Redox chemistry

About oxidation and reduction etc.

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  • Created by: Niki :)
  • Created on: 13-03-13 09:21

The basics

Oxidation numbers - the charge an element would have if the electrons in each bond of the molecule or ion belonged to the more electronegative element.

  • Uncombined element - always 0
  • H = +1 (except when bonded to a metal)
  • O = -2 (except when bonded to F or in peroxides, e.g. Na2O2)

O I L R I G: OXIDATION is loss, REDUCTION is gain. 

Disproportionation - reaction in which the same substance is both oxidised and reduced.

Redox - involves both reduction and oxidation.

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Potassium manganate(VII) titrations

With iron(II) ions: self-indicating

  • reduction: MnO4-(aq) + 8H+(aq) + 5e- --> Mn2+(aq) + 4H2O(l)
  • oxidation: Fe2+(aq) --> Fe3+(aq) + e-

MnO4-(aq) + 5Fe2+(aq) + 8H+(aq) --> Mn2+(aq) + 4H2O(l) + 5Fe3+(aq)

With ethanedioic acid: autocatalysis

  • reduction: MnO4-(aq) + 8H+(aq) + 5e- ---> Mn2+(aq) + 4H2O(l)
  • oxidation: C2O42-(aq) --> 2CO2(g) + 2e-

MnO4-(aq) + 5C2O42-(aq) + 16H+(aq) + 5e- --> Mn2+(aq) + 10CO2(g) + 8H2O(l)

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With sodium thiosulfate

Iodometric

  • 2S2O32-(aq)  → S4O62-(aq) + 2e-
  • I2(aq) + 2e- → 2I-(aq)

I2(aq)  +  2S2O32-(aq) 2I-(aq) + S4O62-(aq)

Chlorine in bleach

ClO-(aq) + 2H+(aq) + 2e- Cl-(aq) + H2O(l)
Cl: from +1 to -1 so it's the oxidising agent

Step 1: ClO-(aq) + 2H+(aq) + 2I-(aq) Cl-(aq) + I2(aq)+ H2O(l)
Step 2: back-titrating with standardised soln of sodium thiosulfate
Step 3: work backwards to find the conc of chlorate(I) in bleach.

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Titraction of copper(II) ions

Step 1: Add excess KI to known volume of the soln containing copper(II) ions. White ppt forms.

  • 2Cu2+( aq) + 4I-(aq) → 2CuI(s) + I2(aq)

Step 2: Titrate the iodine solution with standardised S2O32- to find the conc of iodine

Step 3: Work backwards to find the copper(II) in the original sample.

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