---
title: "The Equilibrium Constant Kc"
description: "At equilibrium the concentrations settle into a ratio that is fixed for a given temperature. Kc puts a number on how far a reaction goes, where Le Chatelier could only say which way it moves."
canonical: https://lightmysky.com/learn/science/the-equilibrium-constant-kc-mt_3G8b6Hyz3u
source: https://lightmysky.com/learn/science/the-equilibrium-constant-kc-mt_3G8b6Hyz3u.md
retrieved: 2026-09-12
---

> **Agent view.** This is the Markdown twin of the page, for tools and assistants.
> When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md
> API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md
> Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt
> Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json
> Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`.

# The Equilibrium Constant Kc

At equilibrium the concentrations settle into a ratio that is fixed for a given temperature. Kc puts a number on how far a reaction goes, where Le Chatelier could only say which way it moves.

Subject: Science · Area: Chemistry · Ages 16 to 18
Page: https://lightmysky.com/learn/science/the-equilibrium-constant-kc-mt_3G8b6Hyz3u

## Ready when they can

- Writes the expression for Kc from a balanced equation, products over reactants, each raised to its coefficient
- Works out the units of Kc for a given equation
- Calculates Kc from equilibrium amounts and the volume of the container
- Reads a very large or very small value of Kc as a statement about where the position lies

## Lesson: Putting a number on equilibrium

The equilibrium constant Kc says how far a reaction goes. Write product concentrations on top and reactant concentrations below, raising each to its balancing number. That products over reactants pattern never changes. A huge Kc means the mixture sits almost all as products, while a tiny one means it barely reacts. Kc says nothing about speed, only about the final position.

To calculate Kc, turn every equilibrium amount into concentration by dividing moles by the container volume in cubic decimetres. Slot the concentrations into the expression and crunch the numbers. Units come from the expression, so they differ from one equation to the next. The classic slip is forgetting to divide by the volume.

**Example.** Try 0.4, 0.4, 0.2 and 0.2 mol at equilibrium in a 2 cubic decimetre vessel, all powers one. Concentrations are 0.2, 0.2, 0.1 and 0.1 mol per cubic decimetre. Then Kc is 0.1 times 0.1 over 0.2 times 0.2, which is 0.25 with no units.

**Tip.** Equilibrium here is dynamic: both reactions keep running at equal speeds while concentrations sit flat, inside a closed system. And Kc belongs to one temperature only. Shift the temperature and the constant itself changes.

**Recap.** Divide by the volume, write products over reactants with powers, and read the size for the position.

## Practice

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

## Needs first

- [The Arrhenius Equation and Finding Activation Energy](https://lightmysky.com/learn/science/the-arrhenius-equation-and-finding-activation-energy-mt_QBQDSNwbXK)
- [Reversible Reactions, Dynamic Equilibrium and Le Chatelier](https://lightmysky.com/learn/science/reversible-reactions-dynamic-equilibrium-and-le-chatelier-mt_Zas-V3kTiR)

## Opens up

- [Kp, Partial Pressures and Gas Equilibria](https://lightmysky.com/learn/science/kp-partial-pressures-and-gas-equilibria-mt_1R8BpA2hfK)
- [Solubility Equilibria and When a Precipitate Appears](https://lightmysky.com/learn/science/solubility-equilibria-and-when-a-precipitate-appears-mt_IMdWLxIAFt)
- [pH, Kw and Strong Acids and Bases](https://lightmysky.com/learn/science/ph-kw-and-strong-acids-and-bases-mt_KJ4yc25Z_l)
- [Weak Acids, Ka and pKa](https://lightmysky.com/learn/science/weak-acids-ka-and-pka-mt_q7fYb7qzMH)
- [Carboxylic Acids, Esters and Hydrolysis](https://lightmysky.com/learn/science/carboxylic-acids-esters-and-hydrolysis-mt_QLxUs5cOH2)