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8.1 Rate of reaction

Syllabus
9701–2028–2029
Topic
8.1
Level
AS

Rate depends on how often collisions are effective

rate=change in reactant or product concentrationtime\mathrm{rate=\frac{change\ in\ reactant\ or\ product\ concentration}{time}}

Term Meaning
rate of reaction change in the amount or concentration of a reactant consumed or product formed per unit time
frequency of collisions number of collisions between reacting particles per unit time
effective collision collision with sufficient energy and a suitable orientation, so reaction occurs
non-effective collision collision with insufficient energy and/or unsuitable orientation, so particles separate without reacting

Reaction rate depends on the frequency of effective collisions, not simply the total collision frequency. Conditions can increase the number of collisions per second, the fraction that meet the energy/orientation requirements, or both.

A collision is not automatically effective, and an effective collision is not defined by high frequency alone. Rate describes change per time; it does not state the final yield or equilibrium position.

Concentration and gas pressure change effective-collision frequency

Change at constant temperature Particle-level effect Rate consequence
increase solution concentration more reacting particles occupy the same solution volume, so collisions occur more frequently more effective collisions per second; rate increases
decrease solution concentration fewer reacting particles occupy the same volume fewer effective collisions per second; rate decreases
increase gas pressure by decreasing volume the same gas particles occupy less space and collide more frequently more effective collisions per second; rate increases
decrease gas pressure by increasing volume particles are farther apart and collide less frequently fewer effective collisions per second; rate decreases

At constant temperature, the energy distribution and activation-energy threshold are not changed by concentration or pressure alone. The qualitative rate change is explained by collision frequency and therefore effective-collision frequency.

Pressure is the relevant concentration-like variable for gaseous reactants, not for an incompressible liquid or solid. Do not explain these changes by saying particles move faster: at fixed temperature their average kinetic energy is unchanged.

Use experimental gradients to calculate average or instantaneous rate

Data representation Rate calculation
two concentration–time readings secant gradient: Δconcentration / Δtime gives average rate over the interval
one point on a concentration–time curve draw a tangent and calculate its gradient for instantaneous rate
mass, moles or gas volume against time calculate change/time in the measured quantity, or convert to concentration if the requested rate requires it

Product concentration has a positive gradient; reactant concentration has a negative gradient. Unless a signed derivative is requested, report the positive magnitude of a reactant-consumption rate and state which species was followed.

A product concentration increases from 0.100 to 0.340 mol dm⁻³ between 20.0 s and 80.0 s. The average rate is (0.340 − 0.100)/(80.0 − 20.0) = 4.00 × 10⁻³ mol dm⁻³ s⁻¹.

For a tangent, choose two well-separated points on the straight tangent—not two points on the curve—and calculate vertical change divided by horizontal change. A shallower concentration–time curve means a smaller rate magnitude.

Do not divide by the final clock reading when the interval begins later, mix minutes with seconds, or label mass-loss rate with concentration-rate units. The numerical unit must follow the measured vertical quantity and time unit.