6.2 Rate of reaction
- Syllabus
- 0620–2026–2027
- Topic
- 6.2
- Level
- —
Learning objectives
6.2.1Rate changes caused by concentration• Describe rate changes caused by concentration, gas pressure, solid surface area, temperature and adding/removing catalysts, including enzymes6.2.2Catalyst increases the rate of a• State: a catalyst increases the rate of a reaction and is unchanged at the end of a reaction6.2.3Practical methods for investigating• Describe practical methods for investigating the rate of a reaction including change in mass of a reactant or a product and the formation of a gas6.2.4Interpret data• Interpret data, including graphs, from rate of reaction experiments6.2.5Collision theory in terms of: (a)• Describe collision theory in terms of: (a) number of particles per unit volume (b) frequency of collisions between particles (c) kinetic energy of particles (d) activation energy, Ea6.2.6Rate changes using collision theory• Explain rate changes using collision theory for concentration, gas pressure, solid surface area, temperature and adding/removing catalysts, including enzymes6.2.7Catalyst decreases the activation• State: a catalyst decreases the activation energy, Ea, of a reaction6.2.8Evaluate practical methods for• Evaluate practical methods for investigating the rate of a reaction including change in mass of a reactant or a product and the formation of a gas
Describe how conditions change reaction rate
| Change | Effect on rate | Reverse change |
|---|---|---|
| increase solution concentration | faster | dilution makes it slower |
| increase gas pressure | faster | lower pressure makes it slower |
| increase solid surface area by using smaller pieces or powder | faster | larger lumps react more slowly |
| increase temperature | faster | cooling makes it slower |
| add a catalyst, including an enzyme | faster | removing it makes the uncatalysed reaction slower |
Reaction rate describes how quickly a reactant is used up or a product is formed. A faster reaction has a greater change in measured quantity per unit time.
When the amount of limiting reactant is unchanged, a faster gas-forming reaction gives a steeper curve and reaches the same final gas volume sooner.
A factor that changes rate does not automatically change the final amount of product. Rate is about how quickly the reaction proceeds; yield depends on the reacting amounts and equilibrium where relevant.
State what a catalyst does
A catalyst increases the rate of a reaction and is chemically unchanged at the end. An enzyme is a biological catalyst.
A catalyst participates in steps of the reaction but is regenerated, so it is not used up overall. It can therefore be recovered with the same chemical identity after the reaction.
| Catalyst changes | Catalyst does not change |
|---|---|
| how quickly products form | the chemical equation |
| time taken to reach completion | the final amount from fixed reactant amounts |
| activation energy | the enthalpy change, ΔH |
A catalyst is not a reactant and does not supply extra product. 'Unchanged' means chemically unchanged at the end, not absent from the reaction pathway.
Measure the rate of a gas-forming reaction
| Method | Apparatus and measurement | Suitable situation |
|---|---|---|
| gas volume | sealed flask connected to a gas syringe; record volume at regular times | a gas forms and can be collected |
| mass loss | open flask on a balance; record total mass at regular times | a gas forms and escapes |
Measure the reactants, assemble the apparatus, add the final reactant and start the timer together, then record gas volume or mass at fixed time intervals until the reading becomes constant. Calculate a rate from change in volume or mass divided by time.
When testing one factor, change only that independent variable. Keep reactant amounts, concentration where not tested, temperature, solid particle size, apparatus, and timing procedure constant.
A bung is essential for gas collection but not for mass-loss measurement, where the gas must escape. Check connections for leaks before starting a gas-syringe experiment.
Interpret reaction-rate data and graphs
On a graph of product formed or reactant used against time, the gradient represents rate. A steeper gradient means a faster rate; a horizontal line means the measured quantity is no longer changing and the reaction has finished.
| Feature | Interpretation |
|---|---|
| steepest section | greatest rate |
| curve becomes less steep | rate is decreasing |
| plateau reached earlier | reaction finishes sooner |
| same plateau height | same final measured amount |
| different plateau height | different final measured amount |
Average rate over an interval = change in measured quantity ÷ time interval. Use the graph scale and include units, such as cm³/s for gas volume or g/s for mass change.
Do not use curve height alone to compare rates. Compare gradients at the same time or over the stated interval; height shows accumulated quantity, not instantaneous speed.
Describe collision theory
A reaction occurs only when reacting particles collide successfully. A successful collision has enough energy to meet or exceed the activation energy, Ea, and a suitable collision arrangement.
| Particle idea | Link to rate |
|---|---|
| particles per unit volume | affects how close particles are |
| collision frequency | more collisions per second create more opportunities to react |
| kinetic energy | faster-moving particles collide more often and with more energy |
| activation energy, Ea | minimum collision energy needed for reaction |
Use this reasoning chain: condition changes particle behaviour → successful collisions per unit time change → reaction rate changes.
Not every collision causes reaction. Increasing concentration raises collision frequency but does not give each particle more kinetic energy or alter Ea.
Explain rate changes using collision theory
| Change | Particle-level cause | Why rate increases |
|---|---|---|
| higher concentration | more particles per unit volume | more collisions per second |
| higher gas pressure | particles are closer; more per unit volume | more collisions per second |
| greater solid surface area | more reactant particles are exposed | more collisions occur at the surface per second |
| higher temperature | particles have more kinetic energy and move faster | collisions are more frequent and a larger fraction meet or exceed Ea |
| add catalyst or enzyme | a lower-Ea pathway is available | a larger fraction of collisions are successful |
For temperature answers, include both effects: particles collide more often and more collisions have sufficient energy. Temperature does not lower the activation energy.
As reactants are used up, their concentration falls, so collision frequency and rate fall. The rate becomes zero when a limiting reactant is completely used up.
Pressure affects gaseous reactants; surface area affects exposed solid. Do not explain either by saying the particles gain energy unless temperature also changes.
Link catalysts to activation energy
A catalyst increases reaction rate by providing an alternative reaction pathway with a lower activation energy, Ea.
At the same temperature, particle kinetic energies are unchanged, but the lower Ea means a larger fraction of collisions have sufficient energy to react. There are therefore more successful collisions per unit time.
| Pathway feature | Without catalyst | With catalyst |
|---|---|---|
| reactant energy | same | same |
| peak height above reactants | higher Ea | lower Ea |
| product energy | same | same |
| ΔH | same | same |
A catalyst does not increase particle energy and does not change ΔH. Only the energy barrier and therefore the rate change.
Evaluate methods for measuring reaction rate
| Method | Strength | Limitation and improvement |
|---|---|---|
| gas syringe | directly measures gas volume and gives many readings | leaks or a sticking plunger lose accuracy; test seals and use a freely moving, suitable-range syringe |
| mass loss on a balance | simple and records continuous change without collecting gas | only works when gas escapes; small changes, drafts and splashes affect readings; use a suitable-precision balance and a cotton-wool plug |
Choose a method by linking it to the reaction and data needed. Consider whether gas forms, whether it is safe to release, the expected volume or mass change, reading frequency, measurement resolution, and the main systematic losses.
For a fair comparison, control all variables except the one tested. Repeat each condition, identify anomalous results, calculate a mean, and collect readings frequently enough to define the steep initial part of the curve.
An evaluation must connect a specific limitation to its effect on the result and a practical improvement. Merely naming apparatus or saying a method is 'more accurate' is not enough.