# Weak Acid and Base Equilibria

Imagine you're cleaning the kitchen counters. You'll probably use cleaning products to keep your house clean! These everyday cleaning products contain weak acids and bases. We are going to explore how weak acids and bases react in equilibrium.

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Weak acid and base equilibrium refer to the equilibrium in weak acid and base reactions because of their partial ionization in a solution.

• First, we are going to broadly discuss the ionization of weak acids and bases
• Then, we will consider the weak acid and weak base equilibrium equations
• After, we will study and work through some acid-base equilibrium problems
• Finally, we will explore some common examples of weak acids and base

If you do not yet have a general sense of what acids and bases are, please check out Acids and Bases to gain a foundational understanding!

## Ionization of Weak Acids and Bases

Before we can get into the specifics of weak acid-base equilibrium, let's consider what makes an acid or base weak and the equilibrium in weak acid-base reactions.

Weak acids are acids that only partially ionize in an aqueous solution.

Weak bases are bases that only partially ionize in an aqueous solution.

This differs from strong acids and bases, which fully ionize in aqueous solutions!

A relatable metaphor to visualize this complete vs. partial ionization can be found in the context of friendships!

• If you have a strong friendship with someone, you may give them most of your trust and accept many gifts from them. But if you have a weaker relationship with someone, you may withhold most of your trust and be slow to accept gifts.
• In the same way, a strong acid completely donates all of its H+ protons, and a strong base readily accepts all H+ protons. Whereas a weak acid only donates some of its H+ protons, and a weak base only accepts some H+ protons.

While weak acids and bases aren't in a friendship, they are definitely in chemical "relationships" partially giving H+ or accepting H+ protons, respectively!

### How are weak acid-base equilibria formed?

Since weak acids and bases don't fully ionize when they react with water, the resulting solution is a mixture of ions. Let's look at an example of a weak base to conceptualize this. When ammonia reacts with water, we get the following chemical equation:

$$NH_{3\ (aq)}+H_{2}O_{(l)}\rightleftharpoons NH_{4\ (aq)}^{+}+OH_{(aq)}$$

In the aqueous solution, we will have:

1. The base: NH3.
2. The hydroxide ion: OH-.
3. The conjugate acid: NH4+ .

Because the reaction doesn't go to 100% completion, we are left with ions and the unionized base species, which create a dynamic equilibrium! This means that the ammonia molecules accept hydrogen ions from the water molecules and form hydroxide ions and ammonium ions at the same rate that hydroxide ions donate hydrogen ions to ammonium ions to reform ammonia molecules and water molecules!

Try to say that sentence five times fast... it's a tongue twister for sure! If you have difficulty understanding equilibrium, check out Dynamic Equilibrium and Reversible Reactions for a refresher!

If weak acids and bases aren't 100% ionizing, how do we know to what percent they are ionizing? Well, that's where equilibrium equations and constants come into play! We will spend time on acids and bases separately as they are each essential concepts to grasp!

## Weak Acid Equilibrium Equation

Let's examine the general weak acid equilibrium equation and constant! In a weak acid reaction, the aqueous acid molecules, HA (aq), react with liquid water by donating H+ ions to the water. This reaction forms aqueous hydronium ions and aqueous anions, A-(aq).

The following generic equation can represent the weak acid reaction:

$$HA_{(aq)}+H_{2}O_{(l)}\rightleftharpoons H_{3}O^{+}_{(aq)}+A^{-}_{(aq)}$$

Where:

• HA = acid species.
• A- = conjugate base of the acid.

Based on this generic equation, we can construct the equilibrium constant expression, referred to as the acid ionization constant, Ka.

The acid ionization constant, Ka, represents the relative strength of an acid. The strength of an acid is determined by the extent to which an acid dissociates in an aqueous solution.

## Weak Acid and Weak Base Examples

Common examples:

### Weak Acids

• Acetic acid (CH3COOH).
• Formic acid (HCOOH).
• Hydrocyanic acid (HCN).

### Weak Bases

• Ammonium Hydroxide (NH4OH).
• Aniline (C6H5NH2).
• Ammonia (NH3).

## Weak acid and base equilibria - Key takeaways

• Weak Acids and Bases only partially ionize
• the extent to which they ionize is quantified by dissociation constants
• Weak Acids can be represented by the generic equation HA (aq) + H2O (l) ⇌ H3O+(aq) + A(aq), and the equilibrium constant ${\mathrm{K}}_{\mathrm{a}}=\frac{\left[{\mathrm{H}}_{3}{\mathrm{O}}^{+}\right]\left[{\mathrm{A}}^{-}\right]}{\left[\mathrm{HA}\right]}$
• Weak Bases can be represented by the generic equation B (aq) + H2O l) ⇌ BH+(aq) + OH-(aq) and equilibrium constant ${\mathrm{K}}_{b}=\frac{\left[B{H}^{+}\right]\left[O{H}^{-}\right]}{\left[B\right]}$
• To find the concentration of H3O+ or OH-, you can use the equation $\phantom{\rule{0ex}{0ex}}{H}_{3}{O}^{+}=\sqrt{{K}_{a}·\left(ConcentrationofHA\right)},orforO{H}^{-}=\sqrt{{K}_{b}·\left(ConcentrationofBase\right)}\phantom{\rule{0ex}{0ex}}$

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What is the equilibrium of a weak acid

Ka = [H3O+][A-}/[HA]

What is an acid base equilibria?

Acid-Base equilibria is the state where the acidic and basic ions in a reaction neutralize each other. At equilibrium is when the rate of the acid/base ionizing into a conjugate base is the same as the conjugate base producing the acid/base species.

why does equilibria favor weak acids and bases?

Equilibria favor weak acids and bases due to their lower energy. Since they have lower energy than strong acids and bases, they are a more stable species.

What are some real-life examples of acid-base reactions?

Some real-life examples of acid-base reactions include the reaction between vinegar and baking soda, antacids you can take for stomach issues, brushing your teeth with basic toothpaste, and so much more!

How is the equilibrium constant for a weak acid

Ka = [H3O+][A-}/[HA]

## Test your knowledge with multiple choice flashcards

If a weak acid reacts with water, which of the following correctly describes the species that will be in the solution?

Which of the following is the weakest base?

The ____ the Ka value, the ____ the pH, the ___ acid

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