Chemical Equilibrium

Introduction to Chemical Equilibrium

Chemical equilibrium occurs when the rates of the forward and reverse reactions in a chemical system are equal, resulting in constant concentrations of reactants and products. Understanding equilibrium is essential for predicting the composition of reaction mixtures and for controlling chemical processes.

• Reversible reactions can proceed in both forward and reverse directions.

• At equilibrium, the concentrations of all species remain constant over time.

• Equilibrium does not mean that reactants and products are present in equal amounts, but that their concentrations are unchanging.

The Equilibrium Constant (Kc)

Definition and Calculation

The equilibrium constant (Kc) is a dimensionless value that expresses the ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of their stoichiometric coefficients. It provides a quantitative measure of the position of equilibrium for a given reaction at a specific temperature.

• For a general reaction:

• The equilibrium constant expression is:

• [X] denotes the concentration of species X in mol L–1.

• Kc is temperature dependent; always specify the temperature when reporting Kc.

• Kc is dimensionless (no units).

• Products appear in the numerator; reactants in the denominator.

Example: Calculating Kc

• Given the reaction: at 472°C

• Equilibrium concentrations: [H2] = 1.2 mol L–1, [I2] = 1.2 mol L–1, [HI] = 0.35 mol L–1

• Expression:

• Calculation:

Significance of the Magnitude of Kc

The value of Kc indicates the extent to which a reaction proceeds at equilibrium:

• Kc >> 1: Products are favored; equilibrium lies to the right.

• Kc << 1: Reactants are favored; equilibrium lies to the left.

• Kc ≈ 1: Significant amounts of both reactants and products are present.

Example: If Kc = 1000, the numerator (products) must be much larger than the denominator (reactants), indicating a product-favored equilibrium.

The Reaction Quotient (Qc)

Definition and Use

The reaction quotient (Qc) is calculated using the same expression as Kc, but with concentrations at any point in time (not necessarily at equilibrium). Qc is used to predict the direction in which a reaction will proceed to reach equilibrium.

• For a reaction:

• Expression: (using current concentrations)

• If Qc = Kc: The system is at equilibrium.

• If Qc > Kc: The system will shift to the left (toward reactants).

• If Qc < Kc: The system will shift to the right (toward products).

Example: Using Qc to Predict Direction

• Given: [N2] = 4.0 × 10–2 mol L–1, [H2] = 8.5 × 10–1 mol L–1, [NH3] = 3.1 × 10–3 mol L–1, Kc = 5.1 × 10–4

• Calculate Qc and compare to Kc to determine the direction of shift.

• If Qc < Kc, the reaction will proceed to form more products.

Manipulating the Equilibrium Constant

Changing the Direction of a Reaction

• If a reaction is reversed, the new Kc is the reciprocal of the original:

Multiplying Coefficients by a Factor

• If all coefficients in a balanced equation are multiplied by a factor n, the new Kc is raised to the nth power:

Adding Chemical Equilibria

• When two or more reactions are added, their equilibrium constants are multiplied:

• If and are for two reactions, then for the sum:

Heterogeneous vs. Homogeneous Equilibria

Homogeneous Equilibria

All reactants and products are in the same phase (e.g., all gases or all aqueous solutions). All species are included in the Kc expression.

Heterogeneous Equilibria

Reactants and products are in different phases (e.g., solids, liquids, and gases). Only species in the gas or aqueous phase are included in the Kc expression; pure solids and pure liquids are omitted because their concentrations are constant and do not affect the equilibrium position.

• Example: For the reaction , only CO2(g) is included in the Kc expression:

• Solids (NaHCO3, Na2CO3) and liquids (H2O) are not included.

Additional info: In all equilibrium expressions, only species with variable concentrations (gases and solutes) are included. The presence of solids or liquids is necessary for equilibrium, but their concentrations are not part of the Kc calculation.