How do we assess the strength of an acid?

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How do we assess the strength of an acid?

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Answer 1 · 2017-03-16T17:02:03

How else but by experiment.........?

Explanation:

Of course we know that $H B r$ $HBr$ is a POTENT $Bronsted acid$ $"Bronsted acid"$ . And all $Bronsted acids$ $"Bronsted acids"$ are by definition potent $Lewis acids$ $"Lewis acids"$ , electron pair acceptors.

Solutions of hydrogen bromide in water are stoichiometric in $hydronium ion$ $"hydronium ion"$ :

$H B r (g) + H_{2} O (l) \to H_{3} O^{+} + B r^{-}$ $HBr(g) +H_2O(l) rarr H_3O^(+) + Br^(-)$ .

With the notable exception of $H F$ $HF$ , ALL of the hydrogen halides, $H X$ $HX$ , are strong $Bronsted acids$ $"Bronsted acids"$ , i.e. the defining equilibrium lies strongly to the RIGHT:

$H X (a q) + H_{2} O (l) ⇌ H_{3} O^{+} + X^{-}$ $HX(aq) + H_2O(l) rightleftharpoons H_3O^+ + X^(-)$

Because the fluoride ion is smaller, and more charge dense, and the $H - F$ $H-F$ bond is strong, $H F$ $HF$ is a relatively weak Bronsted acid, and fluoride salts give rise to (slightly) basic solutions:

$F^{-} + H_{2} O (l) ⇌ H F (a q) + H O^{-}$ $F^(-) + H_2O(l) rightleftharpoonsHF(aq) + HO^-$

This is well-known to be an entropy NOT an enthalpy phenomenon.

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How do we assess the strength of an acid?

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