The concentration of a reactant falls from 1.20 mol dm−3 to 0.90 mol dm−3 in 150 s. What is the average rate of reaction?
- Change in concentration = 1.20 − 0.90 = 0.30 mol dm−3.
- Rate = 0.30 ÷ 150 = 0.0020 mol dm−3 s−1.
Answer: 2.0 × 10−3 mol dm−3 s−1
How fast? The rate of chemical change
Chemistry · Reactivity 2: How much, how fast and how far?. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
The rate of reaction is how fast the concentration of a reactant falls or a product rises. On a graph of concentration, volume or mass against time, the rate at any moment is the gradient of the tangent to the curve.
Collision theory explains rate. Particles react only when they collide with energy at least equal to the activation energy, Ea, and with a suitable orientation. Anything that makes such successful collisions more frequent makes the reaction faster.
The Maxwell–Boltzmann curve shows how many particles have each energy. Heating moves the peak to higher energy and lowers it, so many more particles have at least Ea. A catalyst does not change the curve. It gives a new pathway with a lower Ea.
Rules to remember
- rate = change in concentration ÷ time taken, in mol dm−3 s−1
- Rate at a given time = gradient of the tangent to the curve at that time
- Higher concentration, higher gas pressure or larger surface area: more frequent collisions
- Higher temperature: collisions are slightly more frequent, but mainly a much larger proportion have energy ≥ Ea
- Maxwell–Boltzmann curve: starts at the origin, never touches the energy axis again, and the area under it is the total number of particles
- At a higher temperature the peak is lower and further right; the area stays the same
- A catalyst lowers Ea but does not change ΔH or the energy distribution, and it is not used up
Common mistake
Students say that a catalyst gives the particles more energy, or that heating lowers the activation energy. Neither is true: heating changes the particle energies but not Ea, and a catalyst changes Ea but not the particle energies.
Worked example
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