Rate of reaction, collision theory and the Boltzmann distribution
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 1 worked example, 3 practice questions, 4 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Why food keeps in a fridge
- Rate of reaction
- Reading a concentration-time graph
Read the transcript of the teaching part
Last time
Before we start: last time, in Catalysts, activation energy and industrial processes, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Why food keeps in a fridge
Milk goes off in a day at room temperature, but lasts a week in the fridge. The chemistry that sours it is exactly the same. What changes is its rate, and the explanation is collisions: particles must hit each other, and hit hard enough, for a reaction to happen.
Rate of reaction
Rate of reaction is the change in concentration of a reactant or product per unit time. The unit is moles per dm cubed per second. If a reactant falls from 0.100 to 0.060 mol per dm cubed in 100 seconds, the average rate is 4.0 times ten to the minus four. On a concentration against time graph, the rate at any moment is minus the gradient of the tangent.
Reading a concentration-time graph
The graph for a reactant starts steep and flattens. The gradient decreases because the concentration decreases. The rate is highest at the start, when the reactants are most concentrated. The reaction stops when the limiting reagent runs out, and the curve becomes horizontal. For a limiting reagent question, compare the moles with the equation ratio.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Collision theory · The Boltzmann distribution and temperature.
The whole chapter: Reaction Kinetics, Reaction Kinetics.
Chapter 8: Reaction kinetics
- Rate of reaction, collision theory and the Boltzmann distribution 7 min
Also for A Level Chemistry 9701
Which topics come up most in A Level Chemistry 9701 past papers, counted from every paper since 2004: Paper 2 topic frequency, Paper 4 topic frequency.