# Kp and Kc

For gas reactions, K can be written with partial pressures (Kp) or concentrations (Kc). They are related by Kp = Kc(RT)^Δn, where Δn is moles of gas on the product side minus the reactant side.

Chemistry (university) · Part 07: Chemical equilibrium and solubility · Card 4 of 9 · Last updated 2026-10-19

## Analogy

The same measurement in two units, like miles and kilometres, with a conversion factor.

## Example

**N₂ + 3H₂ ⇌ 2NH₃**

Δn = 2 − (1 + 3) = −2
Kp = Kc (RT)⁻²
If Δn = 0, Kp = Kc

## In practice

Industrial gas reactions are often described with Kp because pressures are easy to measure.

## Practice

### Flashcards

- **Relation between Kp and Kc?** Kp = Kc(RT)^Δn.

## Questions people ask

### When is Kp equal to Kc?

When the number of gas moles is the same on both sides, Δn = 0, and (RT) raised to the power zero is 1. Examples are H₂ + I₂ ⇌ 2HI and N₂ + O₂ ⇌ 2NO. In other reactions the two constants differ.

### Which value of R do I use for Kp?

Use R = 0.08206 L·atm/(mol·K) when Kc uses mol/L and Kp uses atm, with T in kelvin. Keep the units consistent with your data. The conversion Kp = Kc(RT)^Δn only works if Kc is in mol/L and pressure is in atmospheres.

## 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)
- [Reaction quotient](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-7/chemistry-university-part-7-concept-3)
- [ICE table](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-7/chemistry-university-part-7-concept-5)

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