I just learned that:

Acids get stronger with:

  1. Increasing electronegativity of atom bonded to $\ce{H}$.
  2. Size of atom bearing negative charge.
  3. Hybridization of negative charge (more s character, $\ce{e-}$ closer to nucleus, more acidic).
  4. Resonance stabilization (like size, greater distribution of electrons).

So basically, the more concentrated the negative charge (think $\ce{F-}$, $-1$ and very small), the tighter it's going to "hold on" to the proton. With resonance and large negative groups, the charge is less concentrated, so it's not going to "hold on" to the proton as well.

However, electronegativity is the tendency for an atom to "hog" electrons. Therefore, the more electronegative, the more electrons there are in a similarly sized space.

More concentrated electrons = more stable base = weaker acid.

But electronegativity = more concentrated electrons (even though size trumps it).
And electronegativity = stronger acid.

Feel free to edit if you see what I'm trying to say but can say it better!

  • $\begingroup$ You're overlooking the decisive step: An acid is the stronger, the more energy it gains by becoming an anion. With that, you can rationalise. Chemistry is 95% pure logic. $\endgroup$ – Karl Sep 14 '15 at 22:48
  • 1
    $\begingroup$ Hey, more stable base means stronger acid! $\endgroup$ – Mithoron Sep 14 '15 at 23:46
  • $\begingroup$ Well, $\ce{HF}$ is a weak acid ... $\endgroup$ – ParaH2 Sep 5 '17 at 21:09

The main point about strong acids is ‘how stable is the conjugate base?’

The better stabilised the negative charge on a conjugate base of a neutral acid is, the happier the species will be to loose a proton and thus the more acidic the proton is. All of the points you noted just emphasise that.

Therefore, the only wrong assumption you are making is:

more stable base = weaker acid.

The correct way is:

more stable base = stronger acid.


These factors you are talking about are to check the stability of CONJUGATE base. The more stable the conjugate base is, the stronger the acid, and vice versa.

Acidity can depend on mainly 3 factors:

  1. Protic character of the H atom
  2. Polarisation of the H atom

And most important, and the best way, is

  1. Stability of the conjugate base

Or one more simple way is to check for the electron pullers: the stronger the electrons pullers attached to it, the stronger the acid, because the bond with the H atom will be weaker.

But, what do you really mean by strong acid? It's the one which gives its H ion most easily (to be very simple and clear).

If you think according to the conjugate base, it will have a cation (+ charge), at the place where H is given out. Similarly if you put your points you will find it tells about the stability of conjugate base and eventually about the strength of an acid

  • $\begingroup$ I notice you were never really welcomed, so allow me to welcome you to chemistry.stackexchange.com. Please take a tour of the site, and visit the help center to learn more on how it works. I would specifically advise you to visit the page about formatting your posts. We generally prefer high-quality answers, i.e. readable and not using abbreviations like ur. $\endgroup$ – Jan Oct 1 '15 at 17:46

Increasing EN means less acidic. Refer to the $\mathrm{p}K_\mathrm{a}$ values below.

$$ \begin{array}{l|rrrr} \hline \ce{HX} & \ce{HF} & \ce{HCl} & \ce{HBr} & \ce{HI} \\ \hline \mathrm{p}K_\mathrm{a} & 3.2 & -7 & -9 & -10 \\ \hline \end{array} $$


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.