66 QUANTITATIVE AMt/('l'//rr/{.\/, .l,Y.i/,)>7N
Apparent Valence.—In ordinary reactions of double deeomposi-
tion the valences of the dements and radicals which an* being
transposed are measures of the respective ivacfiiiK powers of
these entities. This is not true for reactions of oxidation and
reduction. Here the reacting power of a compound is determined
by the extent of change of valence. As an illustration may be
taken the reversible oxidation of hydriodic acid by ferric chloride:
2FeCl8 + 2HI -»2I'V'I2 + 2IK'I f I,.
The valence of iron in ferric chloride is % but this compound does
not exchange three atoms of chlorine for an equivalent amount of
another radical—it simply parts with one atom of chlorine
which oxidizes hydriodic acid. The* hydrogen equivalent, of
ferric chloride in this reaction is then equal to the chftHyc in
valence of iron. This is 3 — 2 = L
For the purpose of this inspection the actual valence of the
elements undergoing the reaction need not be considered as this
depends upon the structural composition of the compounds,
which is not always known. The ay/wrcnf valence is that which
is indicated by the simplest direct combination of positive and
negative elements and it is therefore* a measure of combining
power. In the above reaction iodine is tin* element that is
oxidized and it is sufficient to regard its apparent valence in
hydriodic acicl as 1 and in the form of the elemi'uf. H,H f). The
hydrogen equivalent of hydriodic acid rn the difference between
these two apparent valences, or 1.
The reaction between potassium jwrmanganute ami oxalic
acid may be inspected similarly. Omitting the roeiiirutntM
given in Eq. (2), page 65, the empirical equation in
H2C204 + KMn04 + H2S04~> K»SO4 -f M»SO4 ~|. Hso + CO,.
Obviously, carbon is the element that in oxidiml mid mniigitiieHC!
is reduced. The apparent valence of an flnnrnt whirh 5* in the
negative radical of an oxyitdd or of itn salt is found by Mibf raeting
the total valence of the positive rudira! from that of the other
elements of the negative radical and dividing tin* twitlf, by the
number of atoms of the element in question.
In oxalic acid the apparent valence of carbon in f lieu ^ ""• •
= 3, while in carbon dioxide it IK 4. The hyiirngi'ii