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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Third Row Elements Page 5

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10.

Acidic and Basic Character of Hydroxides of Third Row Elements

 

The hydroxy compounds of Na, Mg, Al are called hydroxides and those of Si, P, S and Cl are called oxy acids (acids containing oxygen element).

 

 

  • Arrhenius Concept and Ionisation Potential

    The hydroxides of the third row elements can be represented by the general formula
    M-O-H.

    M-O-H will act as an acid if it produces H+ ions in aqueous medium.

M-O-H MO- + H+

M-O-H will act as base if it produces OH- ions in aqueous medium.

M-O-H M+ + OH-

 

 

  • The nature of the hydroxide is governed by the ionisation potential (I.P.) and the electronegativity of M.

 

 

  • The elements Na and Mg are electropositive and have a low I.P. (low electronegativity). Hence, electrons of M are easily lost to the oxygen of the OH group. This weakens the M-OH bond and OH- ions are released.

Thus, hydroxides of these elements are strong bases.

NaOH Na+ + OH-

 

 

  • The elements S and Cl are electronegative with high I.P. Hence, the electrons are not easily lost to oxygen. This weakens the MO-H bond and H+ ions are released. Thus, the hydroxides of these elements are weak bases but strong acids. Chlorine hydroxide is a strong acid.

Cl-O-H H+ + Cl-O-

Si and P have I.P. values lower than S and Cl. Thus, the hydroxides of these elements are weak acids.

The hydroxide of Al shows both acidic and basic character.

Al(OH)3 + 3HCl
AlCl3 + 3H2O

Al(OH)3 + NaOH
NaAlO2 + 2H2O

Thus, in the third row elements, the basic character of the hydroxide decreases and the acidic character of the hydroxy compounds increases with increase in I.P. value.

 

 

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