Acid and Base Dissociation Constants (Ka and Kb) — Calculation

Do you remember when investigated equilibrium? We covered a lot of topics. But, almost all the subjects apply to acids and bases. They also have an equilibrium constant. We call it the acid or base ionization constant. We denote it as $K_{a}$ or $K_{b}$. Wait, don't we already have a constant for acids and bases? You're right! We've already examined the dissociation constants ($pK_{a/b}$). They gave us a magnitude and were easier to compare. But, if we want to do calculations, we'll need the ionization constants.

Let's look at the topics we covered when we studied equilibrium. We'll apply each one to acids and bases.

• We started by defining reversible and irreversible chemical reactions.

• What's the difference between strong and weak acids and bases? If the reaction is reversible or irreversible. Strong acids and bases fully dissociate. So, they are irreversible. But, weak acids and bases are reversible reactions. So, they reach equilibrium!

• Equilibrium occurs when the rates of the forward and reverse reactions are equal.

• This is true of all equilibrium reactions, including weak acids and bases.

• Next, we defined the equilibrium constant. It's the ratio of the rate constants for the forward and reverse reactions. $K_{eq}=\frac{K_{f}}{K_{r}}=\frac{[Products]}{[Reactants]}$.

• This is also true for acid and bases. The generic chemical equation is $HA \rightleftharpoons H^{+} + A^{-}$. So, the constant is $K_{a}= \frac{[H^{+}][A^{-}]}{[HA]}$. Remember, $A^{-}$ is the conjugate base.

• After defining $K_{eq}$, we interpreted its magnitude. The magnitude helped us predict a reaction's direction. We used Le' Chatelier's principle to explain shifts in our products or reactants. These were qualitative considerations.

• The smaller the $K_{a/b}$ value, the weaker the acid. The pH of weak acids is higher when compared to the pH of strong acids.

• Le' Chatelier's principle also applies to acids and bases.

• We can also compare the ionization constant to the reaction quotient.

• Finally, we started using $K_{eq}$ quantitatively. Specifically, we calculated the equilibrium constant from concentrations. Then we learned to calculate reactant and product concentration from the constant.

• Calculating $K_{a/b}$ from concentrations is the same!

• Calculating reactant and product concentrations for weak acids and bases is also the same. We use an ICE table! We almost always solve for hydrogen or hydroxide ion concentration. Then, we can convert molarity to pH or pOH.

Overall the acid and base ionization constants behave just like equilibrium constants. Thankfully, we've already studied equilibrium. So, we're really just applying the knowledge we already have! Unfortunately, acids and bases do have a few tricks. For example, polyprotic acids have multiple ionization constants. So, we'll need a few tricks for calculating the products of polyprotic dissociations.

(Video) 18.2 Acid and base dissociation constants Ka and Kb (HL)

by Mike Sugiyama Jones

In this short video, Mike Jones explains acid and base dissociation constants. He starts with a quick overview of weak acids and bases. Then, he explains their dissociation constants. You'll also learn to compare the magnitude of the constants. This is an excellent short video for understanding acid-base equilibrium.