Acid-Base Strengths

So, when an acid dissolves in water, it will ionize. This is because as it donates hydrogen ions to water, the acid will become a conjugate base, which was a more negative charge than the original acid due to the fact that the acid lost a positively charged hydrogen cation.

Similarly for bases, when they dissolve in water, they ionize by taking a proton from the water molecules. This because as it accepts hydrogen ions from water, the base will become a conjugate acid, with a more positive charge than the original base since the base gained a positively charged hydrogen cation.

The extent to which an acid or base dissociates is known as its strength. If an acid is very strong, then almost all of the acid will dissociate into hydrogen cations(or hydronium cations). If an acid is weak, then most of the original acid will stay intact and only some of it will dissociate into hydrogen cations.

If a base is very strong, then almost all of it will accept protons and form hydroxide ions but if a base is weak, then most of it will stay intact and not as much proton acceptance will occur.

-Dissociation constants + equations(8 min)

-Kb Ka comparison(6 min)

-Some strong acids + bases(3 min)

-Electronegativity(5 min)

We can quantify the relative strengths of acids and bases through their dissociation constants. So, what is an ionization constant? They're basically the equilibrium constants for solutions of certain acids and bases. For example, the acid-ionization constant of a strong acid will be higher than 1, because there will be more conjugate base and hydrogen ions than original acid alone. Similarly, the base-ionization constant of a strong base will be higher than 1, for there will be more conjugate acid and hydroxide ions than original base alone.

However, in general, the higher the acid-ionization constant of an acid is, the stronger the acid is, even between weaker acids. For a general reaction of an acid HA:

HA(aq) + H₂O(l) ⇌ H₃O⁺(aq) + A^-(aq)

The acid-ionization constant K_a is:

In general, the higher the base-ionization constant of a base is, the stronger the base is, even between weaker bases. For a general reaction of a base B:

B(aq) + H₂O(l) ⇌ HB⁺(aq) + OH^-(aq)

The base-ionization constant K_b is:

Below is a chart for comparing different acids and their conjugate bases on relative strength.

So, you may or may not have noticed something interesting about the K_a of an acid and the K_b of its conjugate base(or vice versa). Let's compare them side-by-side:

Alright now let's multiply them. You'll notice that HA is actually equal to the HB⁺ in this reaction. The acid in the acid-ionization constant expression is the same as the conjugate acid in the base-ionization constant. These factors can cancel out. The same goes for A^- and B, as the conjugate base A^- for the acid is the same as the base B. These also cancel out, leaving hydronium and hydroxide. This means that the product of K_a and K_b for a conjugate acid-base pair is equal to K_w , or 1.00 * 10^-14. In other words,