Free Energy-Oxidation State Diagrams V. Rhres-Amau Facultad de Farmacia, Saiamanca, Spain Free energy-oxidation state diagrams (Frost diagrams) are commonly used to visualize the redox properties of chemical substances, and their significance and applications are similar to those of the one-dimensional, Latimer diagrams. Both types of diagrams are throughly discussed in inorganic chemistry textbooks. Moeller's book1 dedicates one chapter to this type of reaction and both diagrams for several elements are discussed. However, in our opinion, there is a significant mistake in the Frost diagram for chlorine in acidic medium displayed in Figure 9-7 (p 577) of the named book (see figure). According to the general theory for redox reactions, when Latimer diagrams are used, those substances for which left potential is lower than the right potential should spontaneously (thermodynamically) disproportionate. On the contrary, if the left potential is higher than the right potential, extreme substances should condense to yield the central suhstance. The same conclusions can be reached from the corresponding Frost diagram. If the point representing a chemical substance lies below the straight line joining those of an oxidized and a reduced suhstance, the condensation reaction is favored, but in the reverse case the disproportionation reaction should take place. From the Latimer diagrams for chlorine in acidic and basic media (p 575 of Moeller') it can he concluded that C1and HOCl should condense to Clz in acidic medium, whereas Cla should disporportionate to hoth substances in basic medium.
acid medium:
-
+ e-
'hCB
CI-
E"(1) = 1.3595 V and F heing 96.6 kJ mol-'.
+
HOCI + H+ eE'(2)
=
-
'hCI2
+ Hz0
and
and (* This value obtained from those corresponding to the electrode potentials of the couples C10-IC1- and ClzIC1-, 0.89 and 1.3595 V, respectively). And for the disproportionation reaction:
C12+ 20H-
-
Cl-
+ ClO- + H20
(4) - (5) = (6)
The Frost diagram for chlorine in hoth media, from the values in the Latimer diagrams, is shown in part a of the figure, following Moeller's Fig. 9-7. In such diagrams the abscissa corresponds to the oxidation state of the substance and the ordinate to the product (Eo n), n heing the number of electrons exchanged in the couple reaction and Eo the electrode potential of such a couple. So, according to this plot, the point representing Clz (coordinates 0,O) always lies below the straight line joining the points representing C1and HOCl in acidic media, or C1- and C10- in basic media, contrary to the conclusion reached using the Latimer diagram. However, when the oxidation state of the element is negative, the ordinate representing such a suhstance should be changed to the contrary value, as shown in part h of the figure, where it can he easily seen that Clz disproportionates to C1- and C10- in a basic medium, according, on the other hand, to our experimental knowledge.
.
(2)
1.63 V
and AG'(2) = -1.63 F and for the condensation reaction: 2HOC1+ C1-
basic medium:
+ H+
+
%Clz e-
-
-
Cb
+ Hz0
C1-
(2)
- (1)= (3)
(4)
I . -1
'
Moeller, Therald. inorganic Chemistry. A A m Introduction. 2nd. ed.: Wiley: New York, 1982.
652
Journal of Chemical Education
I
0
+I
.
-1
0
oxidation state
The Fmst dlagram fw chlorine: (a) correct diagram, (bl incorrect
+I diagram
