More successful collisions per second.

Reactions require collisions with suitable orientation and enough energy. Higher concentration or gas pressure increases encounter frequency; greater exposed solid surface gives more reaction sites. Higher temperature changes the energy distribution so a greater fraction of collisions can exceed the activation barrier.

A catalyst offers a different pathway with lower activation energy and is regenerated overall. It does not change the reaction enthalpy. At fixed temperature a catalyst does not shift the Maxwell–Boltzmann distribution: it changes the threshold. Average rate uses a change over a time interval; instantaneous rate is a tangent gradient.

INTERACTIVE MODEL

A reaction energy profile

Activation energy / kJ mol⁻¹80

READ THE GRAPH

Explore the relationship

Reactants at 100 and products at 60 kJ mol⁻¹. Lowering the barrier leaves ΔH unchanged. Reaction progress is not elapsed time.

Energy / kJ mol⁻¹

Tap or focus a plotted point to read its values.

View plotted data as a table

Explore the relationship
Series	Reaction progress	Energy / kJ mol⁻¹
Energy / kJ mol⁻¹	0	100
Energy / kJ mol⁻¹	0.0166667	104.187
Energy / kJ mol⁻¹	0.0333333	108.362
Energy / kJ mol⁻¹	0.05	112.515
Energy / kJ mol⁻¹	0.0666667	116.633
Energy / kJ mol⁻¹	0.0833333	120.706
Energy / kJ mol⁻¹	0.1	124.721
Energy / kJ mol⁻¹	0.116667	128.669
Energy / kJ mol⁻¹	0.133333	132.539
Energy / kJ mol⁻¹	0.15	136.319
Energy / kJ mol⁻¹	0.166667	140
Energy / kJ mol⁻¹	0.183333	143.571
Energy / kJ mol⁻¹	0.2	147.023
Energy / kJ mol⁻¹	0.216667	150.346
Energy / kJ mol⁻¹	0.233333	153.53
Energy / kJ mol⁻¹	0.25	156.569
Energy / kJ mol⁻¹	0.266667	159.452
Energy / kJ mol⁻¹	0.283333	162.172
Energy / kJ mol⁻¹	0.3	164.721
Energy / kJ mol⁻¹	0.316667	167.094
Energy / kJ mol⁻¹	0.333333	169.282
Energy / kJ mol⁻¹	0.35	171.281
Energy / kJ mol⁻¹	0.366667	173.084
Energy / kJ mol⁻¹	0.383333	174.686
Energy / kJ mol⁻¹	0.4	176.085
Energy / kJ mol⁻¹	0.416667	177.274
Energy / kJ mol⁻¹	0.433333	178.252
Energy / kJ mol⁻¹	0.45	179.015
Energy / kJ mol⁻¹	0.466667	179.562
Energy / kJ mol⁻¹	0.483333	179.89
Energy / kJ mol⁻¹	0.5	180
Energy / kJ mol⁻¹	0.516667	179.836
Energy / kJ mol⁻¹	0.533333	179.343
Energy / kJ mol⁻¹	0.55	178.523
Energy / kJ mol⁻¹	0.566667	177.378
Energy / kJ mol⁻¹	0.583333	175.911
Energy / kJ mol⁻¹	0.6	174.127
Energy / kJ mol⁻¹	0.616667	172.03
Energy / kJ mol⁻¹	0.633333	169.625
Energy / kJ mol⁻¹	0.65	166.921
Energy / kJ mol⁻¹	0.666667	163.923
Energy / kJ mol⁻¹	0.683333	160.64
Energy / kJ mol⁻¹	0.7	157.082
Energy / kJ mol⁻¹	0.716667	153.258
Energy / kJ mol⁻¹	0.733333	149.177
Energy / kJ mol⁻¹	0.75	144.853
Energy / kJ mol⁻¹	0.766667	140.296
Energy / kJ mol⁻¹	0.783333	135.518
Energy / kJ mol⁻¹	0.8	130.534
Energy / kJ mol⁻¹	0.816667	125.357
Energy / kJ mol⁻¹	0.833333	120
Energy / kJ mol⁻¹	0.85	114.479
Energy / kJ mol⁻¹	0.866667	108.808
Energy / kJ mol⁻¹	0.883333	103.004
Energy / kJ mol⁻¹	0.9	97.082
Energy / kJ mol⁻¹	0.916667	91.0583
Energy / kJ mol⁻¹	0.933333	84.9494
Energy / kJ mol⁻¹	0.95	78.7721
Energy / kJ mol⁻¹	0.966667	72.5434
Energy / kJ mol⁻¹	0.983333	66.2803
Energy / kJ mol⁻¹	1	60

Probe along the horizontal axis0Energy / kJ mol⁻¹: 100

Explore: Lower the activation barrier. Check that reactant and product energies stay fixed.

\text{rate}=\frac{\Delta[P]}{\Delta t}=-\frac{\Delta[R]}{\Delta t}

WORKED EXAMPLE

A product concentration increases from 0.10 to 0.34 M over 12 s.

Average formation rate = (0.34 − 0.10)/12.
Rate = 0.020 mol $L^{-1}$ $s^{-1}$ .

Assumed knowledge

Concentration gradients and energy diagrams.

Learning checkpoints & source

YOUR LEARNING CHECKPOINT

Explain temperature, concentration, pressure and surface-area effects.
Interpret rate graphs and catalytic energy profiles.

QCAA Chemistry · Unit 2 · Rates of chemical reactions ↗

READY TO TRY IT?

More successful collisions per second.

A reaction energy profile

Explore the relationship

A product concentration increases from 0.10 to 0.34 M over 12 s.

Put the idea to work.