IB Chemistry: Rates of Reaction & Collision Theory FAQs

Rates of reaction & collision theory is the DP Chemistry topic explaining why reactions speed up or slow down — covering collision theory, activation energy, the Maxwell-Boltzmann distribution, and at HL, rate expressions and the Arrhenius equation. It's examined through Paper 1 multiple-choice, Paper 2 data-response questions, and it works well as an Internal Assessment focus.

Four factors matter: concentration (or pressure for gases), surface area, temperature, and catalysts. Each one changes rate by altering either how often particles collide or what fraction of those collisions carry enough energy to react — collision theory is really just the mechanism behind all four, not a separate fifth idea.

Activation energy (Ea) is the minimum energy colliding particles need for a collision to actually produce products — collision theory states that only collisions meeting both this energy threshold and correct orientation succeed. On a Maxwell-Boltzmann curve, the area beyond Ea represents the fraction of particles able to react at that temperature.

A catalyst provides an alternative reaction pathway with a lower activation energy, so a larger proportion of collisions have enough energy to react — without the catalyst itself being consumed. On a Maxwell-Boltzmann distribution, this shifts the 'successful collision' cut-off leftward, raising rate without changing temperature.

Plot concentration (or volume of gas produced) against time, then draw a tangent at the point you need and find its gradient — that gradient is the instantaneous rate, usually in mol dm⁻³ s⁻¹ or cm³ s⁻¹. For an average rate over an interval, just use the gradient of the straight line joining two points instead.

HL students use $k=A{e}^{-E_a/RT}$ to connect rate constant, temperature and activation energy, most often via the linear form $\ln k=-\frac{E_a}{R}\left(\frac{1}{T}\right)+\ln A$, plotting $\ln k$ against $1/T$ to extract Ea from the gradient. This calculation is HL-only content within Reactivity 2.2.

The three I see most often when marking: confusing rate with position of equilibrium (they're separate ideas — rate is about speed, equilibrium is about position), drawing a tangent that doesn't actually touch the curve at the required point, and forgetting that a catalyst lowers Ea for both the forward and reverse reaction equally.

Pick a genuinely variable independent variable — concentration of sodium thiosulfate is a classic choice — with at least five data points, control temperature tightly since rate depends on it exponentially, and use a proper graphical method (tangent gradient or 1/time as a proxy for rate) rather than a single 'time to disappear' reading.

Rates of reaction and collision theory sit under Reactivity 2.2 in the current DP Chemistry guide (first assessments 2025), within the broader Reactivity 2 theme. SL students cover collision theory, the Maxwell-Boltzmann distribution and the four factors affecting rate; HL adds rate expressions, reaction order and the Arrhenius equation.

It's more conceptually demanding than most kinetics content because it combines graph interpretation (gradients, tangents, areas under curves) with genuine theory, and at HL, logarithmic manipulation of the Arrhenius equation. Students comfortable with algebra and graph work generally find it manageable; students who rely on memorising definitions tend to struggle.

Expect Paper 1 questions distinguishing factors that change Ea (temperature, catalyst) from factors that just change collision frequency (concentration, surface area), Paper 2 questions asking you to sketch or interpret a Maxwell-Boltzmann distribution, and HL-only questions requiring you to calculate Ea from a table of rate constants at different temperatures.

Yes — SL students only need qualitative collision theory: factors affecting rate, the shape of the Maxwell-Boltzmann distribution and activation energy. HL students must additionally handle rate expressions, reaction order, half-life for first-order reactions and full Arrhenius equation calculations, which adds a genuinely mathematical layer SL doesn't require.

It's one of the most commonly chosen IA topics because it's straightforward to design a controlled experiment around — the classic sodium thiosulfate 'disappearing cross' reaction is a well-known example — and it produces clear quantitative data. Examiners reward investigations that calculate a real value, such as a rate constant or Ea, rather than just describing a trend.

Focus revision on three checkable skills: reading a Maxwell-Boltzmann graph correctly, telling rate-of-reaction language apart from equilibrium language, and — at HL — using the Arrhenius equation without a calculator slip. A single focused past-paper session on Reactivity 2.2 usually reveals which of the three is the actual weak point.