Frost diagram

The Frost Diagram is a representation of the standard free energy of formation versus the oxidation number . In this way, the most stable oxidation state corresponds to the species below in the Frost diagram.



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  • 1 Description
  • 2 Construction of a Frost diagram
  • 3 Frost diagram for oxygen, from Latimer diagram, in acidic medium
  • 4 Source


A Frost diagram for an element X, consists of the representation of nE ° for a pair X (N) / X (0) against the oxidation number, N, of the element. As nE ° is proportional to the standard free energy of the conversion reaction of the species X (n) to X (0).

Construction of a Frost diagram

FIG. 1

Frost diagram for oxygen, from Latimer diagram, in acidic medium

FIG. 2.

For the change from O2 to H2O2, from the oxidation number 0 to -1, n = -1 and therefore, nE ° = – 0.70V. For the change from O2 to H2O, from 0 to -2, n = -2, and therefore, nE ° = -2×1.23 0 -2.46V. The slope of the line connecting any two points on a Frost diagram is equal to the normal potential of the pair formed by the two species representing the two points. For example, at the point corresponding to the oxidation state -1 for the Frost diagram, in an acid medium, nE ° = – 0.70, and in the oxidation state nE ° = – 2.46, then the difference Dy is – 1.76. The variation of the oxidation number when passing from H2O to H2O2 is Dn = -1 (Dx), therefore m = Dy / Dx = (- 1.76 / -1) = 1.76V that agrees with the value of E ° of the pair H2O2 / H2O on the Latimer diagram.

FIG. 3.

FIG. 4.

FIG. 5.

The greatest application of Frost diagrams lies in qualitative rapid printing that allows predicting trends in the chemical properties of different species. To interpret the qualitative information contained in a Frost diagram, the following aspects must be taken into account:

  1. The most stable species is found further down on the Frost diagram.
  2. The steeper the line connecting two points on the diagram, the greater the potential of the pair. Hence the spontaneity of the reaction between any two pairs can be deduced by comparing the corresponding lines. The oxidizing agent (oxidized species) of the pair with the most positive slope (highest value of E0) will be reduced and the reducing agent (reduced species) of the pair with the least positive slope (lowest value of E0) will be oxidized.
  3. In a Frost diagram, it can be deduced that an ion or molecule is unstable with respect to its disproportion if it is above the line that joins two contiguous species. The DG0 energy of the intermediate species is above the average value of the two terminal species, which means that it is unstable compared to its disproportion.

FIG. 6.

  1. Two species tend to share an intermediate species if it is below the connecting line that joins them.

FIG. 7.

The free energy of this species is lower than the average value of the other two. === Example ===: NH4NO3 is made up of two ions in which nitrogen has a different oxidation number: – 3 (NH4 +) and +5 (NO3-). As in N2O the nitrogen atom has an oxidation number average to the previous ones (+ 1) and it is observed that it is below the line of NH4 + and NO3-, its proportion is allowed by thermodynamics .

NH4 + (ac) + NO3- (ac) N2O (g) + 2H2O (1)

FIG. 8.

FIG. 9 Frost diagram for nitrogen.

The reaction is kinetically inhibited solution diluted, and usually does not occur. However, in the solid state it is so fast that explosions can even occur. In fact, NH4NO3 is often used in place of dynamite to explode rocks.


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