Molecular Structure: Acid and Base Properties

Factors affecting acid strength

Effect of charge :  It becomes more difficult to take a proton away from something that is already negative.  Thus, a neutral parent acid is always stronger as an acid than its anion.  Here is an example:
H3PO4 : (Ka = 7.5 x 10-3)
H2PO4- : (Ka = 6.2 x 10-8)
HPO42- : (Ka = 2.2 x 10-13)
Binary Acids (HX) -
polarity of the HX bond - the more electronegative X is, the stronger is HX as an acid
CH4 < NH3 < H2O < HF
strength of the HX bond - the weaker the HX bond is, the stronger is HX as an acid
stability of the conjugate base X-, the more stable X- is, the stronger is HX as an acid.
hydrogen halide
bond energy (kJ/mol)
weak acid
strong acid
strong acid
strong acid
OxyAcids (HXOn)
The acidity of a hypohalous acid increases as the electronegativity of the halogen increases:
HClO : (Ka = 3.0 x 10-8)
HBrO : (Ka = 2.5 x 10-9)
HIO : (Ka = 2.3 x 10-11)
The acidity of an oxyacid increases with the number of oxygen atoms:
H2SO4 : (strong acid)
H2SO3 : (Ka = 1.7 x 10-2)
Stability of the conjugate base
This factor (considering the others equal) can play a vital role in deciding the strength of an acid.  The conjugate base results from a loss of a proton, thus, the base is usually an anion, negatively charged.  Therefore, addressing the stability of a conjugate base means considering how well the base can carry an extra electron.


CH3OH (methanol) versus HCOOH (methanoic or formic acid)
The conjugate bases are:
CH3O- (methoxide) versus HCOO- (methanoate or formate)
Clearly, in this case, formate will be more stable.


H3CCH3 (ethane), H2CCH2 (ethene), HCCH (ethyne)
In this set, the difference lies in the hybridization of the C.  In ethane, the C-H bond arises from an sp3 hybrid orbital, in ethene, it is sp2 , and for ethyne, it is sp.  If any one of the above compounds loses a proton, the electron that stays behind will reside in this hybrid orbital.  The greater the s character of a hybrid orbital, the closer are the electrons, on the average, from the nucleus.  Thus, an extra electron in an sp hybrid orbital will see a higher nuclear charge, rendering the anion to be more stable than it would be if the electron is in an sp2 , or sp3 hybrid orbital.  Thus, in terms of acidity:



More about Lewis Acids:

The most general definition of acids and bases, which encompasses the Arrhenius and Bronsted-Lowry definitions is due to our old friend, Lewis and his dot structures. A Lewis acid is defined to be any species that accepts lone pair electrons. A Lewis base is any species that donates lone pair electrons. Thus, H+ is a Lewis acid, since it can accept a lone pair, while OH- and NH3 are Lewis bases, both of which donate a lone pair:

H+ + OH- <=> H2O

Interestingly, however, is that species which have no hydrogen to donate (a la the Bronsted-Lowry scheme) can still be acids according to the lewis scheme. As an example, consider the molecule BF3. If we determine Lewis structure of BF3, we find that B is octet deficient and can accept a lone pair. Thus it can act as a Lewis acid. Thus, when reacting with ammonia, the reaction would look like:

Lewis Acid and Base

In fact octet deficient molecules are often strong Lewis acids because they can achieve an octet configuration by accepting a lone pair from a Lewis base. Compounds involving elements in periods lower then the second period can act as Lewis acids as well by expanding their valence shells. Thus, SnCl4 acts as a Lewis acid according to the reaction:

Figure 2:
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The central tin atom is surrounded by a valence shell of 12 electrons rather than 8.



The variation of pH versus the volume of added titrant is a titration curve. Such a curve can be recorded (by an automatic potentiometer) or be established point by point. The curve is sigmoïd and presents a significant variation of pH at the equivalence point, which allows an easy determination of the latter.


 Note : The more diluted is the acid (or bases) the smaller the pH jump.

 Titration of 50 ml of a strong acid HA 0.1000 M by NaOH 0.1000 M :



Determining the Equialence Point :

a) Graphical method

During the titration of an acid by a base the pH of the solution is recorded versus the volume of added base and the equivalence point can be determined from the graph pH = Vtitrant taking advantage of the approximate symmetry of the curve.

For example, if 25 ml of HCl 0.1 M are titrated by NaOH 0.1 M, the equivalence point occurs at pH = 7.00.