# 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.

Chemistry (university) · Part 07: Chemical equilibrium and solubility · Card 3 of 9 · Last updated 2026-10-19

## Analogy

A satnav: Q is where you are now, K is the destination, and the comparison shows which way to drive.

## Example

**H₂ + I₂ ⇌ 2HI, K = 50**

[H₂] = 0.10, [I₂] = 0.10, [HI] = 0.50
Q = 0.50² / (0.10 × 0.10) = 25
Q < 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.

## Related concepts

- [Dynamic equilibrium](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-7/chemistry-university-part-7-concept-1)
- [Equilibrium constant](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-7/chemistry-university-part-7-concept-2)
- [Kp and Kc](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-7/chemistry-university-part-7-concept-4)
- [ICE table](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-7/chemistry-university-part-7-concept-5)

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