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Chemistry · Ch 6 — Equilibrium
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.
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 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.