Rucete ✏ AP Chemistry In a Nutshell
9. Chemical Equilibrium
This chapter introduces the concept of dynamic equilibrium, the equilibrium constant, and how to use equilibrium expressions to determine the direction and extent of chemical reactions.
Dynamic Equilibrium
• In dynamic equilibrium, the forward and reverse reactions occur at the same rate.
• Concentrations of reactants and products remain constant over time.
• Chemical reactions first undergo concentration changes (kinetic region), then reach a steady state (equilibrium region).
The Equilibrium Expression
• The law of mass action relates concentrations of products and reactants at equilibrium:
• K = [products]^coefficients / [reactants]^coefficients
• The value of K depends on the reaction and temperature.
Types of Equilibrium Constants
• Kc: uses molarity (mol/L).
• Kp: uses partial pressures for gases.
• Ka, Kb: for weak acid and base ionization.
• Ksp: solubility product constant for sparingly soluble salts.
• Kf: formation constant for complex ions.
Writing the Expression
• Coefficients in the balanced equation become exponents in the equilibrium expression.
• Pure solids and liquids are omitted since their concentrations remain constant.
Manipulating the Equilibrium Expression
• Reversing a reaction → invert the equilibrium constant (K′ = 1/K).
• Multiplying the equation by a factor → raise K to that power (K′ = Kⁿ).
• Adding reactions → multiply their K values.
Extent of Reaction and Favorability
• Large K (> 10¹⁰): reaction goes nearly to completion.
• Small K (< 10⁻¹⁰): barely any product formed.
• K ≈ 1: significant amounts of both reactants and products.
• Thermodynamically favorable reactions have K > 1.
Reaction Quotient (Q)
• Q is calculated the same as K but with current concentrations.
• Q = K → system is at equilibrium.
• Q < K → reaction proceeds forward to reach equilibrium.
• Q > K → reaction proceeds in reverse to reach equilibrium.
Equilibrium Tables
• Use I-C-E tables: Initial, Change, Equilibrium.
• Algebraic variable x tracks changes in concentration.
• Allows solving for unknown concentrations or K.
Solving Equilibrium Problems
• When K and initial concentrations are known, use the equilibrium expression to solve for unknown concentrations.
• Often involves setting up a quadratic equation.
• If K is very small or very large, approximation methods can simplify calculations.
Le Châtelier’s Principle
• A system at equilibrium will shift to counteract a disturbance and restore equilibrium.
• Increasing reactant → shifts right (toward products).
• Increasing product → shifts left (toward reactants).
• Decreasing a component causes shift toward the side that produces it.
Temperature Effects
• For exothermic reactions: increasing temperature shifts left (K decreases).
• For endothermic reactions: increasing temperature shifts right (K increases).
• Only temperature changes affect the value of K.
Pressure and Volume Effects
• Only relevant for gaseous equilibria with different numbers of moles on each side.
• Decreasing volume (increasing pressure) shifts toward fewer gas molecules.
• Increasing volume (decreasing pressure) shifts toward more gas molecules.
Catalysts and Equilibrium
• Catalysts speed up the forward and reverse reactions equally.
• Do not shift equilibrium or change the value of K.
• Only reduce the time needed to reach equilibrium.
Applications of Equilibrium
• Industrial: controlling yield in processes like the Haber process (ammonia production).
• Environmental: CO₂ equilibria in oceans affect pH and marine life.
• Biological: weak acid/base equilibria maintain pH in living systems.
In a Nutshell
Chemical equilibrium describes a balance between opposing processes in a reversible reaction. The equilibrium constant expresses the ratio of products to reactants at equilibrium. Changes in concentration, temperature, or pressure can shift the system according to Le Châtelier’s Principle. Mastering these concepts is essential for predicting and manipulating chemical systems in real-world applications.