Question

The oxidising strength of Cr2O72- is greatest in a solution of pH?

The oxidising strength of Cr2O72-
is greatest in a solution of pH
1.0
3.0
7.0
10.0
13.0

Answer 1

Well, typically we deploy dichromate oxidants in AN ACIDIC solution....

Explanation:

And so I suppose that `pH=1.0`

In an acid milieu dichromate undergoes reduction to chromic ion...

`underbrace(Cr_2O_7^(2-))_"orange red" +14H^+ + 6e^(-)rarr`

`underbrace(2Cr^(3+))_"green" + 7H_2O(l)`

Some chromate ion, `CrO_4^(2-)`, MAY be formed in acidic solution...but this is NOT a redox solution...and chromate is still `Cr(VI+)`...

`Cr_2O_7^(2-) +H_2Orarr 2CrO_4^(2-) + 2H^+`

Answer 2

pH 1

Explanation:

You can compare the relative strengths of oxidising agents by consulting a table of standard electrode potentials (`sf(E^@)`).

The more +ve the value, the greater is the ability to take in electrons and cause oxidation.

`sf(Cr_2O_7^(2-)+14H^(+)+6erightleftharpoons2Cr^(3+)+7H_2Ocolor(white)(xxxx)E^@=+1.33color(white)(x)V)`

These are under standard conditions where pH = 0 so pH 1 is the closest.

In high pH conditions chromium(VI) is present as yellow `sf(CrO_4^(2-))` ions:

`sf(CrO_4^(2-)+4H_2OrightleftharpoonsCr(OH)_3+5OH^(-)color(white)(xxxxxx)E^@=-0.13color(white)(x)V)`

As you can see the value is much less +ve so you would not expect it to be such a good oxidising agent at high pH values.