Chemical Equilibrium and Equilibrium Constant

Chemical Equilibrium

• Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate.
• At equilibrium, the forward and reverse reactions are proceeding at the same rate, and the concentrations of reactants and products remain constant.
• Visual representations using a seesaw analogy, and images showing the changing concentrations over time can demonstrate this concept.
• Key players in establishing equilibrium conditions are forward and reverse reaction rates, concentration of the reaction components and constant temperature.
• The equilibrium state is dynamic, where the forward and reverse reactions never stop but happen at equivalent rates.

The Equilibrium Constant

• The equilibrium constant (K) is a ratio of product concentrations to reactant concentrations, each raised to the power of their stoichiometric coefficient in the balanced chemical equation.
• For a generic reaction aA + bB ⇌ cC + dD, the equilibrium constant (K_c) is expressed as [C]^c[D]^d / [A]^a[B]^b. The square brackets denote the concentration of a species.
• The magnitude of K_c indicates the extent of a reaction at equilibrium. A large K_c suggests that products are favored, while a small K_c suggests reactants are favored.
• K_p is the equilibrium constant when partial pressures are used instead of concentrations (for gaseous reactants or products)
• K_p = K_c(RT)^Δn. R (the ideal gas constant) and T (the temperature) are physical constants. Δn is the change in the number of moles of gas between the products and the reactants.

Heterogeneous Equilibrium

• In heterogeneous equilibria, the concentrations of pure solids and pure liquids do not appear in the equilibrium expression.

The Reaction Quotient (Q)

• Q is a ratio of product concentrations to reactant concentrations, similar to K, but calculated using any set of concentrations, not just equilibrium concentrations.
• If Q = K, the system is at equilibrium.
• If Q > K, the reaction proceeds towards reactants to reach equilibrium.
• If Q < K, the reaction proceeds towards products to reach equilibrium.

Le Châtelier's Principle

• Le Châtelier's principle describes how a system at equilibrium responds to a stress (change in conditions).
• If a stress like a change in temperature, pressure, or concentration is applied to a system at equilibrium, the system will shift in a direction that relieves the stress. For example, systems shift to counteract changes of concentrations or temperature to maintain equilibrium.

Haber Process

• The Haber process involves the production of ammonia (NH₃) from nitrogen (N₂) and hydrogen (H₂) under high pressure and elevated temperature, which influences the type and rate of the reaction.

Effects of Pressure Change on Equilibrium

• Increasing the pressure on a reaction vessel will shift the equilibrium in a direction that results in fewer moles of gas and vice versa.
• This principle only applies to gaseous molecules or reactants.

Effects of Temperature Change on Equilibrium

• The change in temperature will shift the equilibrium away from the affected side (either products or reactants) for an endothermic and exothermic reaction. This involves the heat as one of the components in the equilibrium.

Catalysts

• A catalyst speed up both the forward and reverse reaction rates in a reaction vessel and does not change the equilibrium point.

Ideal Solutions: Raoult's Law and Henry's Law

• Raoult's Law describes the vapor pressure of a solvent in a solution, which is directly proportional to the mole fraction of that solvent in the solution.
• Henry's Law describes the solubility of a gas in a solution, which is directly proportional to the partial pressure of the gas above the solution.

Homogeneous and Heterogeneous Systems

• A homogeneous system consists of a single phase throughout.
• A heterogeneous system consists of two or more distinct phases that can be physically separated..

Phase Diagrams

• Phase diagrams visually represent the phases (solid, liquid, gas) of a substance as a function of temperature and pressure. Specific details on the diagram are dependent on the substance. Specific point(s) such as the critical point and triple point are marked on the diagram to indicate phase transition(s).

Solubility Product

• Solubility product (K_sp) is the equilibrium constant for the dissolution of a solid, such as silver chloride, into a solution and vice versa.

Factors Influencing Solubility

• Interactions between solute and solvent molecules, pressure, and temperature significantly affect the solubility of substances.