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Rate of Reactions

Full chapter

Rate of Reactions

Explain why reactions speed up, and read what a progress graph actually proves.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

01

Explain a rate change using collisions

Changing a condition changes how often particles react.

Reaction rate measures how quickly a reactant is used up or a product forms. Reacting particles must collide, and collisions must have enough energy to lead to reaction. An explanation should connect the changed condition to particle behaviour, then to the number of successful collisions per unit time.

Four factors and their explanations
ChangeParticle explanationImportant control
Higher solution concentrationMore reacting particles per unit volume; more frequent collisionsKeep total solution volume and temperature controlled
Higher gas pressure at fixed temperatureGas particles are closer together; more collisions per unit volume per secondApplies to gaseous reactants, not simply pressing on a solid
Smaller solid pieces, same total massGreater exposed surface area offers more places for collisionsGrinding does not create more total moles of solid
Higher temperatureParticles move faster and collide more energetically; more successful collisionsDo not explain temperature solely as an increase in concentration

A larger lump and equal-mass powder can produce the same final amount of gas while reacting at different speeds. In contrast, adding more of a limiting reactant can change the final amount. Keep the question "how fast?" separate from "how much?".

Check your understandingWhy does powdered calcium carbonate react faster than equal-mass chips in excess acid?Think it through, then reveal the answer
The powder exposes more surface area, so acid particles can collide with more carbonate surface per second. The total amount of carbonate, and hence final carbon dioxide amount, is unchanged.
02

Choose a changing quantity

Match the measurement to the reaction.

Ways to follow progress
MeasurementSuitable situationRate evidence
Gas volume against timeGas-producing reaction; gas can be collectedVolume increase per unit time
Mass against timeA gas escapes from an open reaction vesselMass loss per unit time
Time to a fixed visible endpointA precipitate obscures a mark or a colour reaches a chosen endpointShorter time means a faster average approach to that same endpoint

For a gas syringe, check airtight connections and free plunger movement. For mass loss, prevent splashes so that lost liquid is not mistaken for escaped gas. Start timing consistently. A subjective colour or visibility endpoint is less precise than a clear instrumental measurement, so use the same criterion and repeat.

Worked example

Calculate an average rate

A reaction produces 36 cm3 of gas in its first 40 s.

  1. Average rate = change in gas volume / time interval.
  2. 36/40 = 0.90 cm3 s-1.
  3. This is an average over 40 s; the rate may have fallen throughout that interval.
Answer

0.90 cm3 s-1. State both interval and units.

Check your understandingTwo experiments reach the same endpoint in 20 s and 40 s. Which is faster?Think it through, then reveal the answer
The 20 s experiment. For the same defined amount of change, 1/time can compare relative rates; it does not provide an absolute gas-volume rate unless that volume is known.
03

Read slope and final amount separately

A plateau means product formation has stopped, not that nothing happened.

Same yield, different rates

The faster experiment reaches 44 cubic centimetres earlier; both finish at the same volume.

Illustrative results: equal amounts of magnesium, acid in excess, same temperature; concentration differs.

The gradient of a product-volume graph gives rate. A steeper initial slope means faster initial reaction. The curve becomes less steep as reactants are consumed. A horizontal line means no further gas is collected; a reactant may have been exhausted. A tangent estimates the instantaneous rate at a point; a line joining two measured points gives the average over that interval.

For a mass-loss graph, the mass decreases, so the slope is negative. Use the magnitude of the decrease per time for a positive reaction-rate value. Compare final gas volumes only at the same temperature and pressure. A leak can reduce apparent volume without changing the actual chemical yield.

Worked example

Diagnose a changed curve

A repeat reaches its plateau sooner but at the same final volume. What can be concluded?

  1. It formed the measured product more quickly.
  2. The same final amount formed under the stated gas conditions.
  3. A rate factor such as temperature, concentration or surface area may explain it; the curve alone does not identify which.
Answer

Faster rate, same final amount. Do not claim that a catalyst or temperature change is uniquely proved.

Check your understandingA lower plateau appears after a bung leaks. Does that prove less product formed chemically?Think it through, then reveal the answer
No. Some gas may have escaped collection. Apparatus failure changes the measured amount and must be separated from chemical explanations.

Quick revision

Revisit the essentials, then return to an explanation when you need it.

Rate describes change per unit time. Concentration, pressure, temperature and exposed surface area affect successful collisions. On a progress graph, slope indicates rate and the final level indicates measured amount.

Scope and references

Learning outcomes and sources

10. Rate of Reactions (5086 / 5088 / K326 / K328). Use the outcome map to find the explanation for a particular syllabus requirement.

See the learning outcome map

  1. 10(a) Explain rate factors

    • Concentration
    • Gas pressure
    • Particle size/surface area
    • Temperature
    • Collisions between reacting particles

    Explain a rate change using collisions

  2. 10(b) Interpret rate data

    • Volume/mass/time measurements
    • Gradient and plateau
    • Measurement limitations

    Choose a changing quantityRead slope and final amount separately