Chemistry · Ch 6 — Equilibrium
Ionization of Acids and Bases
Ionization of Acids and Bases
The Meaning of Acid and Base Strength
The Arrhenius concept is most useful when we talk about acids and bases in water, because most chemical and biological ionizations happen in aqueous solution. A strong acid, by this definition, is one that dissociates almost completely into its ions in water. For example, hydrochloric acid (HCl) in water exists almost entirely as H₃O⁺ and Cl⁻ ions; very few HCl molecules remain undissociated. The common strong acids are perchloric acid (HClO₄), hydrochloric acid (HCl), hydrobromic acid (HBr), hydroiodic acid (HI), nitric acid (HNO₃), and sulphuric acid (H₂SO₄). Similarly, strong bases like lithium hydroxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH), caesium hydroxide (CsOH), and barium hydroxide (Ba(OH)₂) dissociate completely to give OH⁻ ions in water.
The Brønsted-Lowry concept gives us another way to think about this: a strong acid is a very good proton donor, and a strong base is a very good proton acceptor. These two views are consistent — a substance that completely donates its proton in water is clearly a strong proton donor.
The Dynamic Equilibrium of a Weak Acid
Consider a weak acid HA in water. It sets up the following equilibrium:
Here, HA and H₃O⁺ are both acids (they can donate a proton), while H₂O and A⁻ are both bases (they can accept a proton). The equilibrium is dynamic — protons are constantly being transferred back and forth. The key question is: which direction is favoured? The answer lies in comparing the strengths of the two acids present: HA and H₃O⁺.
The Driving Force: The Weaker Acid Wins
The equilibrium will shift in the direction that produces the weaker acid and the weaker base. Why? Because the stronger acid has a greater tendency to donate its proton, and it will do so to the stronger base. The stronger base accepts the proton, forming the weaker conjugate acid. So the net reaction is: the stronger acid donates a proton to the stronger base, producing the weaker acid and the weaker base.
Let's apply this to our equilibrium. If HA is a stronger acid than H₃O⁺, then HA will be the dominant proton donor. The equilibrium will lie far to the right, meaning the solution will contain mostly A⁻ and H₃O⁺ ions. Conversely, if H₃O⁺ is the stronger acid (which it is for most weak acids), the equilibrium will lie to the left, and the solution will contain mostly undissociated HA molecules.
The equilibrium always shifts in the direction of the weaker acid and the weaker base. The stronger acid donates a proton to the stronger base.
Property 1: Strong Acids Have Very Weak Conjugate Bases
If an acid is strong enough to donate its proton completely in water, the resulting conjugate base must be an extremely weak base — weaker than water itself. Think about it: if the conjugate base were a strong base, it would immediately grab a proton back from H₃O⁺, and the acid wouldn't be fully dissociated. So, for the strong acids listed earlier:
| Strong Acid | Conjugate Base |
|---|---|
| HClO₄ | ClO₄⁻ |
| HCl | Cl⁻ |
| HBr | Br⁻ |
| HI | I⁻ |
| HNO₃ | NO₃⁻ |
| H₂SO₄ | HSO₄⁻ |
All these conjugate base ions (ClO₄⁻, Cl⁻, Br⁻, I⁻, NO₃⁻, HSO₄⁻) are much weaker bases than water. They have almost no tendency to accept a proton.
A common mistake is to think that because an acid is strong, its conjugate base is also strong. The opposite is true: the stronger the acid, the weaker its conjugate base.
Property 2: Very Strong Bases Have Very Weak Conjugate Acids
The same logic applies in reverse. A very strong base, like the amide ion (NH₂⁻), the oxide ion (O²⁻), or the hydride ion (H⁻), is an excellent proton acceptor. Its conjugate acid (NH₃, OH⁻, H₂ respectively) will be an extremely weak acid — it will have almost no tendency to donate that proton back.
Weak Acids and Their Conjugate Bases
A weak acid, like acetic acid (CH₃COOH), hydrofluoric acid (HF), or nitrous acid (HNO₂), is only partially dissociated in water. The equilibrium lies far to the left, meaning the solution contains mostly undissociated HA molecules. Because the acid is weak, its conjugate base is relatively strong — certainly a stronger base than water. For example, the acetate ion (CH₃COO⁻) is a much stronger base than water, which is why it can grab a proton from water in the reverse reaction.
Acid-Base Indicators: A Practical Application
Some organic compounds that are soluble in water behave as weak acids and have the useful property of exhibiting different colours in their acid form (HIn) and their conjugate base form (In⁻). Phenolphthalein and bromothymol blue are classic examples. Their dissociation equilibrium in water is: …