Edexcel A Level Chemistry:复习笔记6.2.12 Autocatalysis

Manganese(II) ions as an Autocatalyst

 

  • Autocatalysis the term used to describe a reaction which is speeded up by a product which acts as a catalyst for the reaction
  • If you plot a rate graph of concentration versus time it has an usual shape

Rate-graph-autocatalysis

Concentration versus time for an autocatalytic reaction

  • The gradient becomes steeper during the course of the reaction which tells you the rate is speeding up, not slowing down over time as the reactants become used up
  • An example of an autocatalysed reaction takes place between manganate(VII) ions and oxalate (ethandioate) ions
  • The overall equation can be deduced from the half equations

6.2-Chemistry-of-Transition-Elements-Oxalate-Permanganate-Overall-Reaction

  • You can see that one of the products is manganese(II) ions - this is the catalyst
  • As more manganese(II) is formed the reaction speeds up
  • Like to the role of iron(II) in the previous section, manganese(II) ions take part in a redox cycle between two different oxidation states (+2 →  +3 → +2)

4Mn2+ (aq)  +  MnO4– (aq) + 8H+ (aq)   →   5Mn3+ (aq)  + 4H2O (aq)

2Mn3+ (aq)  +  C2O42- (aq)  →  2CO2 (g) +  2Mn2+ (aq)

  • The manganese(II) is not present in the beginning of the reaction, but as it is formed is speeds up the reaction and is re-generated during the redox cycle
  • This reaction is easily followed on a colorimeter as the rate at which the purple manganate(VII) ion is consumed accelerates with time

 

 

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