Edexcel IAL Chemistry
Acid–Base Equilibria
🎯 The Big Idea: Acids and bases are all about protons (H⁺) moving between molecules —
and whether that proton-transfer goes all the way (strong) or only partway (weak, an equilibrium)
determines everything else: the pH, how the pH changes when you dilute or titrate, and whether the solution can act as a buffer.
📋 Summary — What This Chapter Covers
- Brønsted–Lowry theory: acids donate protons, bases accept them — and every reaction has a "conjugate" pair linked by that one proton.
- pH is just a compressed way of writing [H⁺] using logs: pH = −log[H⁺].
- Strong acids/bases dissociate almost 100% — an easy calculation. Weak acids/bases only partially dissociate — you need Kₐ.
- Kₐ and pKₐ measure how far a weak acid's equilibrium sits — bigger Kₐ (smaller pKₐ) = stronger acid.
- Kw (ionic product of water) links [H⁺] and [OH⁻] in any aqueous solution: Kw = [H⁺][OH⁻] = 1×10⁻¹⁴ at 298 K.
- Titration curves have a distinct S-shape, and their exact shape (start pH, jump size, equivalence pH) tells you whether you're dealing with strong/weak acid vs strong/weak base.
- Indicators are weak acids themselves — you must pick one whose colour-change range matches the vertical jump of the titration curve.
- Buffers = weak acid + its conjugate base (or weak base + conjugate acid) sitting together, resisting pH change by "soaking up" added H⁺ or OH⁻.
- The Henderson–Hasselbalch equation lets you calculate buffer pH directly, and also lets you design a buffer for a target pH.
1. Brønsted–Lowry Acids & Bases
Forget the old "acids taste sour" definitions. The Brønsted–Lowry theory defines everything in terms of one simple action: passing a proton (H⁺) from one species to another.
- A Brønsted acid = a proton donor. It gives away H⁺.
- A Brønsted base = a proton acceptor. It grabs H⁺.
Think of it like a game of catch with a single ball (the proton). Whoever throws the ball is the acid; whoever catches it is the base. That's genuinely the whole concept.
Conjugate Acid–Base Pairs
Here's the part students often find confusing at first: in a real reaction, the acid and base swap roles as the reaction proceeds, because it's an equilibrium — both directions are happening. A conjugate pair is simply two species (one reactant, one product) that differ by exactly one proton.
Look closely: CH₃COOH and CH₃COO⁻ are a conjugate pair (CH₃COOH loses one H⁺ to become CH₃COO⁻). Likewise H₂O and H₃O⁺ are a pair (H₂O gains one H⁺ to become H₃O⁺).
Memory Trick
"Conjugate" literally means "joined together" — think of conjugate pairs as dance partners connected by a single proton being passed back and forth.
Q1. In the reaction NH₃ (aq) + H₂O (l) ⇌ NH₄⁺ (aq) + OH⁻ (aq), identify the acid, the base, and both conjugate pairs.
2. pH — Compressing a Huge Range of Numbers
is
logarithmic (base 10) scale
10× change