Le Châtelier's principle
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Question
State Le Châtelier's principle.
Answer
If a system at equilibrium is disturbed, the position shifts in the direction that **opposes** (partly cancels) the change.
Question
Adding more of a reactant shifts the position…
Answer
…towards the **products** (right) — the system uses up the added reactant.
Question
Removing a product shifts the position…
Answer
…towards the **products** (right) — the system replaces the lost product.
Question
Effect of increasing pressure on a gas equilibrium?
Answer
The position shifts to the side with **fewer moles of gas**, to reduce the pressure.
Question
What if both sides have equal moles of gas?
Answer
Changing the pressure causes **no shift** in the position.
Question
Effect of raising the temperature?
Answer
The position shifts in the **endothermic** direction (it absorbs the added heat).
Question
Which change is the only one that alters K_{c}?
Answer
A change in **temperature** — concentration, pressure and a catalyst leave K_{c} unchanged.
Question
Effect of a catalyst on equilibrium?
Answer
**No shift** in position and **no change** in K_{c}; it just reaches equilibrium **sooner** (speeds up both directions equally).
Question
Exothermic forward reaction: what does raising T do to K_{c}?
Answer
K_{c} **decreases** (the position shifts towards the reactants).
Question
How do you predict the pressure effect quickly?
Answer
**Count the moles of gas** on each side; the position shifts towards the side with **fewer** gas moles when pressure rises.
Question
Trick for the temperature direction?
Answer
Write **heat** as a species (exo: products + heat; endo: reactants + heat), then treat adding heat like adding that species.
Question
If heating shifts the position towards the products, is the forward reaction exo- or endothermic?
Answer
**Endothermic** — adding heat favours the heat-absorbing direction.
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Full study notes for Le Châtelier's principle
Topic 5.3 hub
How far? The extent of chemical change
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