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Reaction requires collisions with sufficient energy and an appropriate orientation. Increasing concentration or gas pressure usually increases collision frequency; a greater exposed surface area provides more accessible sites for a solid reactant. Temperature increases collision frequency somewhat, but the major kinetic effect is a greater fraction of particles with energy at least the activation energy. State both collision frequency and energetic success when they are relevant rather than saying particles simply become more reactive.
Video by Eliot Rintoul. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Explain factors affecting reaction rate
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What does the area under a Maxwell–Boltzmann curve represent?
Total number of particles. Temperature redistributes particles but does not create them.
After watching 2: How is an instantaneous rate found on a curved concentration–time graph?
Gradient of a tangent. The tangent measures local change per unit time.
What you'll be able to do
Reaction requires collisions with sufficient energy and an appropriate orientation. Increasing concentration or gas pressure usually increases collision frequency; a greater exposed surface area provides more accessible sites for a solid reactant. Temperature increases collision frequency somewhat, but the major kinetic effect is a greater fraction of particles with energy at least the activation energy. State both collision frequency and energetic success when they are relevant rather than saying particles simply become more reactive.
A Maxwell–Boltzmann plot shows the distribution of molecular energies at a stated temperature. Its area represents the total number of particles, not their total energy. Increasing temperature broadens the curve, lowers its maximum and moves the most probable energy to the right. A catalyst provides a different route with a lower activation energy. At the same temperature it does not change the distribution of molecular energies, the reaction enthalpy or the equilibrium constant.
A reaction rate measures change in concentration or another proportional quantity per unit time. The initial gradient gives an initial rate; the gradient of a tangent gives the instantaneous rate later. A clock experiment uses a small fixed detectable change, so inverse time can approximate a relative initial rate if the threshold is identical. Compare experiments while controlling other variables. A gas syringe can leak or respond slowly; a visual endpoint is subjective. Match the method to the reaction and state the limitations of the measured proxy.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Explain why a catalyst increases rate without changing the Maxwell–Boltzmann curve at fixed temperature.
The threshold changes, not the distribution.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.