Chemical equilibrium - the state of a reaction in which all reactants and products have reached constant concentrations in a closed system.

Nitrogen dioxide

Nitrogen dioxide equilibrium

The image shows the changes that take place when nitrogen dioxide gas, NO2 (g), is placed in a sealed, evacuated chamber at 25 °C.

Nitrogen dioxide gas reacts to form dinitrogen tetroxide gas, N2O4 (g).

2NO2 (g) → N2O4 (g)

Nitrogen dioxide is a dark brown gas, whereas dinitrogen tetroxide gas is colourless.

  • Image (a) Initially, the vial contains only molecules of brown nitrogen dioxide gas.
  • Image (b) Some of the nitrogen dioxide gas has been converted to dinitrogen tetroxide gas, which is colourless.
  • Image (c) Eventually, an equilibrium is established, so the gas remains the same colour.

Three physical processes that reach equilibrium are:

• a solid in contact with a solution that contains this solid: for example, sugar crystals in a saturated aqueous sugar solution
• the vapour above a pure liquid: for example, a closed jar that contains liquid water
• the vapour above a pure solid: for example, mothballs in a closed drawer

Two chemical processes that reach equilibrium are:

• a reaction with reactants and products in the same phase: for example, a reaction between two gases to produce a gaseous product. The equilibrium they reach is called homogeneous equilibrium.
• a reaction in which reactants and products are in different phases: for example, an aqueous solution of ions, in which the ions combine to produce a slightly soluble solid that forms a precipitate. The equilibrium they reach is called heterogeneous equilibrium.

The Equilibrium Condition

The Four Conditions That Apply to All Equilibrium Systems:

  1. Equilibrium is achieved in a reversible process when the rates of opposing changes are equal.
  2. The observable (macroscopic) properties of a system at equilibrium are constant. At equilibrium, there is no overall change in the properties that depend on the total quantity of matter in the system. Examples of these properties include colour, pressure, concentration, and pH.
  3. Equilibrium can only be reached in a closed system. A closed system is a system that does not allow the input or escape of any component of the equilibrium system, including energy. For this reason, a system can be at equilibrium only if it is at constant temperature. Small changes to the components of a system are sometimes negligible. Thus, equilibrium principles can be applied if a system is not physically closed.
  4. Equilibrium can be approached from either direction. For a closed chemical equilibrium system in constant environmental conditions, the same equilibrium concentrations are reached regardless of the direction by which equilibrium was reached.

Reversible reaction - a chemical reaction that proceeds in both the forward and reverse directions, setting up an equilibrium in a closed system.

All chemical equilibria are dynamic equilibria.

A dynamic equilibrium is an equilibrium in which the rates of forward and reverse processes are equal.

 Dynamic equilibrium of a reversible reaction

A dynamic equilibrium of a reversible reaction


Equilibrium position - the relative concentrations of reactants and products in a system in dynamic equilibrium.

By convention, the equilibrium position is communicated in reference to the left-hand side (the reactant side) or the right-hand side (the product side).

The left-hand side reaction:

A (95%) ↔ B (5%)

The right-hand side reaction:

A (5%) ↔ B (95%)

It is possible to predict the changes in concentration of reactants and products as a system approaches equilibrium from the coefficients of a balanced chemical equation (the stoichiometry).

Determining Concentrations for Chemical Equilibria

Calculating Equilibrium Concentrations from Initial Reactant Concentrations

Hydrogen fluoride may be synthesized from gaseous hydrogen, H2 (g), and fluorine, F2 (g).

The balanced chemical equation for this reaction is:

H2 (g) + F2 (g) ↔ 2HF (g)

When a chemist starts this chemical reaction in a sealed container at SATP, the initial concentration of gaseous hydrogen and of gaseous fluorine is 2.00 mol/L.

No hydrogen fluoride gas is present initially.

What are the equilibrium concentrations of hydrogen gas and hydrogen fluorine gas, if the equilibrium concentration of floride gas is 0.48 mol/L?


[H2 (g)] initial = 2.00 mol/L; [F2 (g)] initial = 2.00 mol/L;
[HF(g)] initial = 0 mol/L; [F2 (g)] equilibrium = 0.48 mol/L


[H2 (g)] equilibrium ; [HF(g)] equilibrium


H2 (g) + F2 (g) ↔ 2HF (g)

ICE Table for Calculating Equilibrium Concentrations

  H2 (g) F2 (g) 2HF (g)
I 2.00 2.00 0
C -x -x +2x
E 2.00 - x 2.00 - x 2x


 In an ICE table:

  • I stands for “initial” concentrations of reactants and products before the reaction,
  • C for “change” in the concentrations of reactants and products from the start of the reaction to when equilibrium is achieved, and
  • E for “equilibrium” concentrations of reactants and products.

From the balanced equation - H2 (g) and F2 (g) are converted to HF(g) in a 1:1:2 molar ratio.

During the reaction, the concentrations of H2 (g) and F2 (g) decrease as the concentration of HF(g) increases.

Since H2 (g) and F2 (g) are consumed in a 1:1 molar ratio, the decrease in their concentrations is -x mol/L.

Since 2 mol of HF(g) are produced per 1 mol of H2 (g) and of F2 (g), the increase in concentration of HF(g) is +2x.

The equilibrium concentrations of H2 (g) and F2 (g) will be their initial concentrations, 2.00 mol/L, minus the decrease in their concentrations (x mol/L).


Step 1.

Solving the equation for x, using the values in the ICE table.

[F2 (g)] equilibrium was given as 0.48 mol/L.

[F2 (g)] equilibrium is represented by the expression 2.00 - x.

2.00 mol/L - x = 0.48 mol/L

x = 1.52 mol/L

Step 2.

Using the value of x to calculate the equilibrium concentrations of the other two entities.

[H2 (g)] = 2.00 mol/L - x

[H2 (g)] = 0.48 mol/L

[HF(g)] = 2x

[HF(g)] = 3.04 mol/L


The equilibrium concentrations of hydrogen gas and hydrogen fluoride gas are 0.48 mol/L and 3.04 mol/L, respectively.

Related Articles:

Equilibrium Law and the Equilibrium Constant
Solubility Equilibria and the Solubility Product Constant
Changes in Equilibrium Systems
The Nature of Acids and Bases
Strong and Weak Acids and Bases