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Unit 3•Lesson 3 2 min read

Chemical Equilibrium and Keq

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Learning Objectives

Describe chemical equilibrium as a dynamic state.
Write the equilibrium constant expression (Keq) for a given reaction.
Interpret the value of Keq to determine if reactants or products are favored.
Distinguish between Keq and the reaction quotient, Q.

The Dynamic Balance of Reactions

Many chemical reactions are reversible, meaning they can proceed in both the forward and reverse directions. Chemical equilibrium is the state where the rate of the forward reaction is equal to the rate of the reverse reaction.

At equilibrium, the concentrations of reactants and products are constant, but the reactions have not stopped. It is a dynamic state.

The Equilibrium Constant (Keq)

The equilibrium constant (Keq) is a value that expresses the relationship between the concentrations of products and reactants at equilibrium. For a generic reversible reaction:

aA + bB ⇌ cC + dD

The equilibrium constant expression (or law of mass action) is:

Keq = ( [C]ᶜ [D]ᵈ ) / ( [A]ᵃ [B]ᵇ )

[ ] denotes the molar concentration at equilibrium.
The coefficients in the balanced equation become the exponents in the expression.
Important: Pure solids and pure liquids are not included in the Keq expression because their concentration is effectively constant.

Interpreting Keq

The value of Keq tells us about the position of the equilibrium:

If Keq > 1, the concentration of products is greater than reactants at equilibrium. The equilibrium lies to the right, and products are favored.
If Keq < 1, the concentration of reactants is greater than products at equilibrium. The equilibrium lies to the left, and reactants are favored.
If Keq ≈ 1, the concentrations of reactants and products are roughly equal.

The Reaction Quotient (Q)

The reaction quotient (Q) has the exact same mathematical form as Keq, but it is calculated using the concentrations of reactants and products at any given moment, not just at equilibrium.

By comparing Q to Keq, we can predict which way a reaction will shift to reach equilibrium:
If Q < Keq, the ratio of products to reactants is too small. The reaction will shift to the right (forward) to produce more products.
If Q > Keq, the ratio is too large. The reaction will shift to the left (reverse) to produce more reactants.
If Q = Keq, the system is already at equilibrium.

Key Terms

Chemical Equilibrium
The state in which both forward and reverse reactions are occurring at equal rates, so that the concentrations of reactants and products remain constant.
Equilibrium Constant (Keq)
The value of the reaction quotient when the reaction has reached equilibrium. It indicates the relative concentrations of products and reactants at equilibrium.
Law of Mass Action
The principle that the rate of a chemical reaction is proportional to the product of the concentrations of the reactants. It is the basis for the Keq expression.
Reaction Quotient (Q)
A quantity that provides a measure of the relative amounts of products and reactants present in a reaction at any given time.

Check Your Understanding

1

Write the equilibrium constant expression (Keq) for the following reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g).

2

A chemical reaction has an equilibrium constant Keq = 0.005. At equilibrium, are the reactants or the products favored?

3

For the synthesis of ammonia, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), you measure the concentrations and find that Q = 50 and Keq = 100. In which direction will the reaction proceed to reach equilibrium?

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