# Strength of bases and acids

Is it possible that strong acids like $$\ce{H2SO4}$$ behave as weaker ones like $$\ce{CH3COOH}$$ when the concentration is really high and vice versa for weak acid? And the same question for bases. By this I am generally referring to pH values.

Why do concentrated acids react more than dilute ones? The degree of ionization of concentrated acids are lower than diluted acids, so they are supposed to react less. Where am I wrong?

• Actually, strong acids can only partly dissociate (e.g. act as weak acids) when the concentration is really high (see the Debye-Hückel equation) Aug 3 '19 at 16:48
• Sorry, I'll edit it. However, during titration, the pH of acid solution increases slightly before reaching end point if we add base, doesn't it mean that by lowering the concentration, the strength decreased. Aren't degree of ionization and pH the similar things?
– Akil
Aug 3 '19 at 16:50
• @Raphaël Degree of dissociation and pH can describe properly only diluted solutions. Actually dissociation of strong acids only weakens them as undissociated acid is stronger the H3O+. Aug 3 '19 at 18:05
• Actually, yes, concentrated sulfuric acid passivates iron, chromium and aluminium and thus may appear as weaker acid. Aug 6 '19 at 4:03
• Also never forget the concept of solvo-acids and solvo-bases. In a system, say water, the solvo-acid, so the protonated form H3O+ is the strongest acid. All other acids added to water will only form H3O+. Which means that if you system was ammonia, something like acetic acid could be considered a super-acid. If we reduce the amount of H2O in there to basically zero our solvo-acid becomes the protonated form of our acid like H3SO4+. But if no stronger acid is present that can protonate H2SO4 it's up to the autodissociation of H2SO4, hence a low H3SO4+ concentration. Feb 4 '20 at 7:31

In the case of weak acids dissociating like strong ones when they are highly dilute, there is a limit to how far this can go. If you were to try to measure acetic acid dissociation at $$10^{-n}$$ molar for $$n=4,5,6,7,8,...$$ at ambient temperature you would at first rise nicely towards $$100$$% as you might expect, but eventually level off at only about $$99.4$$%.

What stops you is the autoionization of water. As you approach infinite dilution the hydrogen ion concentration approaches not zero but the autoionized value of $$10^{-7}$$ molar. So the ratio of dissociated acid to undissociated acid levels off at $$K_a/10^{-7}$$. Accordingly the limiting degree of dissociation is

$$1/[1+(10^{-7}/K_a)]=K_a/(K_a+10^{-7})$$

For acetic acid with $$K_a\approx1.8×10^{-5}$$ this limit is about $$99.4$$%, but for a much weaker acid such as a typical phenol or hydro-cyanic acid the dissociation is much lower even at extremely low concentrations. When $$K_a<10^{-7}$$ the limiting dissociation is less than $$50$$%; you need a stronger base than water to coax dissociation as a major reaction in aqueous solution.

A strong acid will always have strong characteristics. What makes an acid strong or weak is the minimum pH required to make it's acid hydrogen (the hydrogen that gets ionized in the acidification of the solution) ionized. The stronger the acid the lower the minimum pH, thus making it possible to make a more acid solution (hence the name strong). Same things for bases, but swap the acid hydrogen by the basic OH.

It is not right that diluted acids ionize less. It ionizes by the same amount (acid's kpa) but the solvent makes the average ionization smaller.

For this reason weak acids make tamponing solutions. When they get near their minimum pH they start acting like a base, deionizing the solution as it gets more hydrogen ions, holding the pH.

The characteristic property of all acids and bases is the pH. Their characteristic general reactions are the acid-base reactions. Let's concentrate on this kind of reactions. We don't want to consider here reactions of particular acids and bases (like e.g. oxidation, passivation, complexation).

We have to distinguish between dissociation degree of an acid and acidity (acid strength) of this acid. The degree of dissociation depends on the initial concentration, the acidity not. The acid constant of an acid is a measure for its acidity. An acid is characterized therefore by its acid constant. Acids are grouped into strong acids, medium strong acids and weak acids according to their acid constants.

You are right, higly concentrated strong acids can have lower dissociation degree like weak acids.

But for the behavior of an acid in an acid-base reaction, the reaction velocity is crucial. It depends on the hydrogen ion concentration in the solution. The measure for this concentration is the pH of the solution.

Highly diluted strong acids can have the same pH like less diluted weak acids and therefore the same reaction velocity in acid-base reactions.

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The analog holds for bases.