The Reaction Quotient and Le Chatelier's Principle
Introduced| English | Meaning |
|---|---|
| reaction quotient/rɪˈækʃn ˈkwəʊʃənt/ | An expression with the same form as an equilibrium constant but evaluated using the current composition, which need not be at equilibrium. |
Why the shift happens
- Le Chatelier tells you which way a system moves.
- But why does adding a reactant push it forward?
- The ratio $Q$ suddenly changes, breaking the balance.
- Comparing $Q$ to $K$ explains every shift precisely.
Disturbing Q
- Adding or removing a substance instantly changes the reaction quotient 反应商 $Q$.
- Now $Q \neq K$, so the system is no longer balanced.
- The reaction must move to fix that.
Le Chatelier's principle: the equilibrium shifts to oppose the change made
Moving back to K
- If the stress made $Q < K$, the reaction shifts forward.
- If it made $Q > K$, the reaction shifts backward.
- Either way, it moves until $Q = K$ again.
A stress makes $Q < K$. The reaction shifts...
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$Q < K$ means too few products, so it moves forward.
After a stress, the reaction shifts until $Q$ equals ____ again.
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The system moves back to $Q = K$.
A stress makes $Q > K$. The reaction shifts...
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$Q > K$ means too many products, so it moves backward.
The same answer, explained
- Adding reactant lowers $Q$ (a bigger denominator), so it shifts forward.
- Adding product raises $Q$, so it shifts backward.
- This is exactly what Le Chatelier predicted, now with a reason.
Adding reactant changes $Q$ by...
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More reactant in the denominator lowers $Q$, so it shifts forward.
The $Q$-versus-$K$ rule and Le Chatelier's principle give the same predictions.
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One is the quantitative reason for the other.
Adding reactant makes $Q$ drop below $K$. Which way does the reaction go?
- $Q < K$ means too few products.
- The reaction shifts forward to raise $Q$ back to $K$.
Predict the shift
For N2 + 3H2 -> 2NH3 (exothermic), sort each stress by which way it shifts the equilibrium.
Adding more of a reactant changes the value of $K$.
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False. Adding a substance changes $Q$, and the system shifts until $Q = K$ again; only temperature changes $K$.
A concentration or pressure change moves $Q$ but leaves $K$ unchanged — only temperature changes $K$. The reaction shifts to bring $Q$ back to the same $K$. So Le Chatelier and the $Q$-versus-$K$ rule always agree; one is the reason for the other.
A stress instantly changes the reaction quotient $Q$ without changing $K$, so $Q \neq K$ and the reaction shifts to restore $Q = K$: forward if $Q < K$, backward if $Q > K$. This is the quantitative reason behind every Le Chatelier shift.