Thursday, 26 February 2015

3.4 Redox Reactions in Electrolytic and Chemical Cells

Redox reaction in an electrolytic cells
■ Redox reaction in electrolysis

Both molten state and aqueous solution:
Cathode: Reduction occurs (electron acceptance occurs)
Anode: Oxidation occurs (electron loss occurs)

Electrons flow through the external circuit from the anode of a reducing agent to the oxidising agent in the cathode.
■ Summary of the redox reaction for the electrolysis of molten lead(II) bromide

Anode → electron loss occurs

Cathode → electron acceptance occurs

Half ionic equation:
4OH(aq) → O2(g) + H2O(l) + 4e (oxidation → reducing agent: hydroxide ions, OH)
2H+(aq) + 2e → H2(g) (reduction → oxidising agent: hydrogen ions, H+)

Overall equation:
4OH(aq) + 4H+(aq) → O2(g) + H2O(l) + 2H2(g)

Colourless gas bubbles are liberated at cathode electrode anode electrode.

Gas liberated at cathode electrode rekindles a glowing splinter. Gas liberated at anode produces 'pop' sound when tested with a lighted splinter.


Redox reaction in chemical cells
■ Simple voltaic cell (chemical cell)

Consists of two different metal electrodes.

Connected to the end of the wire and the other end dipped into an electrolyte solution.

Potential difference generated depends on the difference in the position of the metal used in the electrochemical series.
Cathode: Reduction occurs(electron acceptance occurs) → positive terminal
Anode: Oxidation occurs(electron loss occurs) → negative terminal
■ Daniel cell

Consists of two different metal electrodes immersed in two separate electrolyte solutions.

Electrolyte solution can be separated either by a porous pot or a salt bridge

Function of porous pot or salt bridge: to separate two electrolyte solutions but allow ions to pass through to complete a circuit.

Chemical energy → electrical energy.
■ Summary of the redox reaction for a zinc-copper Daniel cell

Arrangement of a Daniel cell (Electrolyte solution separated by a salt bridge or porous pot)

Negative terminal → electron loss occurs

Positive terminal → electron acceptance occurs

Half ionic equation:
Zn(s) → Zn2+(aq) + 2e (oxidation → reducing agent: zinc)
Cu2+(aq) + 2e → Cu(s) (reduction → oxidising agent: copper(II) ions, Cu2+)

Overall equation:
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

Observation:
Zinc rod dissolves in the solution at negative terminal.
Blue copper(II) sulphate solution fades and a brown solid is formed at positive terminal.
■ This video contains information on the redox reaction in chemical cell



Redox reaction in other voltaic cells
■ Redox reaction in other voltaic cells

The electron flow from negative terminal to positive terminal to produces electrical energy.
■ Summary of the redox reaction for dry cell

Negative terminal → electron loss occurs

Positive terminal → electron acceptance occurs

Half ionic equation:
Zn(s) → Zn2+(aq) + 2e (oxidation → reducing agent: zinc)
2MnO2(s) + 2NH4+ + 2e → Mn2O3(s) + 2NH3(g) + H2O(l) (reduction → oxidising agent: manganese (IV) oxide, MnO2)

Overall equation:
Zn(s) + 2MnO2(s) + 2NH4+ → Zn2+(aq) + Mn2O3(s) + 2NH3(g) + H2O(l)
■ Summary of the redox reaction for alkaline cell

Negative terminal → electron loss occurs

Positive terminal → electron acceptance occurs

Half ionic equation:
Zn(s) → Zn2+(aq) + 2e (oxidation → reducing agent: zinc)
2MnO2(s) + H2O(l) + 2e → Mn2O3(s) + OH(aq) (reduction → oxidising agent: manganese (IV) oxide, MnO2)

Overall equation:
Zn(s) + 2MnO2(s) + H2O(l) → Zn2+(aq) + Mn2O3(s) + OH(aq)
■ Summary of the redox reaction for mercury cell

Negative terminal → electron loss occurs

Positive terminal → electron acceptance occurs

Half ionic equation:
Zn(s) → Zn2+(aq) + 2e (oxidation → reducing agent: zinc)
HgO(s) + H2O(l) + 2e → Hg(l) + 2OH(aq) (reduction → oxidising agent: mercury (II) oxide, HgO)

Overall equation:
Zn(s) + HgO(s) + H2O(l) → Zn(OH)2(aq) + Hg(l)
■ Summary of the redox reaction for accumulator

Negative terminal → electron loss occurs

Positive terminal → electron acceptance occurs

Half ionic equation:
Pb(s) → Pb2+(aq) + 2e (oxidation → reducing agent: lead)
PbO2(s) + 4H+(aq) + 2e → Pb2+(aq) + 2H2O(l) (reduction → oxidising agent: lead (IV) oxide, PbO2)

Overall equation:
Pb(s) + PbO2(s) + 4H+(aq) → 2Pb2+(aq) + 2H2O(l)
■ Summary of the redox reaction for nickel-cadmium cell

Negative terminal → electron loss occurs

Positive terminal → electron acceptance occurs

Half ionic equation:
Cd(s) + 2OH → Cd(OH)2(s) + 2e (oxidation → reducing agent: cadmium)
NiO2(s) + 2H2O(l) + 2e → Ni(OH)2(s) + 2OH (reduction → oxidising agent: nickel (IV) oxide, NiO2)

Overall equation:
Cd(s) + NiO2(s) + 2H2O(l) → Cd(OH)2(s) + Ni(OH)2(s)
■ This video contains information on the redox reaction in an electrolytic cell
Laboratory Activity 3.4.1: Redox reactions in electrolytic and chemical cells


Differences between Chemical Cells and Electrolytic Cells
■ Similarities of electrolytic cell and voltaic cell

Oxidation occurs in the anode while reduction occurs in the cathode

Electrons transfer from anode to cathode

Oxidation involves the lost of electrons while reduction involves obtaining electrons
■ Electrolytic cell differs from voltaic cell in some aspects:

Differences Electrolytic cell Voltaic cell
Cell structure
Made of Carbon or different/same metal immersed in the electrolyte Two different metals immersed in the electrolyte
Electric current Produces chemical reaction Produced by chemical reaction
Energy changes Electrical energy to chemical energy Chemical energy to electrical energy
Cathode Negative terminal (Cations receive electrons → reduction) Positive terminal (Electrons are accepted → reduction)
Anode Positive terminal (Anions release electrons → oxidation) Negative terminal (Electrons are released → oxidation)
Requirement of battery Uses battery Does not use battery
Electron flow From positive terminal to negative terminal From negative terminal to positive terminal
■ This video contains information on the redox reaction in an electrolytic cell
Worked-example 3.4(a)
Choose true or false for the statement given.
Question Answer
An electrolytic cell produces electrical energy from chemical energy
The cathode is the negative terminal for both electrolytic and chemical cell.
The flow of electrons in a chemical cell is opposite to the flow of electrons in electrolytic cell.
The oxidation process occurs at the anode for both electrolytic and chemical cell.
In both electrolytic and chemical cell, the transfer of electrons occurs from the reducing agent to oxidising agent.


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