Balance each redox reaction occurring in acidic aqueous solution. K(s) + _Cr^3-(

Question

Balance each redox reaction occurring in acidic aqueous solution. K(s) + _Cr^3-(aq) right arrow _Cr(s) + _K^+(aq) _ Al(s) + _Fe^2+(aq) right arrow _AI_3(aq) ^+ _Fe(s) The following redox reaction is used in acidic solution in the Breadthalyzer test to determine the level of alcohol in the blood: H^+(aq) + Cr_2O_7^2- (aq) + C_2H_5OH(aq) right arrow Cr^3+ (aq) + C_2H_4O(aq) + H,OO) Identify the elements undergoing changes in oxidation state and indicate the initial and final oxidation numbers for these elements. Write and balance the oxidation half-reaction. Write and balance the reduction half-reaction. Combine the half-reactions to produce a balanced redox equation.

Explanation / Answer

Que 1)

a)

Balanced redox reaction :

3K(s) + Cr3+(aq)    --->   Cr(s) + 3K+(aq)

K = K+ + 1e- ( oxidation)
Cr3+ + 3e- = Cr ( reduction)
3 K (s) + Cr3+(aq) = Cr(s) + 3 K+(aq)

b)

Balanced redox reaction :

2Al(s) + 3Fe2+(aq)    --->   2Al3+(aq) + 3Fe(s)

Al = Al 3+ + 3e- ( oxidation)

2Al = 2Al 3+ + 6e-
Fe2+ + 2e- = fe ( reduction)

3Fe2+ + 2e- = 3fe

2 Al(s) + 3 Fe2+(aq)2 Al3+(aq) + 3 Fe(s)

Que 2)

Balanced equation:
6H+(aq) + 3Cr2O7 2- (aq) +18 C2H5OH (aq) 6Cr3+(aq) + 18 C2H4O(aq) +21 H2O (l)

Chrimium (Cr) under goes change in oxidation state.initial oxidation state of Cr is +6.

Final oxidation state of Chrimium (Cr) is Cr3+ i.e. +3

Reduction Cr2O72-(aq) Cr3+(aq)

Oxidation C2H5OH(l) C2H4O(aq)

Cr2O72-(aq) + 14H+(aq) + 6e- ---> 2Cr3+(aq) + 7H2O(l)

and

C2H5OH --> C2H4O + 2H(+) + 2e(-)

18 C2H5OH -->18 C2H4O + 36H(+) + 18e(-)
you have to multiply the top (reduction) equation by 3 so that the electrons cancel out
Cr2O72-(aq) + 14H+(aq) + 6e- ---> 2Cr3+(aq) + 7H2O(l)

you get

3 [Cr2O7(-2) + 42H(+) + 18e(-) --> 6Cr(3+) + 21H2O]


and your oxidation equation is still
18 C2H5OH -->18 C2H4O + 36 H(+) + 18 e(-)
now your electrons are equal and thus cancel out, combine the equations
you get

3Cr2O7(-7) + 42H(+) + 18e(-) + 18 C2H5OH --> 6Cr(3+) + 21H2O + 18 C2H4O + 36H(+) + 18e(-)

the electrons cancel out, and some H+'s also cancel out
your final combined redox equation would be

6H+(aq) + 3Cr2O7 2- (aq) +18 C2H5OH (aq) 6Cr3+(aq) + 18 C2H4O(aq) +21 H2O (l)

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