88 ACIDS AND BASES: OXIDATION AND REDUCTIONHF HC1- HX(aq) -> HX(g) 48 18
- HX(g) -> H(g) + X(g) 566 431
- H(g)->H + (g) + e~ 1318 1318
- X(g) + e~ -> X"(g) -333 -364
- H + (g)-+H + (aq) -1091 -1091
- X~(g)->X-(aq) -515 -381
- HX(aq)-»H+(aq)4- X~(aq) -7 -69
Clearly, the higher enthalpy of solution 1 and bond dissociation
energy 2 of hydrogen fluoride outweigh the greater hydration
enthalpy of F", 6, and AHf^ 8 for HF? 7 is quite small; this means a
smaller pKa value than for HC1. Clearly, one important factor in
determining acid strength is the strength of the X—H bond; in
many inorganic substances, this is in fact an O— H bond, for example
in water (a weak acid) and in HNO 3 , H 2 SO 4 (strong acids). For
water, the strength of the O—H bond is decreased (and the acid
strength increased) by co-ordination of the water to a small highly
charged cation. This means that species such as [A1(H 2 O) 6 ]^3 + are
quite strongly acidic; the relevant equilibria have already been
discussed in some detail.
Many of the inorganic oxoacids are strong (i.e. have negative
pKa values) in aqueous solution. But, as we have seen, use of a
solvent with a lower proton affinity than water (for example pure
ethanoic (acetic) acid makes it possible to differentiate between the
strengths of these acids and measure pKa values. The order of
strength of some typical oxoacids is then found to be (for HnX -»
increasing
strengthH 2 CO 3 carbonic acid OC(OH) 2
H 3 PO 4 phosphoric(V) acid OP(OH) 3
H 2 SO 4 sulphuric acid O 2 S(OH) 2
HC1O 4 chloric(VII) (perchloric) acid O 3 C1(OH)If the formulae of the acids are written as shown on the right, it
becomes apparent that acid strength increases as the number of
oxygen atoms not involved in O—H bonding increases.
THE EFFECT OF STRUCTURE: BASE STRENGTH
A base must be capable of accepting protons; for this, at least one