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
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
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)
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