If a reaction requires a "strong" acid - what does this mean? Does it mean any acid of a specific concentration or pH? Are only certain acids classed as "strong"? I have read on Wikipedia that it needs to ionise completely in an aqueous solution - in this case, is a strong acid (for example, HCl) still classed as a strong acid if it is in a 0.00001M solution?


The strength or weakness of an acid is independent of its concentration. A strong acid dissociates completely in solution, so $\ce{HCl}$ at any molarity would be considered strong, and $\ce{HCOOH}$ (for example) at any molarity would be weak. It is more enlightening to consider $K_a$ values, which are essentially the equilibrium constants for the ionization of an acid $\ce{HA}$. $$K_a=\frac{[\ce{H}^+][\ce{A}^-]}{[\ce{HA}]}$$ This value will either be very small (<<1) or very large (>>1000). Acids with very small $K_a$ are considered weak, and acids with very large $K_a$ are considered strong (the $K_a$ of strong acids is usually not stated and assumed to be infinite).

Addendum: The $K_a$ value is concentration-independent because you are only interested in how well the acid dissociates from its molecular form. If you had $0.0000001\ \text{mol}\cdot\text{dm}^{-3}\ \ce{HCl}$, practically every molecule of $\ce{H-Cl}$ would dissociate into $\ce{H^+}$ and $\ce{Cl^-}$ no matter how many there are. But if you had $12 \ \text{mol}\cdot\text{dm}^{-3}\ \ce{HCOOH}$ (methanoic acid), only a small proportion of the $\ce{HCOOH}$ would dissociate into $\ce{H^+}$ and $\ce{HCOO}^-$, irrespective of how many you had to start.

  • $\begingroup$ Would it not be simpler to just say that a high concentration of hydronium causes acidity? And, would it matter which definition of acid we are using (Arrhenius and Bronsted-Lowry)? (I am still learning, so this is a legit question that I have) $\endgroup$ – Carlos Carlsen Jun 7 '16 at 16:13
  • 1
    $\begingroup$ Nitric acid Ka is about 24, but it is considered a strong acid. Instead of "assumed to be infinite", a strong acid is more like Ka > 1 $\endgroup$ – DavePhD Jun 7 '16 at 16:18
  • $\begingroup$ Also, having a $K_\mathrm{a} > 1$ is not equal to ‘fully dissociated in solution’. It is ‘fully dissociated in aquaeous solution.’ Tiny, but important difference. $\endgroup$ – Jan Jun 7 '16 at 16:45

What makes an acid strong is also dependent on the context. E.g. lactic acid is considered strong in body fluids because lactate is completely ionized in body pH 7.4.


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.