Chemical Equilibrium and Reversible Reactions

Chemical Equilibrium

• Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, and the concentrations of reactants and products remain constant.
• Equilibrium between two phases of the same substance is called physical equilibrium.
• Reversible reaction: aA + bB ⇌ cC + dD, where a, b, c, and d are stoichiometric coefficients.

Reversible Reaction

• Stoichiometric coefficients represent the number of moles of each substance involved in the reaction.
• The rates of the forward and backward reactions are equal at equilibrium.
• The concentrations of the reactants and products remain constant at equilibrium.

Equilibrium Constant, K_c

• The equilibrium constant, K_c, relates the concentrations of products and reactants at equilibrium.
• K_c = [C]^c[D]^d / [A]^a[B]^b
• The equilibrium constant expression depends only on the stoichiometry of the reaction, and not on its mechanism.

Homogeneous Equilibrium

• In homogeneous equilibrium, all the reacting species are in the same phase (e.g., all gases).
• The general equilibrium constant expression involves the concentrations of all species.

Heterogeneous Equilibrium

• In heterogeneous equilibrium, the reactants and products are in different phases (e.g., solid, liquid, and gas).
• Pure solids and pure liquids are excluded from the equilibrium constant expression.

Reaction Quotient, Q_c

• Qc is a measure of the relative amounts of products and reactants present in a reaction at any given time.
• Qc = [C]^c[D]^d / [A]^a[B]^b
• Comparing Qc to Kc:
  • If Qc < Kc, the reaction will proceed to the right to reach equilibrium.
  • If Qc > Kc, the reaction will proceed to the left to reach equilibrium.
  • If Qc = Kc, the reaction is at equilibrium.

Le Chatelier's Principle

• If a change (stress) is applied to a system in equilibrium, the system will shift in a way that relieves the stress.
• Types of stress: changing concentration, pressure, volume, and temperature.

Effect of Changes in Concentration

• Adding a reactant shifts the equilibrium towards the product side.
• Removing a reactant shifts the equilibrium towards the reactant side.
• Adding a product shifts the equilibrium towards the reactant side.
• Removing a product shifts the equilibrium towards the product side.

Effect of Changes in Pressure

• Shifting the equilibrium based on the number of moles in the gaseous reactants and products involved.
  • Increasing pressure: shifts towards the side with fewer moles.
  • Decreasing pressure: shifts towards the side with more moles.

Effect of Changes in Temperature

• Changes in temperature alter the equilibrium constant K_c. Endothermic reactions = increase in K when temperature is increased, exothermic; decrease in K when temperature is increased.

The Effect of a Catalyst

• Catalysts do not affect the equilibrium position, but they do affect the rate of reaction (speed) - equally to both forward to backward. they lower the activation energy for both reactions

Summary of Guidelines for Writing Equilibrium Constant Expressions

• Concentrations of condensed phase species are expressed in mol/L.
• Concentrations of gaseous species are expressed in atm.
• Pure solids, pure liquids, and solvents do not appear in the equilibrium constant expression.

Test your knowledge on chemical equilibrium and reversible reactions in this quiz. Explore the concepts of equilibrium constants and stoichiometric coefficients while assessing your understanding of equilibrium in chemical processes.