Electrolysis and Redox Reactions
Redox Reactions
Redox Reactions:
Where reduction and oxidation tend to take place together.
Oxidation is the loss of electrons, while reduction is the gain of electrons.
OILRIG
Oxidation:
The addition of oxygen to a substance or the loss of electrons from an atom of a molecule.
Example: lead oxide + carbon → lead + carbon monoxide
Example: Fe → Fe^2+ + 2e^-
Reduction:
The removal of oxygen from a substance or the gain of electrons.
The oxidation state of the element decreases.
Example: Cu^2+ + 2e^- → Cu
Redox Equations:
Represents the overall redox reaction.
Balancing involves ensuring the number of electrons lost equals the number gained.
Example: 2Fe + Cu^2+ → 2Fe^2+ + Cu.
Electrolysis
Definition: The decomposition of an ionic compound, when molten or in aqueous solution, by the passage of electric current.
How it works: Electric current is passed through an electrolyte, causing anions(-) to move to the anode(+) and cations(+) to move to the cathode(-).
Anode Reaction
At the anode, oxidation occurs.
Anions move towards the anode.
Anions lose electrons and form new substances.
Cathode Reaction
At the cathode, reduction occurs.
Cations move towards the cathode.
Cations gain electrons and form new substances.
Overall Reaction
The overall reaction involves the transfer of ions from the electrolyte to the electrodes.
Electrodes: Inert(unreactive) electrodes usually are used so they don’t react with the ions in the electrolyte. Graphite and Platinum
Electrolytes:
Electrolytes are substances that conduct electricity when dissolved in water or molten state.
In electrolysis, an electric current is passed through an electrolyte to bring about chemical changes.
The electrolyte dissociates into ions, allowing the flow of electric current.
Common electrolytes used in electrolysis include salts, acids, and bases.
The ions in the electrolyte migrate towards the electrodes to undergo oxidation or reduction reactions.
The type of ions present in the electrolyte determines the products formed during electrolysis.
Half equations:
Anode: 2Cl⁻ → Cl₂ + 2e⁻
Cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻
Substances produced:
Anode: Non-metals or halogens are produced.
Cathode: Metals or hydrogen gas are produced.
Rules for cathode:
If metal ions are present, they will be discharged first.
Hydrogen ions are discharged if no metal ions are present.
Rules for anode:
Halide ions are discharged first, forming halogens.
If no halide ions are present, oxygen or other non-metal ions are discharged.
Electrolysis of Copper Chloride
Process: Electrolysis is the process of using an electric current to drive a non-spontaneous chemical reaction. In the case of copper chloride (CuCl2), when an electric current is passed through a solution of copper chloride, the copper ions (Cu2+) and chloride ions (Cl-) are attracted to the oppositely charged electrodes.
Anode Reaction: At the anode, chloride ions (Cl-) lose electrons and are oxidized to form chlorine gas (Cl2) and release electrons. The half-equation at the anode is:
2Cl- - 2e^-=Cl2
Cathode Reaction: At the cathode, copper ions (Cu2+) gain electrons and are reduced to form solid copper (Cu) which plates out onto the cathode. The half-equation at the cathode is: Cu2+ + 2e- -> Cu
Overall Reaction: The overall reaction for the electrolysis of copper chloride is: Cu2+ (aq) + 2Cl- (aq) -> Cu (s) + Cl2 (g)
Products: At the end of the electrolysis process, copper metal will be deposited at the cathode, and chlorine gas will be evolved at the anode.
Purpose: Electrolysis of copper chloride is used in industries for copper purification and electroplating processes.
Electrolysis II
Aluminium Extraction in Electrolysis
Aluminium is extracted from bauxite ore through electrolysis.
Bauxite is purified to Pure Aluminium Oxide(Al2O3) , which is dissolved in molten cryolite.
The electrolysis process occurs in a cell with carbon electrodes.
At the cathode, aluminium ions are reduced to molten aluminium and are drained off from the electrolysis chamber
At the anode, oxygen is formed from oxide ions but due to the high temperatures, the oxide ions react with the graphite anodes and form carbon dioxide.
Copper Purification and Electroplating
Copper is purified through electrolysis to remove impurities.
Impure copper is the anode, pure copper is the cathode, and copper sulfate is the electrolyte.
At the cathode, the copper ions will be reduced to form pure copper.
To rebalance the imbalance charge, the copper atoms from the anode oxidize to form copper ions and will move into solution to replace the converted ions until all the copper in the solution have been converted into copper metal.
Any impurities in the copper anode fall out as “Anode sludge”
Half Equation at the Cathode:
Cathode Reaction: Cu²⁺(aq) + 2e⁻ → Cu(s)
What Forms at the Cathode: Copper metal is deposited at the cathode.
Half Equation at the Anode:
Anode Reaction: Cu(s) → Cu2+(aq)+2e−
What Forms at the Anode: Oxygen gas is formed at the anode.
Process Overview:
In the electrolysis of copper sulfate solution, copper ions are reduced at the cathode to form solid copper metal, while water molecules are oxidized at the anode to produce oxygen gas and protons.
The impurities in the copper anode settle as sludge beneath the anode, leading to the purification of copper during the electrolysis process.
Electroplating
Electroplating involves the deposition of a thin layer of metal onto a surface.
At the cathode, the metal from the anode will coat it.
The electrolyte will contain the same metal as the one in the anode.
The metal in the anode is oxidized, moves across the solution, and is then reduced back into its elemental metal on the surface of the metal on the cathode.
Electroplating with Tin
Cathode: Tin ions (Sn²⁺) from the electrolyte are reduced and deposited as solid tin on the cathode.
Anode: Tin metal from the anode dissolves into the electrolyte as tin ions (Sn²⁺).
Half Equations:
Cathode: Sn²⁺ + 2e⁻ → Sn (Reduction)
Anode: Sn → Sn²⁺ + 2e⁻ (Oxidation)