AP Chemistry · Unit 7: Equilibrium
7.10 Reaction Quotient and Le Châtelier’s Principle
In short
You must understand that a disturbance to a system at equilibrium causes the reaction quotient (Q) to differ from the equilibrium constant (K), taking the system out of equilibrium. The system responds by bringing Q back into agreement with K to establish a new equilibrium state.
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What else do you need to know about Reaction Quotient and Le Châtelier’s Principle?
You must distinguish between stresses that change Q only (such as changes in concentration) and stresses that change K (such as a change in temperature). In either case, you must understand that the concentrations or partial pressures of species redistribute to bring Q and K back into equality.
Can you do these?
- I can explain that a disturbance to a system at equilibrium causes Q to differ from K, and that the system responds by bringing Q back into agreement with K.
- I can identify that changes in concentration change Q only, while changes in temperature change K, and describe how species redistribute to equalize Q and K.
How is Reaction Quotient and Le Châtelier’s Principle tested on the exam?
You are assessed on your ability to calculate, estimate, or predict an unknown quantity from known quantities by selecting and following a logical computational pathway (e.g., performing dimensional analysis and attending to significant figures). You must be able to connect what is happening at the molecular level to a model for a system at equilibrium, specifically connecting the value of the equilibrium constant (K) and the reaction quotient (Q) to the dominant direction of the reaction. A common pitfall is using ineffective mathematical routines instead of connecting the value of the equilibrium constant to the experimental data or observations provided.
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Reaction Quotient and Le Châtelier’s Principle: common questions
What do you need to know about Reaction Quotient and Le Châtelier’s Principle for AP Chemistry?
You must understand that a disturbance to a system at equilibrium causes the reaction quotient (Q) to differ from the equilibrium constant (K), taking the system out of equilibrium. The system responds by bringing Q back into agreement with K to establish a new equilibrium state.
How is Reaction Quotient and Le Châtelier’s Principle tested on the AP Chemistry exam?
You are assessed on your ability to calculate, estimate, or predict an unknown quantity from known quantities by selecting and following a logical computational pathway (e.g., performing dimensional analysis and attending to significant figures).
How do I know I have mastered Reaction Quotient and Le Châtelier’s Principle?
Check you can do each of these: explain that a disturbance to a system at equilibrium causes Q to differ from K, and that the system responds by bringing Q back into agreement with K; identify that changes in concentration change Q only, while changes in temperature change K, and describe how species redistribute to equalize Q and K.
Where does Reaction Quotient and Le Châtelier’s Principle fit in AP Chemistry?
Reaction Quotient and Le Châtelier’s Principle is topic 7.10 in Unit 7: Equilibrium of AP Chemistry.
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