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Predict how changes in conditions affect equilibrium position.
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When a system at equilibrium is disturbed, it will shift to partially counteract the disturbance and establish a new equilibrium.
| Disturbance | Shift | Effect on K |
|---|---|---|
| Add reactant | Forward โ | No change |
| Remove reactant | โ Reverse | No change |
| Add product | โ Reverse | No change |
| Remove product |
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| Section | Format | Questions | Time | Weight | Calculator |
|---|---|---|---|---|---|
| Multiple Choice | MCQ | 60 | 90 min | 50% | โ |
| Free Response (Long) | FRQ | 3 | 69 min | 30% | โ |
| Free Response (Short) | FRQ | 4 | 36 min | 20% | โ |
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mol of reacts with mol of . How many grams of water are produced? Which is the limiting reagent? ()
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| Forward โ |
| No change |
K does not change when concentrations change (at constant T).
| Disturbance | Shift Direction |
|---|---|
| Decrease volume (increase P) | Toward fewer moles of gas |
| Increase volume (decrease P) | Toward more moles of gas |
| Add inert gas (constant V) | No shift |
If , pressure changes have no effect.
Temperature is the only factor that changes K.
| Reaction Type | Increase T | Decrease T |
|---|---|---|
| Exothermic () | Shift left, K decreases | Shift right, K increases |
| Endothermic () | Shift right, K increases | Shift left, K decreases |
Think of heat as a product (exothermic) or reactant (endothermic).
A catalyst:
Adding an ion already present in solution shifts the equilibrium away from that ion.
Adding NaCl (source of Clโป) shifts left โ less AgCl dissolves.
AP Chemistry Tip: Always state both what happens (shift direction) AND why (Le Chatelier's principle counteracts the stress).