Today I learned that $\ce{H2S}$ is more acidic than $\ce{H2O}$ because $\ce{H2S}$ has a larger molecule size. But $\ce{Fe^3+}$ is more acidic than $\ce{Fe^2+}$ and $\ce{Fe^3+}$ is smaller than $\ce{Fe^2+}$. Likewise, formic acid is more acidic than acetic acid but formic acid is smaller than acetic acid.

How does this go together?

  • $\begingroup$ Are you trying to compare Brønsted-Lowry acidity and Lewis acidity? $\endgroup$
    – ringo
    Feb 27 '17 at 3:09

It is key to learn about the applicability of certain concepts — or why does a concept work in a certain case — along with the concepts themselves. Most notably, atom size (it should not be molecule size, but for $\ce{H2O/H2S}$ it does not matter due to hydrogen’s tinyness) is only of limited help in a few select cases to determine acidity.

When comparing the acidity of two compounds you should always actually look at the following equation and then compare the anionic products of two different acids:

$$\ce{HA + H2O <=> A- + H3O+}\tag{1}$$

Whichever generates the more stable anion is the stronger acid. Thence, a number of concepts have been developed to determine the more stable anion from a given list of alternatives — one of them is the atomic size. It is useful only when directly comparing two otherwise group-identical species — like oxygen and sulfur which are in the same group but different periods or chlorine and iodine. Basically, under otherwise identical conditions (same valency and same charge) the larger anion is able to stabilise the negative charge better. Thus, $\ce{H2S}$ is a stronger acid than $\ce{H2O}$ and $\ce{HI}$ is stronger than $\ce{HCl}$ but we cannot say anything about the difference between $\ce{H2S}$ and $\ce{HCl}$ or even $\ce{H2S}$ and $\ce{HF}$ from that.

All the other pairs you listed do not fit into that difference concept at all. The acidity of $\ce{Fe^n+}$ is not explained simply by looking at the metal ion. And the difference between acetic and formic acid is on a much more subtle scale since both are oxyanions. To reiterate: do not only learn concepts but learn what concepts are for.

  • 1
    $\begingroup$ Since OP discussed formic and acetic acids, it should also be pointed out that once past about propanoic acid that additional $\ce{-CH2 -}$ groups don't change pKa much. So for organic compounds molecular size means little. For organic compounds it is more about what type of functional group. Carboxylic acid, phenol, alcohol etc. (This is obviously much further down the rabbit hole than the points you raised.) $\endgroup$
    – MaxW
    Feb 27 '17 at 5:47

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