Question

Consider the following unbalanced chemical equation for the reaction which is used to determine blood alcohol levels: H1+ + Cr2O72- + C2H6O → Cr3+ + CO2 + H2O In the forward reaction, is chromium oxidized and/or reduced? reduced oxidised both

Answer 1

The crystals go from orange `Cr_2O_7^(2-)` to green in the event of a positive test. And then you are really in the poo had you been driving a vehicle.

Explanation:

The metal undergoes reduction from `Cr(VI+)rarrCr(+III)`..

`Cr_2O_7^(2-)+14H^+ +6e^(-) rarr 2Cr^(3+) + 7H_2O`

And ethanol is OXIDIZED up to acetic acid....

`H_3C-stackrel(-I)CH_2OH +H_2O rarr H_3C-stackrel(+III)CO_2H+4H^+ +4e^(-)`

To retire the electrons, we add two of the former to THREE of the latter....

`2Cr_2O_7^(2-)+28H^+ +12e^(-) +3H_3C-stackrel(-I)CH_2OH +3H_2Orarr 4Cr^(3+) + 14H_2O+3H_3C-stackrel(+III)CO_2H+12H^+ +12e^(-)`

To give after cancellation....

`underbrace(2"Cr"_2"O"_7^(2-))+16"H"^+ +3"H"_3"CCH"_2"OH" rarr "4Cr"^(3+) +3"H"_3"CCO"_2"H" + 11"H"_2"O"`

...and this I think is balanced, but what do I know?

And the associated colour change...

`underbrace(Cr_2O_7^(2-))_("orange")`

`rarr` `underbrace(2Cr^(3+))_"(green)"`

And see this old answer.