SPM Chemistry Form 5 Notes – Terminology and Concepts: Oxidation and Reduction (Part 4)

by BerryBerryTeacher

in Berry Reference (Notes)

Welcome back to Berry Berry Easy. Today, the Berry Berry Teacher will like to share with all Berry Readers, Part 4 of the SPM Form 5 Chemistry notes on “Oxidation and Reduction”. In the previous part, oxidation and reduction were viewed in terms of changes made to the oxidation numbers. However, we also mentioned that it can also be understood through electron transfer. So yes, you guessed it! This part will focus on redox reaction in terms of electron transfer. Similar to very easy “change in oxidation number”, understanding redox reactions in terms of electron change are just as easy.

Do also try to memorise the table presented at the end of this post. Most of the time, objective questions will ask about the colours of the precipitation when iron (II) or iron (III) ions are present in the various reagents. So don’t forget this tips. Do pop quiz with your friends to memorise the table below.

SPM Chemistry Form 5 – Terminology and Concepts: Oxidation and Reduction (Part 4)

Rusting of Iron Nails

Rusting of Iron Nails

Oxidation and Reduction in Terms of Electron Transfer

2I- (aq) –> I2 (aq) + 2e
Oxidation: Iodide ion, I- is a reducing agent because it donates/loses electrons to become I2.

Br2 + 2e –> 2Br- (aq)
Reduction: Bromine water, Br2 is an oxidising agent because it receives/accepts electrons from I- to form bromide ions, Br-.

–> Overall reaction: 2I- (aq) + Br2 –> I2 (aq) + 2Br- (aq)

Conversion of Fe2+ Ions to Fe3+ Ions and Vice Versa

A) Common oxidising agent (change Fe2+ ions to Fe3+ ions):

  • bromine, Br2
  • chlorine, Cl2
  • hydrogen peroxide, H2O2
  • concentrated nitric acid, HNO3
  • acidified potassium manganate(VII), KMnO4 solution
  • acidified potassium dichromate(VI), K2Cr2O7 solution

Fe2+ (aq) –> Fe3+ (aq) + e
Oxidation: Iron(II) ion, Fe2+ is a reducing agent because it donates/loses one electron to become Fe3+.

Br2 (aq) + 2e –> 2Br- (aq)
Reduction: Bromine water, Br2 is an oxidising agent because it receives/accepts electrons from Fe2+ to form bromide ions, Br-.

–> Observation: iron(II) sulphate solution changes colour from pale green to yellowish-brown.
–> Overall reaction: 2Fe2+ (aq) + Br2 (aq) –> 2Fe3+ (aq) +2Br- (aq)

B) Common reducing agent (change Fe3+ ions to Fe2+ions):

  • zinc powder, Zn
  • aluminium, Al
  • Magnesium, Mg
  • Calcium, Ca
  • Sulphur dioxide, SO2
  • Hydrogen sulphide, H2S
  • Sodium sulphide solution, Na2SO3
  • Tin(II) chloride solution, SnCl2

Zn (s) –> Zn2+ (aq) + 2e
Oxidation: Zinc powder, Zn is a reducing agent because it donates/loses two electrons to form zinc ions, Zn2+.

Fe3+ (aq) + e –> Fe2+ (aq)
Reduction: Iron(III) ion, Fe3+ is an oxidising agent because it receives/accepts one electron to become Fe2+.

–> Observation: iron(III) sulphate solution changes colour from yellowish-brown to pale green.
–> Overall reaction: 2Fe3+ (aq) + Zn (aq) –> 2Fe2+ (aq) + Zn2+ (aq)

C) Investigate the presence of iron(II) and iron(III) ions

Reagent Ions Observations
NaOH solution / NH3 solution Fe2+ Green precipitate,insoluble in excess alkali
NaOH solution / NH3 solution Fe3+ Brown precipitate,insoluble in excess alkali
Potassium hexacyanoferrate(II) solution Fe2+ Light blue precipitate
Potassium hexacyanoferrate(II) solution Fe3+ Dark blue precipitate
Potassium hexacyanoferrate(III) solution Fe2+ Dark blue precipitate
Potassium hexacyanoferrate(III) solution Fe3+ Greenish-brown solution
Potasium / Ammonium thiocyanate solution Fe2+ Pale red colouration
Potasium / Ammonium thiocyanate solution Fe3+ Blood-red colouration

The next Part in this series contains arguably the most important and memorable “series” in your SPM Chemistry studies, namely the Electrochemical Series. It’ll be something that you’ll memorise even after you leave school. So stay tune and log in frequently to Berry Berry Easy.

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