---
title: "Kp, Partial Pressures and Gas Equilibria"
description: "For gases the equilibrium constant is written in partial pressures. Each gas contributes its mole fraction of the total pressure, and the conditions chosen in industry are argued from there."
canonical: https://lightmysky.com/learn/science/kp-partial-pressures-and-gas-equilibria-mt_1R8BpA2hfK
source: https://lightmysky.com/learn/science/kp-partial-pressures-and-gas-equilibria-mt_1R8BpA2hfK.md
retrieved: 2026-09-12
---

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# Kp, Partial Pressures and Gas Equilibria

For gases the equilibrium constant is written in partial pressures. Each gas contributes its mole fraction of the total pressure, and the conditions chosen in industry are argued from there.

Subject: Science · Area: Chemistry · Ages 17 to 18
Page: https://lightmysky.com/learn/science/kp-partial-pressures-and-gas-equilibria-mt_1R8BpA2hfK

## Ready when they can

- Calculates mole fractions and partial pressures from equilibrium amounts and a total pressure
- Writes Kp for a gas reaction and works out its units
- Explains why raising the total pressure shifts the position while leaving Kp untouched
- Uses Kp to argue the conditions picked for an industrial process such as ammonia synthesis

## Lesson: Kp from partial pressures

Kc put a number on how far a reaction goes; for gases we write that number with pressures instead of concentrations, and it tells industry how to run its plants. In a gas mixture each gas pushes as if the others were not there. Its mole fraction is its moles over the total moles, and its partial pressure is that fraction times the total pressure.

**Example.** For nitrogen plus hydrogen making ammonia, Kp is the product pressures over the reactant ones, each raised to its balancing number: p(NH3) squared over (p(N2) times p(H2) cubed), with both reactant terms in the denominator. To find the units, put pressure units into that expression and cancel them down.

Raise the total pressure and the system shifts toward the side with fewer gas moles to ease the strain. Kp itself does not move, because Kp answers only to temperature.

That is why ammonia plants run at high pressure: it pushes the yield up without touching Kp. The temperature is a compromise, hot enough for a decent rate but cool enough to protect the yield.

**Recap.** Share out the pressure by mole fraction, write Kp with powers, and shift position with pressure while Kp stays fixed.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [The Equilibrium Constant Kc](https://lightmysky.com/learn/science/the-equilibrium-constant-kc-mt_3G8b6Hyz3u)
- [Gas Volumes and the Ideal Gas Equation](https://lightmysky.com/learn/science/gas-volumes-and-the-ideal-gas-equation-mt_I1fz_6wjuD)

## Opens up

- [pH, Kw and Strong Acids and Bases](https://lightmysky.com/learn/science/ph-kw-and-strong-acids-and-bases-mt_KJ4yc25Z_l)