Question

How many electrons are transferred in the redox reaction taking place in basic solution:

Cr(s) + ClO₄^-(aq) --> ClO₃^-(aq) + Cr³⁺(aq)?

Answer 1

let us calculate the oxidation state of Cl in ClO₄^-

let oxidation state be x

charge over each oxide is -2

so we can right

x+(4*-2)=-1 as the overall charge is -1

x-8=-1

x=-1+8=7

so oxidation state of Cl is +7

let us calculate the oxidation state of lr in CrO₃^-1

let oxidation state be y

charge over each oxide is -2

so we can right

y+(3*-2)=-1 as the molecule is negatively charge or overall charge is -1

y-6=-1

y=-1+6=5

so oxidation state of Cl is +5

we see oxidation state of Cl goes from +7 to +5 there fore it gain of electron and is getting reduced

the change oxidation state for chromium # from0 so it is losing electron and is getting oxidised

half cell oxidation

Cr^{$\rightarrow$} Cr³⁺

charge on left side is 0 and right side is +3 so if we add +3e on right side we can balance the charge

we get

Cr^{$\rightarrow$} Cr³⁺ +3e equation 1

^{half cell reduction}

ClO₄^-$\rightarrow$ClO₃^-

we see the number of oxygen atom on left side is 4 and right side is 3

so in basic condition

for every excess of oxygen 1 H₂O added to side which have more oxygen atom and 2OH^- molecule on the side which is deficient in it

therefore in this case 1H₂O added to left side  and 2OH^- molecule on the right side to balance O atoms

so we get

ClO₄^- +1H₂O $\rightarrow$ClO₃^- +2OH^-

we see all atoms are balance (mass balanced equation)

charge on left side is -1 and right side is -2 +(-1)=-3  so if we add +2e on left side we can balance the charge

we get

ClO₄^- +1H₂O + 2e $\rightarrow$ClO₃^- +2OH^- equation 2

Cr^{$\rightarrow$} Cr³⁺ +3e equation 1

now multiply equation 1 with 2 and equation 2 with 3 to make the electron count same

3ClO₄^- +3H₂O + 6e $\rightarrow$3ClO₃^- +6OH^- equation 4

2Cr^{$\rightarrow$} 2Cr³⁺ +6e equation 3

net equation adding equation 3 and 4

3ClO₄^- +3H₂O +2Cr $\rightarrow$3ClO₃^- +6OH +2Cr³⁺

thus 6 electrons are transferred in the redox reaction taking place in basic solution