Modern inorganic chemistry

104 ACIDS AND BASES: OXIDATION AND REDUCTION

lor which AG = AG(7; + AG(%

= ( - 1467) + ( -f 1312)

• 
  
  
  
  • 155 kJ
    Thus chloride ions are oxidised to chlorine by manganate(VII) under
    standard conditions
    (b) Cr 2 CH~(aq) + 6C1"(aq) + 14H 3 O + -> 2Cr^3 + (aq)
    4- 21H 2 O -f 3Cl 2 (aq)
  
  
  
  

Reaction AG~* = -nh'c

(i) Cr 2 O= (aq) + 6e + 14H 3 CT +1,33 jf} = ^6 x 96487 x (+1.33)

-> 2Cr^3 '(aq) + 21H 2 O "

k1 2 |aq) + f " -> CP(aq) +1.36

Cl'(aq) ->• K'Maq) + f' -1,36 -1 x 96 487 x ( - 1,36)

(ii) 6C1 (aq) ->~3Cl 2 (aq) + 6t>" -1.36 AG(T*» = -6 x 96487 x (-1,36)

Hence (i) and (ii) give

6CP(aq)

21H 2 O + 3Cl 2 (aq)

for which AG - AGg -f

= ( -769)

= + 18 kJ

( + 787)

Thus under standard conditions chloride ions are not oxidised to

chlorine by dichromate(VI) ions. However, it is necessary to empha-

sise that changes in the concentration of the dichromate(VI) and

chloride ions alters their redox potentials as indicated by the Nernst

equation. Hence, when concentrated hydrochloric acid is added to

solid potassium dichromate and the mixture warmed, chlorine is

liberated.

Equilibrium constants from electrode potentials

We have seen that the energetic feasibility of a reaction can be
deduced from redox potential data. It is also possible to deduce the
theoretical equilibrium position for a reaction. In Chapter 3 we saw
that when AG = 0 the system is at equilibrium. Since AG = — nFE,
this means that the potential of the cell must be zero. Consider once
again the reaction

+ Zn(s) -* Cu(s) + Zn^2 f (aq)