What is the colour change when aqueous iron(II) ions are treated with an acidified solution of dichromate(VI) ?

1 Answer
Apr 13, 2016

The colour change is from pale blue/green to yellow/brown.


Dichromate(VI) is a powerful oxidising agent and takes in electrons according to the 1/2 equation:

#Cr_2O_(7(aq))^(2-)+14H_((aq))^(+)+6erarr2Cr_((aq))^(3+)+7H_2O_((l))" "color(red)((1))#

The iron(II) ions give up their electrons according to:

#Fe_((aq))^(2+)rarrFe_((aq))^(3+)+e" "color(red)((2))#

To get the electrons to balance you can see that we need to multiply #color(red)((2))# by 6 then add this to equation #color(red)((1))rArr#

#Cr_2O_(7(aq))^(2-)+14H_((aq))^(+)+cancel(6e)+6Fe_((aqa))^(2+)rarr6Fe_((aq))^(3+)+cancel(6e)+2Cr_((aq))^(3+)+7H_2O_((l))" #

Iron(II) solutions contain the hexaaquoiron(II) ion #[Fe(H_2O)_(6)]^(2+)# which is a pale blue/green colour.

In the solid, hydrated state they look like this:


In aqueous conditions a solution of iron(III) contains the hexaaaquoiron(III) ion #[Fe(H_2O)_6]^(3+)]#. The solution looks a yellow/brown colour:


The colour is actually yellow due to the hydrolysis of the ion in water:


It is the #[Fe(H_2O)_5(OH)]^(2+)# that gives the yellow/brown colour.

Pure iron(III) ions are actually pale violet. This is what they look like in the solid state:


However, these colours will be masked when you carry out the reaction.

Potassium chromate(VI) is orange in colour and that's what you see before the reaction. These are converted to green #Cr^(3+)# ions so you will see a mixture of these and the iron(III) species after the reaction.