There are three factors that affect the acidity of the generic bond X-H in a nonmetal hydride.

1.  The POLARITY of the X-H bond

• If all other factors are constant, the polarity of the X-H bond is directly proportional to the strength of the acid.  Polarity is measured by the difference in electronegativities between the atoms in a bond.
• When an acid dissociates in water, the X-H bond is broken to form the ions H⁺ and X^- ions.  The greater the polarity of the bond, the easier it is to form these ions, and thus the stronger the acid.

2.  The SIZE of the X atom

• The strength of acids increases is directly proportional to the size of the X atom increases because the strength of the X-H bond diminishes as the size of the X atom increases.  The relative strengths of the X-H bond in various compounds can be compared by measuring the bond-dissociation enthalpy.
  • Bond-dissociation enthalpy (BDE) - The amount of energy required to break a bond, specifically in the gas phase of a compound.

3.  The CHARGE on the acid or base

• Compounds becomes less acidic as the negative charge increases.
• Conversely, compounds become more basic as the negative charge increses.
• This can be explained by the fact that it is easier to remove a positive ion (H⁺) from a neutral atom or molecule than a negatively charged one because the force of attraction between the positive and negative particles hinders the separation of the oppositely charged particles.  Bases (H⁺ acceptors) become stronger as their negative charge increases because they have a stronger force of attraction for pulling in extra H⁺ ions.

The OXIDATION STATE of the central atom

When the polarity, size, and charge of two compounds are all the same (e.g. when comparing the strengths of oxyacids of the same element) we must find another way to measure the relative strengths of these acids.

• Oxyacid - An acid in which the acid hydrogen atoms are attached to an oxygen atom
  • Examples of oxyacids of the same element:

    H₂SO₄ (sulfuric acid) and H₂SO₃ (sulfurous acid)
    HNO₃ (nitric acid) and HNO₂ (nitrous acid)

• As the oxidation state of the central atom in an oxyacid increases, the strength of the acid increases significantly.  As the oxidation state of an atom increases, its tendency to draw electrons in a bond toward itself increases.  In an oxyacid, the central atom pulls electrons away from the oxygen, consequently making the oxygen more electronegative.  The O-H bond, therefore becomes more polar, making it easier to form ions and thus increasing the strength of the acid.

Next:  "Equilibrium:  Acid-Dissociation Equilibrium Constants"