Order in the increasing order of acidity:HCl, H2SO4, HF, HCl, HI, HBr, HNO3, HBrO, HClO, HClO3, HClO4, H2S, H3PO4(?)

1 Answer
Truong-Son N.
Jan 10, 2018

Well, you should know seven of these at least roughly...

...and for the rest, why don't you use the acid dissociation constants your book provides? Higher #K_a# = stronger acid.

The seven common strong acids are:

#"HClO"_4#, #" "" "K_a ~~ 10^10#
#"HI"#, #" "" "" "" "K_a = 3.2 xx 10^9#
#"HBr"#, #" "" "" "K_a = 1.0 xx 10^9#
#"HCl"#, #" "" "" "K_a = 1.3 xx 10^6#
#"H"_2"SO"_4#, #" "" "K_(a1) = 10^3#
#"HNO"_3#, #" "" "K_a = 24#
#"HClO"_3#, #" "" "K_a ~~ 10#

Of these, the binary acids are some of the strongest, with one exception being #"HClO"_4#.

We explain after-the-fact that in general,

  • a more electronegative central atom pulls electron density towards itself.
  • a larger atom makes a longer (weaker) bond with the #"H"#;

Whatever trends there are follow from those two observations.

As examples,

  • #"HI"# is a stronger acid than #"HBr"#; the primary reason is that #"I"# is larger than #"Br"#, and makes a weaker bond with #"H"# (there is a counteracting electronegativity trend, which is not significant here). Weaker bond = stronger acidity.
  • #"HNO"_3# is a stronger acid than #"HClO"_3#; the primary reason is that #"N"# is more electronegative than #"Cl"#, so it pulls electron density towards itself better. Thus, the #"H"#, attached to an outer #"O"#, gets less of it and thus the #"O"-"H"# bond is weakened. Weaker bond = stronger acidity.

Setting aside the ones we picked, the remaining ones are:

#"H"_3"PO"_4#, #" "" "K_(a1) = 7.52 xx 10^(-3)#
#"HF"#, #" "" "" "K_a = 7.2 xx 10^(-4)#
#"H"_2"S"#, #" "" "" "K_(a1) = 9.1 xx 10^(-8)#
#"HClO"#, #" "" "K_a = 3.0 xx 10^(-8)#
#"HBrO"#, #" "" "K_a = 2.0 xx 10^(-9)#

Of these, the only ones that are conceptually reasonable to explain are #"HClO"# vs. #"HBrO"#.

  • In direct contrast with #"HCl"# vs. #"HBr"#, #"HClO"# is a stronger acid than #"HBrO"#, because #"Cl"# is more electronegative, which dominates over the size difference between #"Cl"# and #"Br"# due to the presence of the oxygen.

Therefore, the #"H"-"Cl"# bond is weakened more from more uneven sharing of electrons, and weaker bond = stronger acidity.

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