Le Chatelier's principle and acid-base titration curves
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 1 worked example, 4 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
- A colour change you can control
- Le Chatelier's principle
- Predicting shifts
Read the transcript of the teaching part
Last time
Before we start: last time, in Equilibrium constants and calculations, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
A colour change you can control
Yellow chromate ions turn orange when you add acid, and turn back to yellow when you add alkali. The equilibrium has not been destroyed, it has shifted. Le Chatelier's principle lets you predict which way a system will move when you change the conditions, and acid-base ideas fit alongside it.
Le Chatelier's principle
When a change is made to a system in dynamic equilibrium, the position of equilibrium shifts to oppose the change. It tells us the direction of the shift, not the speed, and not the value of the equilibrium constant. A catalyst speeds up the forward and reverse reactions equally. It does not change the position of equilibrium. It helps equilibrium to be reached sooner.
Predicting shifts
Increase the concentration of a reactant: the position shifts right, to use up the added reactant. Remove a product: shifts right. Increase the pressure: the system shifts to the side with fewer gas molecules. Decrease the pressure: it shifts to the side with more. Increase the temperature: the system shifts in the endothermic direction. Decrease it: the exothermic direction. Only a temperature change alters the value of the equilibrium constant.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Le Chatelier's principle and dynamic equilibrium.
The whole chapter: Equilibria, Equilibria.
Chapter 7: Equilibria
- Equilibrium constants and calculations 7 min
- Le Chatelier's principle and acid-base titration curves 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.