Acid (Base) Strength

We rank the relative strength of different acids in terms of the magnitude of their acid equilibrium constant K_a. The larger the K_a, the greater the hydronium ion concentration for a solution of that acid. If we compare a series of different acid solutions with the same concentration, the solution made from the acid with the largest K_a has the greatest hydronium ion concentration. We would refer to this acid as the "strongest" as it dissociates the most.

Typically this comparison is made only for "weak" acids. Strong acids have K_a's that are so large they are rarely tabulated (or utilized). It is generally assumed that strong acid K_a's are essentially infinitely large.

In contrast, weak acids do not ionize 100%. Take this table of a few weak acids that have been sorted by the K_a values.

Acid Name	Formula	Ka
Hydrofluoric	HF	3.5 x 10-4
Formic	HCOOH	1.8 x 10-4
Acetic	CH3COOH	1.8 x 10-5
Hypochlorous	HClO	3.0 x 10-8
Hydrocyanic	HCN	4.9 x 10-10

Of these acids the strongest is HF as it has the largest K_a. Hydrocyanic is the weakest of the group as it has the smallest K_a. This means that HF will dissociate to a greater extent when placed in water than the other acids. HCN will dissociate the least.

An analogous comparison can be made for weak bases looking at K_b. The largest K_b will protonate to the greatest extent in water and form the highest concentration of the hydroxide ions. The smallest K_b will form the lowest concentration of hydroxide in water.

Finally, a safety note. Weak acids/bases are not "weak" in the sense that they are not reactive. Weak acids and bases can be extremely dangerous especially in high concentrations. Don't let the name fool you. It is merely a means to rank how much they react in water to form hydronium or hydroxide ions. The stronger the acid, the larger the K_a, the more H₃O⁺. The stronger the base, the larger the K_b, the more OH^-.

© 2013 mccord/vandenbout/labrake

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