Acid-Base Reactions and Buffers

Definition of Acid-base Reactions and Buffers

For starters, let's take a look at the definition of an acid-base reaction, and a buffer solution.

Now that we've covered the basic definitions, let's look at what acid-base reactions look like!

Acid-base Reaction Equations

Acid-base reactions follow a basic structure: $$HA + B \rightarrow A^- + HB$$

Here, HA is our acid, which donates a proton to the base, B. The symbol, A^-, is the conjugate base and HB is the conjugate acid.

While all acid-base reactions follow this same structure, the reaction is slightly different based on the strength of the acid and/or base. Strong acids/bases dissociate completely. Here is an example:

$$HCl \rightleftharpoons H^+ + Cl^-$$

Hydrochloric acid (HCl) is a strong acid, so it will completely dissociate into its ions: H⁺ and Cl^-. After the dissociation, the concentration of the ions is equal to the initial concentration of the strong acid/base.

Weak acids/bases work differently because they do not dissociate completely. Here's another example: $$CH_3COOH \rightleftharpoons CH_3COO^- + H^+$$

Acetic acid is a weak acid, so it only partially dissociates into an H⁺ ion and the conjugate base. Since the dissociation isn't complete, you will still have the weak acid, and in a much higher concentration than its ions.

Here is a diagram to explain:

Fig. 1 - Strong acids dissociate completely, while weak acids only partly dissociate.

When a strong acid (or base), dissociates it will be completely converted into its ions. For a weak acid/base, however, there is only a partial conversion.

The difference in how these reactions work is why buffers are made from weak acids and bases and not strong ones.

Types of Buffer Solution

Before we get into why buffer solutions use weak acids/bases, let's talk about the different types of bases. There are two types of buffers:

1. Acidic buffer solutions
2. Alkaline buffer solutions

Acidic buffer solutions are used in systems that have a pH below 7 (i.e. acidic systems). These are made up of a weak acid and its conjugate base. Alkaline buffer solutions are used in systems with a pH above 7 (basic solutions). These are made up of a weak base and its conjugate acid.

Here is a table of some common buffers:

Now that we know the different types, we can learn how buffer solutions keep the pH stable.

Neutralization Reaction of Acids and Bases for Buffers

The goal of a buffer is to neutralize any added acid or base as much as possible, therefore these buffers participate in neutralization reactions.

The reactions that happen are dependent on what is being added to the buffer system. Let's start by looking at acidic buffers. The system we will be using is acetic acid/acetate (HC₂H₃O₂/C₂H₃O₂^-).

When a strong base is added, the OH^- will be neutralized by the weak acid:

$$HC_2H_3O_2 + OH^-\rightarrow H_2O + C_2H_3O_2^-$$

The buffer "absorbs" the OH^- ions, so the pH won't change much.

When a strong acid is added, the H⁺ ions will combine with the conjugate base to reform the weak acid: $$H^+ + C_2H_3O_2^-\rightarrow HC_2H_3O_2$$

Here is a graphic showing what's going on in solution:

Fig. 2 - The effects of adding a strong acid or base on a buffer system.

The formation of the weak acid/conjugate base will only change the pH slightly, due to their weakness. However, if this was a strong acid or base, that would be a different story. Buffers are made from weak acids/bases because of this.

Alkaline buffers work in the same way since they also contain a weak acid and weak base.

pH of Buffer Solution

We know that buffers are designed to keep the pH stable, but the pH will still be altered slightly when a strong acid or base is added. The way we measure the pH of a buffer solution is by using the Henderson-Hasselbalch equation.

Let's work on a quick example problem.

When the concentrations of the acid and base are equal, then the logarithm will be equal to zero. This means that the pK_a= pH

The concentration of acid and base listed in the equation is the total concentration, not just for the buffer. So if HCl was added to a buffer sample, the concentration of the acid would be a sum of the concentrations of the weak base and the HCl. In the next section, we will walk through these types of examples.

Examples of Acid-base Reactions and Buffers

Now that we know how to calculate the pH of buffer solutions, we can learn how to calculate the pH when a strong acid/base is added. Let's walk through some examples together.

Now let's try an example where a base is added.

Let's do one more problem. This time we'll be calculating the change in pH.

Now, I hope that you feel more confident in your understanding of acid-base reactions and buffers!