Describe the oxidising action of potassium dichromate and write the ionic equations for its reactions with:
(a) iodide, (b) iron(II) solution, (c) ${{H}_{2}}S$

Hint: Potassium dichromate is a strong oxidising agent. Oxidising agents take up electrons and reduce themselves while oxidising the other substance. We can write ionic equations by balancing the number of net charges on both the sides.

Complete answer:
Potassium dichromate is a very strong oxidising agent. We know that oxidising agents are the substance that gets reduced in order to oxidise other substances by gaining electrons.
The formula of potassium dichromate is ${{K}_{2}}C{{r}_{2}}{{O}_{7}}$ and it is a very strong oxidising agent in acidic medium.
An ionic equation is similar to a molecular equation but here we write the molecules in their dissociated ion forms i.e. the form in which they exist in solutions. The net charge on both the sides is equal.
Now let us discuss the oxidising action of potassium dichromate for the reactions with the given solutions.
Firstly, we have iodide. Potassium dichromate oxidises iodide to iodine. We can write the ionic equation as-
     \[\begin{align}
  & C{{r}_{2}}{{O}_{7}}^{2-}+14{{H}^{+}}+6{{e}^{-}}\to 2C{{r}^{3+}}+7{{H}_{2}}O \\
 & 2{{I}^{-}}\to \left. {{I}_{2}}+2{{e}^{-}} \right]\times 3 \\
 & C{{r}_{2}}{{O}_{7}}^{2-}+14{{H}^{+}}+6{{I}^{-}}\to 2C{{r}^{3+}}+7{{H}_{2}}O+3{{I}_{2}} \\
\end{align}\]
Next, we have an iron(II) solution. Iron (II) solution has ferrous ions and potassium dichromate oxidises then to ferric ions i.e. it oxidises iron (II) in the solution to iron (III). We can write the ionic equation as-
     \[\begin{align}
  & C{{r}_{2}}{{O}_{7}}^{2-}+14{{H}^{+}}+6{{e}^{-}}\to 2C{{r}^{3+}}+7{{H}_{2}}O \\
 & F{{e}^{2+}}\to \left. F{{e}^{3+}}+{{e}^{-}} \right]\times 6 \\
 & C{{r}_{2}}{{O}_{7}}^{2-}+14{{H}^{+}}+6F{{e}^{2+}}\to 2C{{r}^{3+}}+6F{{e}^{3+}}+7{{H}_{2}}O \\
\end{align}\]
And lastly, we have hydrogen sulphide. It takes up the hydrogen and oxidises hydrogen sulphide to sulphur. We can write the ionic equation as-
     \[\begin{align}
  & C{{r}_{2}}{{O}_{7}}^{2-}+14{{H}^{+}}+6{{e}^{-}}\to 2C{{r}^{3+}}+7{{H}_{2}}O \\
 & {{H}_{2}}S\to \left. S+2{{H}^{+}}+2{{e}^{-}} \right]\times 3 \\
 & C{{r}_{2}}{{O}_{7}}^{2-}+14{{H}^{+}}+3{{H}_{2}}S\to 2C{{r}^{3+}}+7{{H}_{2}}O+3S \\
\end{align}\]

Note:
Potassium dichromate acts as a strong oxidising agent in acidic medium because it exists as dichromate ion in acidic medium. It has a negative charge of -2 therefore, it readily takes up electrons and oxidises other compounds. In alkaline medium it exists as potassium chromate instead of potassium dichromate.