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

1 Answer
Apr 13, 2016

Answer:

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

Explanation:

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:

image.made-in-china.com

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:

www.alevelchem.com

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

#[Fe(H_2O)_6]^(3+)+H_2O_((l))rightleftharpoons[Fe(H_2O)_5(OH)]^(2+)+H_3O^(+)#

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:

http://www.oxfordchemserve.com

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.