SPM Chemistry Form 4 Notes – Electrochemistry (Part 8 )

by BerryBerryTeacher

in Berry Reference (Notes)

Half-equations are components that make up an overall reaction equation. Two half-equations (usually written as empirical equations) can be written as a pair in which overall reaction equation is form (must be balanced according to electron charge). The two that can form a pair must each be an oxidation and reduction half-reactions. Usually students taking SPM level chemistry will see this for redox reactions with ions or in acqueous solutions. Berry Berry Easy would like to clear all doubts from Berry Readers regarding this half-equations through Part 8 of the summarised notes for SPM Chemistry Form 4 on Electrochemistry. Also in this notes is the application of what you have learnt through the comparison of electrolytic cells and voltaic cells.

[Tips: Students occasionally forget to use minimum stoichiometric values when combining two half-equations as a pair. Although questions like this occur rarely in SPM level, but they do exist. The reason behind the requirement to write in minimum stoichiometric values is due to the half-equations being empirical in nature.]

SPM Chemistry Form 4 Notes – Electrochemistry (Part 8 )

Voltaic Cell

Voltaic Cell

Berry Important Half Equations

Anode:

  • 2Cl - –> Cl2 + 2e
  • 2Br - –> Br2 + 2e
  • 2I - –> I2 + 2e
  • 4OH- –> 2H2O + O2 + 4e

Cathode:

  • 2H+ + 2e –> H2
  • Zn2+ + 2e –> Zn
  • Fe2+ + 2e –> Fe
  • Pb2+ + 2e –> Pb
  • Cu2+ + 2e –> Cu
  • Ag+ + e –> Ag

How to write overall reaction equation?

Example 1:

  • Anode: 2I - –> I2 + 2e
  • Cathode: 2H+ + 2e –> H2
  • Overall: 2I - + 2H+ –> I2 + H2

Example 2:

  • Anode: 2Br - –> Br2 + 2e
  • Cathode: Pb2+ + 2e –> Pb
  • Overall: 2Br - + Pb2+ –> Br2 + Pb

Example 3:

  • Anode: 4OH- –> 2H2O + O2 + 4e
  • Cathode: 2H+ + 2e –> H2 (x2)
  • Overall: 4OH- + 4H+ –> 2H2O + O2 + 2H2

Example 4:

  • Anode: 4OH- –> 2H2O + O2 + 4e
  • Cathode: Cu2+ + 2e –> Cu (x2)
  • Overall: 2Cu2+ + 4OH- –> 2Cu + 2H2O + O2

.

Comparison of Electrolytic Cells and Voltaic Cells

Similarities:

  • Two electrodes involves in the reaction
  • Electrons flow through the external circuit (connecting wires)
  • Anode (oxidation): loss of electrons
  • Cathode (reduction): gain of electrons

Differences:

Electrolytic Cell Voltaic Cell
Main basic structures A battery is needed to supply electrical energy Battery is not needed.
Energy conversion Electrical energy –> chemical energy Chemical energy –> electrical energy
Transfer of electrons at the positive terminal Anode (positive terminal): Oxidation – anions lose electrons at the anode Cathode (positive terminal): Reduction
Transfer of electrons at the negative terminal Cathode (negative terminal): Reduction – cations accept electrons from the cathode Anode (negative terminal): Oxidation

In the next part, Part 9 of short notes from Berry Berry Easy on Electrochemistry for SPM Chemistry Form 4 students, the main focus will be on the Electrochemical Series which all students love to hate. :-)

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September 8, 2012
September 9, 2012

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