Firstly you must understand that a redox reaction will only occur if the reductant (reducing agent) is higher in the reduction half-equation table than the oxidant (oxidising agent). Now how do we find that out?
In any chemistry exam you are given a periodic table and another sheet containing "some standard potentials". Now the rule is that as you go up the 'table' you will find that the element becomes more reactive. Therefore as you go up the stronger the reductant (reducing agent) is and the weaker of an oxidant it becomes. The opposite is also true. As you go down that table you will find that the element becomes a weaker reductant i.e stronger oxidant (oxidising agent). So going by this logic. For this particular question, you look at the standard potential table and see which one is the lowest out of the list. Therefore the answer would be Fe3+.
To summarise:
- Strong reductants are weak oxidants.
- Strong oxidants are weak reductants.
- As you go up the table the stronger the reductant is (i.e the weaker the oxidant is) and the more reactive that element is.
- As you go down the table the stronger the oxidant is( i.e the weaker reductant is)
- The greater the difference between the two reactants in that table the more vigorous the reaction is.
capisce?
