Explain to me why Chromium was reduced and why Iron was oxidized in this equation?

Question

A solution of potassium dichromate is added to an acidified solution of iron (II) chloride.

I know it ends up looking like this Cr2O72− + H+ + Fe2+ --> Cr3+ + H2O + Fe3+

But I do not understand why.

Serious question. No retarded answers please.

The equation does not need to be balanced for the moment.

Answer 1

Reduction means gaining electrons, thus a decrease in oxidation number. Oxidation is the opposite.
In this reaction you have

Chromium going from +6 to +3->decrease in oxidation number->reduction

Iron from +2 to +3 ->increase in oxidation number->oxidation.

Do you mean why the reaction is giving these products?
Let's see if something else could happen.

Always when a compound takes electrons and is reduced there must be a donor to give these electrons. So a redox reaction will always have one compound being reduced and one being oxidized.

Cr at +6 is at its highest oxidation level. It can go to different lower ones, but under acidic conditions usually it is reduced to +3. Since it is at its maximum oxidation level it cannot be oxidised further, so it can only be reduced.

Fe +2 can either be reduced to Fe 0 (elemental iron) or oxidised to Fe +3.

Thus the only possible reaction is one where Fe+2 can give more electrons and chromium receives them (chromium can't give more electrons).

This is over-over-simplified. A proper answer to why the reaction occurs this way would involve oxidation potentials (as mentioned above), deltaG, etc. Probably you've just begun learning about redox.

Answer 2

Because the oxidation potentials are different (Sounds like a tautology, but it isn't.) Every ionic species in water solution has an oxidation potential; the Handbook of Chemistry and Phyics has a list. The oxidation potentials describe how much energy is absorbed or released in creating the ion, and if energy is released, the reaction will proceed in the direction to do so.

Answer 3

Your final equation is wrong. There must be an equal number of elements on each side of the eq. Once you have ballanced the equation (for example, total of 6 Chromium atoms per side), then you can clarity the electron status.

[edit] P.S. I don't see the potassium or chlorine anywhere in you equation.

Answer 4

If I knew the answer, I would be a chemist. But unfortunately I am a Computer Information Systems graduate who works at Mcdonalds so I don't know.

Answer 5

my opinion:

you need to look up the redox potentials for Cr(VI)/Cr(III) couple and the redox potentials for Fe(III)/Fe(II) couple. Since here, we are taking into consideration the higher vs the lower oxidation state, i.e.,
Cr(VI)/Cr(III) couple for example, the potential we are referring to is reduction potential. On the other hand, if it is written as Cr(III)/Cr(VI), the potential value is oxidation potential.

Now, back to your question, if you have the values of reduction potentials of both the Cr(VI)/Cr(III) couple and Fe(III)/Fe(II) couple,
you'll see that the value is higher for the Cr(VI)/Cr(III) couple. What this means is that the Cr(VI)/Cr(III) couple has higher propensity to be reduced compared to the Fe(III)/Fe(II) couple.

Therefore, when you have a dichromate and an iron(II)chloride, chromium, due to its higher reduction potential, will go ahead and grab the electrons from iron and get reduced, oxidising the iron from oxidation state 2 to ox state 3.

the best way to double check is as follows:
now look up the oxidation potentials of the couples, Fe(II)/Fe(III) vs Cr(III)/Cr(VI). you'll see that the oxidation potential is higher for the iron couple compared to the chromium couple. therefore, in a solution of cr(VI) and Fe (II) , iron will have higher propensity to get rid of its electrons and get oxidised to +3 state, and since the solution stays neutral, chromium has to eat up the electrons, thereby getting reduced.

Answer 6

Just remember "LEO goes GER"

Loss of Electrons means that thing was Oxidized. (LEO)
Gain of Electrons means that thing was Reduced. (GER)


Iron in this reaction must have lost electrons and Chromium must have gained electrons.

Answer 7

You should learn thermodynamics. The chemical reaction takes place spontanously when that reaction need no energy or even generates energy.

Answer 8

well since u have the answer already i cant help u but i do have a question

ITS SUMMER

WHY R U DOING CHEM?