Chemistry unit guide
How much, how fast and how far
Rates, rate expressions, mechanisms, acids and bases, pH, buffers and titration curves.
Reactivity 2 is the most calculation-dense unit in the course and the one where HL and SL diverge most sharply. The acid-base half is heavily examined in both papers because it combines a calculation, a graph reading and an explanation in a single question. Titration curves in particular appear almost every session, and students who have practised reading them lose almost no marks, while students who have only read about them lose most.
Topics in this unit
What gets examined
- Deducing rate order from initial rate data and writing the rate expression.
- Linking a rate expression to a proposed mechanism and identifying the rate-determining step.
- pH, pOH, [H⁺] and [OH⁻] conversions, including for weak acids using Ka.
- Buffer calculations and explaining how a buffer resists pH change when acid or base is added.
- Reading equivalence point, half-equivalence point and pKa off a titration curve.
- Choosing an appropriate indicator by matching its range to the pH at equivalence.
How to revise it
Read titration curves off the axes, not from memory
The equivalence point is the steepest part, and the half-equivalence point sits at half that volume where pH equals pKa. Practise on printed curves until finding both takes seconds.
Keep strong and weak separate
For a strong acid, [H⁺] equals the concentration. For a weak acid it does not, and needs Ka. Deciding which case you are in before calculating prevents the single most common error in this unit.
Work orders one reactant at a time
Find two experiments where only one concentration changes, and compare. Trying to reason about several at once is where the mistakes come from.
Learn what a buffer answer must contain
The equilibrium, what happens when H⁺ is added, what happens when OH⁻ is added. Mark schemes want both directions, and answering only one is a routine half-mark loss.
Where marks get lost
- Treating a weak acid as fully dissociated when calculating pH.
- Confusing the equivalence point with the neutral point - equivalence is not always pH 7.
- Giving pH to the wrong precision; the digits before the decimal are not significant figures.
- Writing a rate expression that includes a reactant that does not appear in the rate-determining step.
- Choosing an indicator by colour preference rather than by its pH range.
Practise this unit
Reading about a unit only goes so far. Work through real IB-style questions on it, with mark schemes.