Hydrogen acids. The acid strength increases with increase in the size of atoms. The per
cent ionic character of the bonds in these compounds is in the reverse order to their acid
strengths, e.g.
HI > HBr > HCl > HF
H2Te > H2Se > H2S > H2O
The gas phase acidity increases across a period and down a group in the p-block binary acids,
HF is a stronger acid than H2O and HI is the strongest of the hydrogen halides.
All these halide ions except F-
have proton affinity smaller than H2O which is constant with
all the hydrogen halides except HF, being strong acids in water.
Another factor in proton transfer is hydrogen bonding. Thus water has great stabilizing effect
on small, highly electronegative ions, particularly for F-
, Cl-
, OH-
to which it can act as
hydrogen-bond acceptor. NH+
4 is stabilized by hydrogen bonding and has reduced acidity.
An example is the increased acidity of HCl in CH3OH which can stabilize Cl-
by forming
Cl… H-OCH3 in comparison with HCl in dimethylformamide (CH3)2NCHO which does not
have significant hydrogen-bond donor properties.
11
10.1 Acids Strength and Molecular Structure: In order to understand the relationship
between acid strength and molecular structure, acids may be divided into two types, hydrides
and oxoacids.
(i)Hydrides: Two factors influence the acid strength of the hydride of an element- the electro
negativity of the element and the atomic size of the element. These may be understood by
making a comparison of the hydrides of the element in a period and in a group.
(a) Hydrides of the elements of a period: Consider the hydrides of nitrogen, oxygen and
fluorine of the second period. The electronegativity of these element increases in the order.
N < O < F
And acid strength 0f the hydrides increases in the same order
NH3 < H2O < HF
Similarly, the electronegativities of the elements of the third period change in the order
P < S < Cl
The acid strength of the hydrides of these elements increases in the same order
PH3 < H2S < HCl
PH3 does not react with water, H2S is a weaker acid and HCl is a strong acid.
(b) Hydrides of the elements in a group: The acidity of the hydrides of the elements of a
group increases with increasing size of the central atom. Consider the hydrides of group 16
and 17 elements
H2O < H2S < H2Se < H2Te
HF < HCl < HBr < HI
This order is the reverse of that expected on the basis of the electronegativity. The first
hydrides of each series (H2O and HF) is the weakest acid of the series and is formed by the
element with the highest electronegativity.
Two factors that influence acid strength are the electronegativity of the central atom and the
size of the central atom. When these factors work against each other, the effect of atomic size
outweighs the electronegativity effect. A proton is more easily removed from a hydride in
which the central atom is larger than from the one in which the central atom is small.
Consider for example the hydrides of carbon, sulphur, and iodine. The electronegativity of C,
S and I, which belong to different groups are about the same (2.5). The atomic radius of C is
77 pm, of S is 103 pm and of I is 133 pm. There is a marked increase in the acidity of the
hydrides with increase in size of the central atom. Methane, CH4, does not dissociate in
water, H2S is a weak acid, and HI is a strong acid.