Arrange in descending order of acidityof H3PO2,H3PO3 and H3PO4.Give reasons

Here, the O atoms draw electrons away from Z atom and make it more positive. The Z atom, therefore, becomes more effective in withdrawing electron density. In turn, electrons of the H – O bond are drawn more strongly away from the H atom. The net effect makes it easier for the proton to dissociate and increases the acidity of the compound. With more number of 0 atoms bonded to Z, the compound becomes a stronger acid.

The acid strength is also explained by the oxidation number of central atom ( here P )

H3PO2  -      +1

H3PO3  -     +3

H3PO4  -     +5

The acid strength is also explained by counting the number of oxygen atoms bonded to P

hence order of acidity is : H3PO4   > H3PO3  >H3PO2

--

Regards

Ramesh

For this question, first draw the Lewis structures of each acid. You will see that for H3PO3 one of the hydrogens is bonded directly to phosphorus. This is not an acidic hydrogen and therefore H3PO3 is diprotic. On the other hand, H3PO4 is triprotic, all hydrogens bonded to oxygens. Looking at the pKa values we see that pKa1 for H3PO3 is 1.3 where as pKa1 for H3PO4 is 2.1. Also the pKa2 for H3PO3 is greater than pKa2 for H3PO4. From this experimental data we can see that H3PO3 is more acidic. Another way of thinking about this is to consider the polarization of each acid. H3PO3 is more polar, so the O-H bonds will be weaker, making the H+ easier to come off, and causing H3PO3 to be more acidic than H3PO4. HenceH3PO4

The correct acidic strength order is H3PO2>H3PO3>H3PO4 because number of P-OH bond increases the acidity of species decreases. Also we can check by stability of conjugate bases.

The acidity is the following order H3PO2>H3PO3>H3PO4....because the electron withdrawing p=o is one for all the acids but the effect is minium for H3PO4 due to the effect is distributed over 3 OH- group but effect is maximum in H3PO2 due to the effect is distributed in one OH ....