Equilibrium Composition Relation

What is this formula?

The equilibrium composition relation calculates the amount of a chemical species at equilibrium from its initial amount, stoichiometric coefficient, and equilibrium extent of reaction.

When to use it

Use this formula when building equilibrium tables, performing material balances, or calculating final species amounts in reversible reactions.

Example

Reaction:

N2 + 3H2 ⇌ 2NH3

For NH3:

Initial NH3 = 0 mol

Stoichiometric coefficient = +2

Equilibrium extent = 0.40 mol

Formula:

nie = ni0 + vξ

Substitution:

nie = 0 + 2×0.40

Result:

nie = 0.80 mol

At equilibrium, 0.80 mol of NH3 are present.

Applications

Chemical equilibrium calculations

Reaction engineering

Material balances

Thermodynamics

Industrial process analysis

Note

The stoichiometric coefficient must include its sign: negative for reactants and positive for products. This relation is exact for a single reaction written with a defined stoichiometric equation. Systems with multiple simultaneous reactions require one extent variable for each independent reaction.