AP® Chemistry · Unit 7 · Topic 7.10
Reaction Quotient and Le Chatelier's Principle
The reaction quotient Q has the same form as K but uses whatever concentrations exist right now. Comparing Q to K tells you which direction the reaction must shift to reach equilibrium.
Q and K are calculated the exact same way: products raised to their coefficients over reactants raised to theirs. The difference is timing. K is the value of that ratio once the system has settled, at a given temperature. Q is the value of that same ratio right now, using whatever concentrations happen to exist at this instant, equilibrium or not. That is why K is fixed at a given temperature and Q is not: Q is a snapshot, and snapshots change as the reaction runs.
Comparing the two tells you where the system stands relative to its resting point. If Q is less than K, there are not yet enough products relative to reactants, so the forward reaction runs faster than the reverse, consuming reactants and building products until Q climbs up to meet K. If Q is greater than K, the reverse reaction dominates until Q falls back down to K. Once Q equals K, the forward and reverse rates are equal and concentrations stop changing.
This is the machinery behind Le Chatelier's Principle. Adding reactant, removing product, or changing volume all work by knocking Q away from K, and the system responds by shifting in whichever direction drives Q back toward K. Temperature changes are the exception: they actually change the value of K itself, so the whole target point moves rather than just the current position.
Key terms
- Reaction quotient (Q)
- The value of the equilibrium expression calculated using concentrations at any given moment, not necessarily at equilibrium.
- Equilibrium constant (K)
- The value of the same expression once the system has reached equilibrium, fixed at a given temperature.
- Le Chatelier's Principle
- The idea that a system at equilibrium responds to a disturbance by shifting in the direction that relieves it.
- Shift toward products
- A net forward reaction that occurs when Q is less than K, increasing product concentrations until Q rises to meet K.
Watch it explained
- How it worksHow does a system with Q smaller than K walk itself toward more products?
- Worked exampleFind Q for a mixture and predict the shift when K equals 50 and Q comes out 5.
- How it worksGraph Q rising toward K over time and mark the moment the shift stops.
- Common mistakeWhy isn't Q a fixed number the way K is at a given temperature?
Check yourself
End of topic.