IB Chemistry: The Reactivity Series & Displacement — Your Questions Answered

The reactivity series ranks metals and halogens by how readily they lose or gain electrons — the more reactive element is oxidised more easily. In a displacement reaction, the more reactive species pushes a less reactive one out of its compound because it's the stronger reducing (or oxidising) agent.

Identify which species is higher in the reactivity series using the data booklet — that one gets oxidised and displaces the other. Write the full balanced equation, then the ionic equation, cancelling spectator ions. Examiners want oxidation states and the words 'oxidised'/'reduced' stated explicitly, not just 'more reactive'.

Write the full molecular equation first and balance it. Then split every aqueous ionic compound into its separate ions, and cross out anything that appears unchanged on both sides — those are spectator ions. What's left is the ionic equation, and it's usually what paper 2 mark schemes actually credit.

A more reactive halogen displaces a less reactive halide ion because it's the stronger oxidising agent — reactivity falls down group 17, so chlorine displaces both bromide and iodide, while bromine only displaces iodide. Examiners expect precise colour observations, not just 'a colour change happened'.

Every year I see students write 'X is more reactive so it displaces Y' with no mention of oxidation states or electron transfer — that earns almost nothing against a 'deduce' or 'explain' command term. Always name what's oxidised, what's reduced, and cite the reactivity series or electrode potential values used.

No — you're not expected to memorise it from a blank page. The IB Chemistry data booklet gives a table of standard electrode potentials, and you rank reactivity by comparing those values: the more negative the potential, the more easily that species is oxidised, and the more reactive it is.

Assign an oxidation number to every atom before and after the reaction — if any element's number changes, it's a redox reaction. In displacement, the free element (Mg, Cl₂, Zn) starts at oxidation state 0 and ends non-zero, confirming it's been oxidised or reduced.

The reactivity series itself isn't different — both levels use the same data-booklet electrode potentials and the same displacement reasoning. What changes at HL is depth: Reactivity 3.2 adds quantitative electrode potential calculations and cell diagrams that SL students are never examined on.

The reactivity series is a qualitative ranking built from observed displacement reactions; standard electrode potentials give that same ranking a number, in volts, measured against a standard hydrogen half-cell. HL students use those E° values to calculate cell voltage and predict whether a reaction is actually feasible.

It's one of the more forgiving topics once you learn the logic rather than memorising facts — most lost marks come from vague wording, not difficult chemistry. Students who can write a correct half-equation and name the oxidising and reducing agent under time pressure usually pick up full marks here.

Practise past paper questions using the exact command term given — 'deduce', 'identify' and 'explain' each expect a different depth of answer. Always show the oxidation state change explicitly, name the oxidising and reducing agent, and use data-booklet electrode potentials to justify feasibility rather than assuming it.

You don't need a chemistry background to help. Ask your child to explain out loud which species is oxidised, which is reduced, and which data-booklet value they used to decide — if they can teach it back clearly and unprompted, they've genuinely understood it, not just memorised the equation.

Past papers matter, but for a narrow sub-topic like this, targeted practice grouped by concept works better than hunting through mixed papers. On RevisionPrep, the Chemistry question bank and Topical Worksheets sort displacement and redox questions by sub-topic, so students can drill the reasoning repeatedly instead of searching for relevant questions.