Chemical Equilibrium

Introduction to Chemical Equilibrium

Chemical equilibrium is a fundamental concept in chemistry describing the state in which the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products. This chapter explores the dynamic nature of equilibrium, the equilibrium constant, and how systems respond to disturbances.

Chemical Kinetics and Reaction Rates

Chemical kinetics studies the factors affecting reaction rates and mechanisms. The reaction rate is the speed at which reactants are converted to products, defined as:

• Rate = –Δ[reactant]/Δ[time]

• Rate = +Δ[product]/Δ[time]

• Rates are always positive, regardless of direction.

Dynamic Equilibrium

Dynamic equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction. Although concentrations of reactants and products remain constant, they are not necessarily equal. This balance is dynamic, as molecules continue to react but with no net change in concentration.

• Forward reaction slows as products accumulate.

• Reverse reaction accelerates as products increase.

• At equilibrium, both rates are equal and concentrations are constant.

Equilibrium Constant (K)

The Law of Mass Action

The equilibrium constant, K, quantifies the ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of their coefficients in the balanced equation. The law of mass action is expressed as:

• K = [Products]/[Reactants] at equilibrium

• K is unitless and can be expressed in terms of concentration (Kc) or partial pressure (Kp).

Writing Equilibrium Constant Expressions

For a general reaction: aA + bB ⇌ cC + dD, the equilibrium constant expression is:

• Coefficients become exponents in the expression.

Heterogeneous Equilibria

In heterogeneous equilibria, solids and liquids are omitted from the equilibrium constant expression because their concentrations are constant.

• Only gases and aqueous species are included.

Interpreting K Values

K > 1: Product-Favored Equilibrium

If K is greater than 1, the equilibrium favors products, meaning more product molecules are present at equilibrium.

K < 1: Reactant-Favored Equilibrium

If K is less than 1, the equilibrium favors reactants, meaning more reactant molecules are present at equilibrium.

Example: Solubility Product

For sparingly soluble salts, the equilibrium constant (Ksp) is often very small, indicating a reactant-favored equilibrium.

Relationships Between K and Chemical Equations

Reversing and Scaling Equations

• Reversing a reaction inverts the equilibrium constant:

• Multiplying coefficients by a factor raises K to that power:

• Adding equations multiplies their equilibrium constants:

Calculating Keq from Measured Equilibrium Concentrations

ICE Tables

ICE tables (Initial, Change, Equilibrium) are used to organize and solve equilibrium problems. They help track changes in concentrations and calculate equilibrium values.

• Initial: Starting concentrations

• Change: Amounts gained or lost

• Equilibrium: Final concentrations

Example: ICE Table for N2O4 ⇌ 2 NO2

Solving Quadratic Equations in Equilibrium Calculations

When equilibrium calculations lead to a quadratic equation, use the quadratic formula:

Reaction Quotient (Q) and Predicting Direction

Comparing Q and K

The reaction quotient, Q, is calculated the same way as K but with current concentrations. Comparing Q to K predicts the direction the reaction will proceed:

• Q > K: Reaction shifts left (reverse)

• Q < K: Reaction shifts right (forward)

• Q = K: System is at equilibrium

Le Châtelier’s Principle

Disturbing and Restoring Equilibrium

Le Châtelier’s principle states that if a system at equilibrium is disturbed, it will shift to minimize the disturbance. Common disturbances include changes in concentration, volume, pressure, and temperature.

• Adding reactants: shifts equilibrium right

• Removing reactants: shifts equilibrium left

• Adding products: shifts equilibrium left

• Removing products: shifts equilibrium right

• Decreasing volume (for gases): shifts to side with fewer gas molecules

• Increasing temperature (exothermic): shifts left; (endothermic): shifts right

The Effect of Catalysts

Catalysts and Equilibrium

Catalysts increase the rate of both forward and reverse reactions equally, but do not affect the position of equilibrium or the value of K. They simply help the system reach equilibrium faster.

Self-Assessment Quiz: Key Points

• At equilibrium, the rates of forward and reverse reactions are equal.

• At equilibrium, concentrations of products and reactants are constant but not necessarily equal.

• Keq represents the law of mass action.

• Keq expression for 2HI(g) ⇌ I2(g) + H2(g):