Modern inorganic chemistry

ACIDS AND BASES: OXIDATION AND REDUCTION 103

We can determine the energetic feasibility for this reaction from the
two half-reactions:

Reaction E~~(\) AG"^ — —nFE^

Fe^3 + (aq)-4-

Fe^2 '(aq)-»

;. (i) 2Fe^2 + (aq

iBr 2 (aq) + <

;. (ii) Br 2 (aq) -

e -»Fe^2 + (aq)

Fe^3 + (aq) + e~

)^2Fe^3 "(aq) 4- 2e~

?~ -> Br(aq)

\-2e~ -»2Br~(aq)

+ 0.76

-0.76

-0.76

+ 1,07

1.07

-1 x 96487 x (+0.76)

-1 x 96487 x (-0.76)

-2 x 96487 x (-0.16)

AG(7; = + 146.7kJ

-1 x 96487 x ( + 1.07)

-2 x 96487 x ( + 1.07)

AG^ = - 206.5 kJ

Hence (i) and (ii) give

2Fe^2 + (aq) 4- Br 2 (aq) -» 2Fe^3 + (aq) -f 2Br~(aq)

AG = AG^ -f GJ^}

• 
  
  
  
  • 146.7 -h ( - 206.5)
    = - 59.8 kJ
  
  
  
  

Thus the reaction is energetically feasible and does indeed take
place. It is interesting at this point to investigate the reasons why
iron(II) ions in aqueous solutions are quantitatively estimated by
titration using potassium manganate(VII) (permanganate) when
chloride ions are absent but by potassium dichromate(VI) when
chloride ions are present. The data for the oxidation of chloride ions
to chloride by (a) manganate(VII) and (b) dichromate( VI) ions under
standard conditions are given below:
(a) 2MnO4 (aq) -f 10Cl~(aq) + I6H 3 O +
4- 24H 2 O 4- 5Cl 2 (g)

Reaction

MnO 4 (aq)+ 5e~ -f 8H^O +

-> Mn-(aq) + 12H 2 O

(i) 2MnO4 (aq) + I0e~ + 16H 3 O*

— » 2Mn^2 "^(aq) 4- 24H 2 O

iC! 2 (aq) + e~ ->Cl~(aq)

CP(aq) -»• 4Cl 2 (aq) + e"

(ii) lOCl'(aq) -> 5Cl 2 (aq) 4- We~

E~(V)

+ 1.52

4- 1.52

4-1.36

-1.36

-1.36

AG*=-WJ

-5 x 96487 x

AG(t = - 10 x

x (+1.52) =

• 1 x 96 487 x
  AGJ^, = — 10 x
  x (-1.36) =

*E

( + 1.52)

96487

-1467kJ

(-1.36)

96487

+ 1312kJ

Hence (i) and (ii) give

2MnC>4 (aq) + 10C1" (aq) + 16H 3 O+ ->

2Mn2+(aq) + 24H 2 O 4- 5Cl 2 (aq)