# Equilibrium constant

For aA + bB ⇌ cC + dD, the equilibrium constant is K = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ). Pure solids and liquids are left out. K is large if products are favoured and small if reactants are favoured. It changes only with temperature.

Chemistry (university) · Part 07: Chemical equilibrium and solubility · Card 2 of 9 · Last updated 2026-10-19

## Analogy

A scoreboard that always lands on the same ratio: no matter how the game starts, the final ratio is fixed.

## Example

**Write K**

N₂ + 3H₂ ⇌ 2NH₃
Kc = [NH₃]² / ([N₂][H₂]³)
K > 1: mostly products
K < 1: mostly reactants

## In practice

The Haber process for making ammonia is designed around K for N₂ + 3H₂ ⇌ 2NH₃.

## Practice

### Flashcards

- **Which species are left out of K?** Pure solids and pure liquids.
- **What is the only thing that changes K?** Temperature.

### Interview questions

**For H₂ + I₂ ⇌ 2HI with K = 64, find equilibrium concentrations starting from 1.0 M H₂ and 1.0 M I₂.**

- Set up an ICE table
- (2x)²/(1−x)² = 64
- Take square root: 2x/(1−x) = 8
- x = 0.80, so [HI] = 1.6 M and [H₂] = 0.20 M

## Questions people ask

### Why are solids and pure liquids left out of K?

Their concentration does not change in a meaningful way, because it is fixed by their density. So they would be a constant that just folds into K. Only gases and dissolved species, whose concentrations really change, appear in the expression. Water as a solvent is also left out.

### What happens to K if I reverse the equation or multiply it?

If you reverse the reaction, the new K is 1/K. If you multiply all coefficients by n, the new K is K raised to the power n. If you add two reactions, you multiply their K values. These rules are used to build K for complex reactions.

## Related concepts

- [ICE table](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-7/chemistry-university-part-7-concept-5)
- [Dynamic equilibrium](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-7/chemistry-university-part-7-concept-1)
- [Reaction quotient](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-7/chemistry-university-part-7-concept-3)
- [Kp and Kc](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-7/chemistry-university-part-7-concept-4)

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