Reaction quotient
The reaction quotient Q has the same form as K but uses the concentrations right now. Compare Q to K. If Q < K, the reaction goes forward. If Q > K, it goes in reverse. If Q = K, it is at equilibrium.
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Analogy
A satnav: Q is where you are now, K is the destination, and the comparison shows which way to drive.
Example
[H₂] = 0.10, [I₂] = 0.10, [HI] = 0.50Q = 0.50² / (0.10 × 0.10) = 25Q < K, so the reaction shifts right
In practice
A plant chemist measures Q on a reactor sample to see if the process has finished or needs more time.
Practice
Flashcards
- What does Q > K tell you?The reaction will go in reverse, toward reactants.
Questions people ask
Why does Q < K mean the reaction goes forward?
If Q is smaller than K, there are too few products compared with equilibrium. The system makes more products by moving forward until Q rises to equal K. If Q is larger than K, there are too many products, and the system reverses.
Can Q be calculated before any reaction has started?
Yes. Q uses whatever concentrations you have, so you can work it out at the very start. If only reactants are present, Q is zero. If only products are present, Q is huge. Either way, the reaction moves toward K.