is the compound that is broken down during electrolysis
○ A molten ionic compound
○ A concentrated aqueous solution of ions
● Electrodes: These are metal or graphite rods conduct electricity to the electrolyte and
away from the electrolyte
○ Positive electrode: Anode
○ Negative electrode: Cathode
● The power supply: is direct current
Electrolysis of Molten Ionic Compounds
● An ionic compound consists of both a cation and anion
○ Cation: Positive charge
○ Anion: Negative charge
● Since opposite charges attract
○ Cation moves to the anode
○ Anion moves to the cathode
, ANODE CATHODE
Its charge Negative (-ve) Positive (+ve)
It attracts Cations (+ve) Anions (-ve)
Type of reaction Oxidation Reduction
Electron involvement Loses electron Gains electrons
Electrolysis of aqueous solutions
● In an aqueous solution, beside the ions of the solution itself, we have Hydrogen and
hydroxide ions (H+ and OH–)
● Water is a weak electrolyte
● The result of the electrolysis depends on
○ The electrode potential
■ The ion with higher electrode potential is deposited at the electrode
○ The concentration of ions
■ Ions with a higher concentration are more likely to be discharged
■ If the solution is dilute: oxygen is deposited at the Anode
○ Reactivity compared to hydrogen
■ If it is more reactive, hydrogen is deposited
■ If it is less reactive, it is deposited
, 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 passes
○ The amount of charge that passes through the electrolyte (strength of electric
current)
○ Q = Charge in Coulombs / C
○ I = Current in amperes / A
○ t = Time in seconds / s
● The amount or the quantity of electricity can also be expressed by the faraday's unit
● One faraday is the amount of electric charge carried by 1 mole of electrons
○ 1 faraday = 96’500 C.mol-1
○ F = Faraday’s Constant / 96’500 C.mol-1
○ L = Avogadro's Constant / 6.022x1023
○ e = Charge on the electron / 1.6x10-19
Determining Avogadro's constant by electrolysis
● Moles of electrons = Charge / Faraday's constant
