ORNL-1816 - the Molten Salt Energy Technologies Web Site

I ANP QUARTERLY PROGRESS REPORT
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The protons, having an exceptional polarization
potential, react almost completely with the sol-
vent, Eq. 4, represented by the other half of the
hydroxyl ions in Eq. 2. Water, the product of this
reaction, thus may be regarded as a solvated
proton (onium species) in fused hydroxide so-
lution. Using the protonic-solvent theory, the
quantitative extent of reaction is regarded as
being controlled by the relative proton affinities
of the two bases involved in any acid-base equi-
I i br ium.
Protonic-Solvolysis-Type Reactions. At least
three types of protonic-solvolytic reactions might
be expected. First, the hydride ion provides a
special case:
OH- + HB-+ B- + H,O
where B- is taken as a generalized representation
of an anionic species. Third,
(8) OH- + B-+BH + 0--
The second type of solvolytic reaction requires
that the hydroxyl ion be a stronger proton acceptor
than the B- ion. There are, of course, many
examples of this, such as the obvious reaction
(9) OH- + HCI+CI- + H,O
which, in fused hydroxides, is a solvolytic re-
action and not a neutralization. Equation 9 repre-
sents an obvious reaction because of the ex-
ceedingly weak base character of the chloride
ion. There should, however, be a series of such
reactions involving bases with proton affinities
lying between those of CI- and OH-, such as
HPO,-- and HC0,-, so that the reaction
(10) HC0,- + OH'--+ COS-' + H,O
might occur; if it did occur, it would be an ex-
ample of Eq. 2.
The third type of solvolytic reaction, Eq. 8,
requires that the B- ion be a stronger base than
the 0" ion. At best, there will be very few
instances of this.
Proton ic-Neutra lizat ion-Ty pe Reactions. Neu-
tralization reactions in fused hydroxides which
can be treated by the protonic theory are also of
three types: (1) the reaction of water with the
oxide ion according to the formula
(11) H20 + O--+ 20H-
where the water may come from the dissociation
of another compound; (2) the reaction of water
with an ion which is a stronger base than the
hydroxyl ion but a weaker base than the oxide
ion, that is, essentially the reverse of Eq. 7,
(12)
H20 + B---+ BH + OH-
and (3) the reverse of Eq. 8,
(13) 0" + H B d B- + OH-
An obvious example of the reaction given by
Eq. 13 is
(14) 0-- + HCI + CI- + OH-
Kinetically, this reaction undoubtedly would go
by two steps,
HCI + OH-+ CI- + H,O
H,O + 0--+20H'
if the oxide ion were present in low concen-
trations, with the second step controlling the rate.
However, the equilibrium would be represented
by Eq. 14. Equation 13 represents a wider range
of reactions than Eq. 8 because the permissible
proton affinity range for B- goes all the way up
to that of the oxide ion; therefore the reaction
(15) HC0,- + 0-- + CO,-- + OH-
is undoubtedly an example in which the bicar-
bonate ion behaves like an acid.
OxidicJystem Concept. There are a great many
reactions in fused hydroxides which cannot be
usefully treated in terms of the protonic-solvent
theory. Consider, for example, the familiar re-
action
(16) CO, + 20H-+CO,-- + H,O
The application of the Bransted theory here requires
a rather complex, formal treatment. The
actual proton exchange is between the bases OHand
O--. The weaker base, OH-, serves as the
proton acceptor, and the stronger base, O--,
serves as the proton donor. This is the reverse
of the neutralization reaction given in Eq. 11 and
would not be predicted by the protonic-solvent
theory. This seeming contradiction may be re-
i-
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