Phosphoric acid has basicity of 3 i.e it can loose 3 $\ce{H+}$ while phosphorous acid has basicity of 2. Why is phosphorous acid more acidic than phosphoric acid? Acidity refers to the ability to liberate protons. Phosphoric acid liberates more protons than phosphorous acid (as the basicity of phosphoric acid is 3 and that of phosphorous acid is 2). Why then is phosphorous acid more acidic than phosphoric acid?
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1$\begingroup$ Consider that proticity is not the only factor governing acidity. For example, hydrochloric acid is considered to be strongly acidic, but it only has the capacity to donate one proton. At the same time, carbonic acid is considered to be weakly acidic, though it has two protons available for donation. $\endgroup$– T. KentCommented Dec 25, 2016 at 20:33
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$\begingroup$ @T.Kent does carbonic acid really exists ? ^^ $\endgroup$– ParaH2Commented Dec 25, 2016 at 22:48
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2 Answers
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You’re using a wrong definition of acidity. Acidity and its adjective acidic do not mean ‘number of protons liberated eventually.’ Instead, acidity is well defined as the equilibrium constant $K_\mathrm{a}$ of the corresponding first deprotonation reaction $(1)$:
$$\begin{align}\ce{H_nA &<=> H_{($n-1$)}A- + H+}\tag{1}\\[0.6em] K_\mathrm{a} &= \frac{[\ce{H_{($n-1$)}A-}][\ce{H+}]}{[\ce{H_nA}]}\tag{2}\end{align}$$
For phosphoric acid, the corresponding deprotonation is reaction $(3)$ and the equilibrium constant is as written; for phosphorous acid, see $(4)$.
$$\begin{align}\ce{H3PO4 &<=> H2PO4- + H+} &&K_\mathrm{a} = 7.11 \times 10^{-3}\tag{3}\\[0.8em] \ce{H3PO3 &<=> H2PO3- + H+} && K_\mathrm{a} = 5.01 \times 10^{-2}\tag{4}\end{align}$$
Phosphorous acid has the higher acidity constant corresponding to a lower $\mathrm{p}K_\mathrm{a}$ value and is thus more acidic than phosphoric acid.
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I'm pretty sure it is because phosphorous acid is more polar than phosphoric acid. This is because in the phosphoric acid there are more OHs and an O, so it balances out more towards the center.
In phosphorous acid, there is a H directly bonded to the central phosphorus. Hydrogen doesn't pull as much on electrons as oxygens do; hence, that part of the molecule becomes more positive, causing a stronger dipole moment than that in phosphoric acid.
answered Dec 25, 2016 at 20:37