Question

In each row, check the box under the compound that can reasonably be expected to be more acidic in aqueous solution, e.g. have the larger Ka
a) HCH3CO2 vs HCH3SO2

b) H3PO4 vs H3PO3

c HClO2 vs HClO

Please explain why.

Answer 1

Answer:

$HCH_{3}SO_{2}$

$H_{3}PO_{3}$

$HClO_{2}$

Explanation:

Every acid (HA) tends to disolve into proton ($H^{+}$) and anion ($A^{-}$) in aqueous solution. Acid strength can be determined by measuring this tendency to separate into proton an anion. Strength of an acid can be quantified by its acid dissociation value - Ka. A strong acid will have a tendency to easily release proton and will have larger Ka value and smaller logarithmic value (pKa = - logKa) similar to calculating pH of the solution. So the easiest way to resolve this issue is by looking for Ka or pKa value of the acid (This table may be useful in more complex tasks and is attached below). However, stronger acid can be determined elsehow.

a) Carbon is element 14 with 4 valent electrons and sulfur is element 16 with 6 valence electrons. Thus, sulfur has stronger electronegativity (tendency to attract bonded electrons towards itself). This means that sulfur will hold oxygen tighter to itself so the hydrogen bond to it can be more easily separated from it. $HCH_{3}SO_{2}$ is more acidic in aqueous solution.

b) In $H_{3}PO_{4}$, phosphorus holds one double bond with oxygen and three OH group equally. To show an acidic tendency, phosphorus would need to let go one hydrogen out of one of OH groups. In $H_{3}PO_{3}$, phosporus holds two double bong with oxygen, one OH and one hydrogen, all single and lonely, ready to leave phosphorus and show acidic characteristics in aqueous solution. Thus, $H_{3}PO_{3}$ is more acidic compound.

C) In all Cl acids, the electron density is placed around Cl so the more oxygen around Cl, the more acidic will be the chemical. This is comparable to an oxidation state - the bigger oxidation state, the stronger acid will be:

$HClO_{4} ^{+7} >HClO_{3}^{+5} >HClO_{2}^{+3} >HClO_{}^{+1}$

$HClO_{2}$ can reasonably be expected to be more acidic in aqueous solution.