SPM Chemistry Form 5 Notes – Terminology and Concepts: Oxidation and Reduction (Part 11)

by BerryBerryTeacher

in Berry Reference (Notes)

What is the differences between an electrolytic cell and a chemical cell? Most student will answer that electrolytic cells involves electrical supply while a chemical cell do not. While structurally that is the correct answer but most students do not remember the other differences or similarities between the two. So,the Berry Berry Teacher summarises everything for our Berry Readers, in turn creating Part 11 of the SPM Form 5 Chemistry notes on “Oxidation and Reduction” – redox reactions in electrolytic cell and chemical cell. This subtopic is a popular topic in SPM Chemistry exams, so students should understand and memorise the tables of differences given below. Do also understand and memorise the similarities between the two different type of cells. Examples of redox reactions in the electrolytic cell and chemical cell are given after the table which is useful for elaboration of points in an essay or strutural question.

SPM Chemistry Form 5 – Terminology and Concepts: Oxidation and Reduction (Part 11)

Rusting of Iron Nails

Rusting of Iron Nails

Redox Reactions in Electrolytic Cell and Chemical Cell

Similarities

  • redox reaction.
  • Anode: oxidation
  • Cathode: reduction
  • Electrons flow from anode to cathode in the external circuit

Differences

Differences Electrolytic Cell (Electrolysis) Chemical Cell / Voltaic Cell
Structure With electrical supply. No electrical supply.
Electrodes Can be the same or difference metal (graphite or platinum). Must be two different metals.
Flows of electrons From anode to cathode through external circuit. From more electropositive metal to less electropositive metal through external circuit.
Transformation of energy Electrical energy to chemical energy. Chemical energy to electrical energy.
At positive terminal Anode.Oxidation occurs. Anions release electrons at the anode. Cathode.Reduction occurs.

Oxidising agent gain electrons.

At negative terminal Cathode.Reduction occurs.

Cations gain electrons from the cathode.

Anode.Oxidation occurs.

Reducing agent releases electrons.

1) Redox Reactions in Electrolytic Cell

Example 1: Electrolysis of molten zinc chloride

  • Electrodes: Carbon
  • Ions present: Cl- and Zn2+
  • Anode: Oxidation / 2Cl-(l) –> Cl2(g) + 2e / Cl- ions act as reducing agent.
  • Cathode: Reduction / Zn2+(l) + 2e –> Zn(s) / Zn2+ ions act as oxidising agent.

Example 2: Electrolysis of copper(II) sulphate solution.

  • Electrodes: Carbon
  • Ions present: Cu2+, SO42-, H+, OH-
  • OH- ions are discharged because OH- ion is below SO42- ion in electrochemistry series.
    Anode
    : Oxidation / 4OH-(aq) –> O2(g) + 2H2O(l) + 4e / Oxygen gas is liberated.
  • Cu2+ ions are discharged because Cu2+ ion is below H+ ion in electrochemistry series.
    Cathode
    : Reduction / Cu2+(aq) + 2e –> Cu(s) / Cu2+ ions are reduced to copper metal (brown layer formed).
  • –> Overall equation: Cu2+(aq) + 4OH-(aq) –> O2(g) + 2H2O(l) + Cu(s)

Example 3: Electrolysis of copper(II) sulphate solution.

  • Electrodes: Copper
  • Ions present: Cu2+, SO42-, H+, OH-
  • OH- ions  and SO42- ion are not discharged.
    Anode
    : Oxidation / Cu(s) –> Cu2+(aq) + 2e / Copper anode (electrode) dissolves.
  • Cu2+ ions are discharged because Cu2+ ion is below H+ ion in electrochemistry series.
    Cathode
    : Reduction / Cu2+(aq) + 2e –> Cu(s) / Cu2+ ions are reduced to copper metal.

Example 4: Electrolysis of concentrated sodium chloride solution.

  • Electrodes: Carbon
  • Ions present: Na+, Cl-, H+, OH-
  • Cl- ions are discharged because of the higher concentration. (Concentration of Cl- ion is high, the ion is selectively discharged rather than the OH- ion, the one that is placed below the electrochemical series.)
    Anode
    : Oxidation / 2Cl-(aq) –> Cl2(g) + 2e / Chlorine gas (green gas with choking smell) is liberated.
  • H+ ions are discharged because Na+ ion is below H+ ion in electrochemistry series. (H+ ions and Na+ ion are placed very far apart in the electrochemical series, the concentration factor becomes unimportant.)
    Cathode
    : Reduction / 2H+(aq) + 2e –> H2(g) / Hydrogen gas is liberated.
  • –> Overall equation: 2Cl-(aq) + 2H+(aq) –> Cl2(g) + H2(g)

2) Redox Reactions in Chemical Cell

Example 1: Daniel cell

  • Anode (negative terminal): Oxidation / Zinc strip immerses in zinc sulphate solution.
    Zn(s) –> Zn2+(aq) + 2e / Zinc strip becomes thinner.
  • Cathode (positive terminal): Reduction / Copper strip immerses in copper(II) sulphate solution.
    Cu2+(aq) + 2e –> Cu(s) / A brown layer formed around copper strip. / Concentration Cu2+ ions decreases cause the intensity blue colour of solution decreases.
  • Zinc is more electropositive than copper. Electrons are flowed from zinc strip to copper strip through the external circuit. (Note: Conventionally, electrons flow in the opposite direction of electrical current).
  • –> Overall equation:  Zn(s) + Cu2+(aq) –> Zn2+(aq) + Cu(s)

Good news to all Berry Readers, as the series involving “Oxidation and Reduction” will end with the next post with the various chemical cells that you would encounter in daily life.

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September 7, 2012

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