Loading
Unit 8
11–15% of examAcid-base reactions are equilibria in which a proton (H⁺) moves from one particle to another. In this unit you calculate pH for strong and weak acids and bases, read titration curves, and learn how buffers hold pH steady. It's one of the most heavily weighted units on the exam, so it's worth plenty of practice.
Longer videos that cover the whole unit. Good for a first pass or a final review.
Water ionizes very slightly into H₃O⁺ and OH⁻, and Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C. pH and pOH are the negative logs of those concentrations, so at 25 °C they add up to 14 and neutral water has a pH of 7; at other temperatures Kw changes, so neutral pH isn't exactly 7.
Key terms
The strong acids to know are HCl, HBr, HI, HNO₃, HClO₄ and H₂SO₄. They ionize completely in water, so for an acid like HCl that gives up one H⁺, [H₃O⁺] is simply the acid's concentration. Strong bases are the group 1 and group 2 hydroxides: NaOH gives one OH⁻ per formula unit and Ba(OH)₂ gives two, so pH and pOH take just one step.
Key terms
A weak acid only partly ionizes, so most of it stays as un-ionized HA, and its Ka tells you how far the ionization goes. You use Ka (or Kb for a weak base) with an ICE table to find pH and percent ionization, and for any conjugate pair, Ka × Kb = Kw.
Key terms
When you mix an acid and a base, let them react completely first, then look at what's left. Leftover strong acid or base sets the pH; leftover weak acid together with its conjugate base makes a buffer; and equal moles of a weak acid and a strong base give a basic solution, because only the conjugate base is left.
Key terms
In a titration you add a solution of known concentration until the reaction is complete, and the titration curve plots pH against the volume added. At the equivalence point, you've added just enough titrant to react with all the acid or base you started with, and at the half-equivalence point of a weak acid titration, pH = pKa.
Key terms
You can often predict acid strength from structure: an acid is stronger when its conjugate base is more stable, for example because nearby electronegative atoms or resonance spread out the negative charge. Carboxylic acids are a common family of weak acids, and ammonia and similar nitrogen compounds are common weak bases.
Key terms
Compare a solution's pH with the acid's pKa: below the pKa, the acid form (HA) is the main form, and above it, the base form (A⁻) is. An indicator is a weak acid whose two forms have different colors, so it switches color near its own pKa. For a titration, choose an indicator whose pKa matches the pH you expect at the equivalence point.
Key terms
A buffer contains large amounts of both a weak acid and its conjugate base. Added base is used up by the acid and added acid is used up by the conjugate base, so the pH changes only a little.
Key terms
For a buffer, pH = pKa + log([A⁻]/[HA]). When the acid and its conjugate base are present in equal amounts, the log term is zero and pH = pKa, and adding a little acid or base barely changes the ratio, so the pH barely moves.
Key terms
Buffer capacity is how much acid or base a buffer can absorb before its pH changes a lot. A more concentrated buffer with the same acid-to-base ratio has the same pH but can absorb more, and a buffer with more acid than base can handle more added base than added acid.
Key terms
If one of a salt's ions is a weak acid, a weak base (like F⁻ or CO₃²⁻) or hydroxide, the salt's solubility depends on pH. For example, adding acid uses up OH⁻ or a basic anion, so by Le Châtelier's principle more of the salt dissolves.
Key terms