🧂Physical Chemistry II Unit 5 – Chemical Equilibrium & Phase Transitions

Key Concepts

• Chemical equilibrium occurs when the rates of forward and reverse reactions are equal, resulting in no net change in concentrations over time
• Equilibrium constants ($K$) quantify the relationship between reactant and product concentrations at equilibrium and depend on temperature
• Le Chatelier's principle predicts the direction of shift in equilibrium when a system is disturbed by changes in concentration, pressure, or temperature
• Phase transitions involve changes in the physical state of matter (solid, liquid, or gas) and are accompanied by changes in enthalpy and entropy
• Phase diagrams illustrate the conditions (pressure and temperature) under which different phases of a substance exist in equilibrium
• Gibbs phase rule relates the number of components, phases, and degrees of freedom in a system at equilibrium
• Kinetics and equilibrium are interconnected concepts that describe the rates and extent of chemical reactions and the final state of a system
• Real-world applications of chemical equilibrium and phase transitions include industrial processes (Haber-Bosch process for ammonia synthesis), environmental phenomena (carbon dioxide solubility in oceans), and biological systems (oxygen binding to hemoglobin)

Thermodynamic Foundations

• Thermodynamics provides the framework for understanding chemical equilibrium and phase transitions by relating macroscopic properties to microscopic behavior
• Gibbs free energy ($G$) is a key thermodynamic quantity that determines the spontaneity of a process at constant temperature and pressure
  • A negative change in Gibbs free energy ($\Delta G < 0$) indicates a spontaneous process, while a positive change ($\Delta G > 0$) indicates a non-spontaneous process
• At equilibrium, the change in Gibbs free energy is zero ($\Delta G = 0$), and the system has reached its most stable state
• The relationship between Gibbs free energy and the equilibrium constant is given by $\Delta G^\circ = -RT \ln K$, where $\Delta G^\circ$ is the standard Gibbs free energy change, $R$ is the gas constant, and $T$ is the absolute temperature
• Enthalpy ($H$) and entropy ($S$) contribute to the Gibbs free energy through the equation $\Delta G = \Delta H - T\Delta S$
  • Enthalpy represents the heat absorbed or released during a process, while entropy represents the degree of disorder or randomness in a system
• The temperature dependence of the equilibrium constant is described by the van 't Hoff equation, $\frac{d \ln K}{dT} = \frac{\Delta H^\circ}{RT^2}$, which relates the change in equilibrium constant with temperature to the standard enthalpy change of the reaction

Equilibrium Constants

• Equilibrium constants ($K$) are dimensionless quantities that express the ratio of product concentrations to reactant concentrations at equilibrium, each raised to their stoichiometric coefficients
• For a general reaction $aA + bB \rightleftharpoons cC + dD$, the equilibrium constant is given by $K = \frac{[C]^c[D]^d}{[A]^a[B]^b}$, where the square brackets denote molar concentrations
• Equilibrium constants can be expressed in terms of partial pressures ($K_p$) for gaseous reactions or molar concentrations ($K_c$) for reactions in solution
• The magnitude of the equilibrium constant indicates the extent of the reaction at equilibrium
  • A large $K$ value ($K \gg 1$) implies that the reaction favors product formation, while a small $K$ value ($K \ll 1$) implies that the reaction favors reactants
• Equilibrium constants are temperature-dependent but independent of initial concentrations, pressure, or the presence of a catalyst
• The reaction quotient ($Q$) has the same form as the equilibrium constant but is calculated using instantaneous concentrations rather than equilibrium concentrations
  • Comparing $Q$ to $K$ allows for predicting the direction of a reaction to reach equilibrium

Le Chatelier's Principle

• Le Chatelier's principle states that when a system at equilibrium is subjected to a disturbance, the system will shift its equilibrium position to counteract the disturbance and establish a new equilibrium
• Changes in concentration, pressure, or temperature can disturb a system at equilibrium, causing it to respond according to Le Chatelier's principle
• Concentration changes
  • Adding a reactant or removing a product will shift the equilibrium towards the products, while removing a reactant or adding a product will shift the equilibrium towards the reactants
• Pressure changes (for gaseous reactions)
  • Increasing the pressure will shift the equilibrium towards the side with fewer moles of gas, while decreasing the pressure will shift the equilibrium towards the side with more moles of gas
• Temperature changes
  • For exothermic reactions ($\Delta H < 0$), increasing the temperature will shift the equilibrium towards the reactants, while decreasing the temperature will shift the equilibrium towards the products
  • For endothermic reactions ($\Delta H > 0$), increasing the temperature will shift the equilibrium towards the products, while decreasing the temperature will shift the equilibrium towards the reactants
• Catalysts do not affect the equilibrium position but instead accelerate the rate at which equilibrium is reached by lowering the activation energy of the forward and reverse reactions equally

Phase Diagrams

• Phase diagrams are graphical representations of the equilibrium relationships between the solid, liquid, and gas phases of a substance as a function of pressure and temperature
• The lines in a phase diagram represent the conditions under which two phases coexist in equilibrium (solid-liquid, liquid-gas, or solid-gas)
  • The solid-liquid equilibrium line is called the melting or freezing curve, the liquid-gas equilibrium line is called the vaporization or condensation curve, and the solid-gas equilibrium line is called the sublimation or deposition curve
• The triple point is the unique pressure and temperature at which all three phases (solid, liquid, and gas) coexist in equilibrium
• The critical point is the highest temperature and pressure at which the liquid and gas phases can be distinguished
  • Above the critical point, the substance exists as a supercritical fluid with properties intermediate between those of a liquid and a gas
• Phase diagrams can be used to predict the phase changes a substance will undergo as pressure and temperature are varied, as well as to determine the conditions needed to maintain a desired phase
• Different substances have distinct phase diagrams that reflect their unique intermolecular forces and molecular structures (water, carbon dioxide)

Gibbs Phase Rule

• The Gibbs phase rule is a fundamental relationship that describes the number of degrees of freedom ($F$) in a system at equilibrium as a function of the number of components ($C$) and the number of phases ($P$)
• The Gibbs phase rule is expressed as $F = C - P + 2$, where the "2" represents the two intensive variables (pressure and temperature) that can be independently varied
• The number of components ($C$) is the minimum number of chemically independent species required to describe the composition of all phases in the system
• The number of phases ($P$) is the number of physically distinct and homogeneous regions in the system (solid, liquid, or gas)
• The degrees of freedom ($F$) represent the number of intensive variables (pressure, temperature, or composition) that can be independently varied without changing the number of phases in the system
• For a single-component system ($C = 1$), the Gibbs phase rule simplifies to $F = 3 - P$, meaning that a maximum of two phases can coexist in equilibrium at a given pressure and temperature (triple point), and a single phase has two degrees of freedom (pressure and temperature)
• The Gibbs phase rule is a powerful tool for analyzing the behavior of multi-component systems, such as binary alloys or aqueous solutions, and for designing processes that involve phase transitions and separations

Kinetics and Equilibrium

• Chemical kinetics and equilibrium are closely related concepts that describe the rates and extent of chemical reactions, respectively
• Kinetics deals with the rates of chemical reactions, including the factors that influence reaction rates (concentration, temperature, pressure, and catalysts) and the mechanisms by which reactions occur
• Equilibrium is the state reached by a system when the rates of the forward and reverse reactions are equal, resulting in no net change in concentrations over time
• The approach to equilibrium is governed by the relative rates of the forward and reverse reactions
  • If the forward reaction is faster than the reverse reaction, the system will shift towards the products until equilibrium is reached
  • If the reverse reaction is faster than the forward reaction, the system will shift towards the reactants until equilibrium is reached
• The time required to reach equilibrium depends on the reaction rates and the initial concentrations of reactants and products
• Catalysts accelerate the approach to equilibrium by lowering the activation energy of the forward and reverse reactions equally, but they do not affect the equilibrium position or the equilibrium constant
• The relationship between kinetics and equilibrium is expressed by the law of mass action, which states that the rate of a reaction is proportional to the product of the concentrations of the reactants, each raised to a power equal to its stoichiometric coefficient
• Understanding the interplay between kinetics and equilibrium is crucial for optimizing chemical processes, such as industrial syntheses or enzymatic reactions, and for predicting the behavior of complex systems, such as atmospheric chemistry or biochemical networks

Real-World Applications

• The Haber-Bosch process for ammonia synthesis is a classic example of the industrial application of chemical equilibrium
  • The process involves the reaction of nitrogen and hydrogen gases at high pressure and temperature in the presence of an iron catalyst to produce ammonia, which is used in fertilizers and other chemicals
  • The equilibrium constant for the reaction is small, so the process is designed to shift the equilibrium towards the products by using high pressure, moderate temperature, and continuous removal of ammonia
• The solubility of carbon dioxide in oceans is a critical factor in the global carbon cycle and climate change
  • The dissolution of atmospheric carbon dioxide in seawater is governed by Henry's law, which relates the partial pressure of a gas to its concentration in solution at equilibrium
  • As the concentration of carbon dioxide in the atmosphere increases due to human activities, more carbon dioxide dissolves in the oceans, leading to ocean acidification and potential impacts on marine ecosystems
• The binding of oxygen to hemoglobin in red blood cells is a vital example of equilibrium in biological systems
  • Hemoglobin is a protein that contains four iron-containing heme groups, each of which can reversibly bind one molecule of oxygen
  • The binding of oxygen to hemoglobin is cooperative, meaning that the binding of one oxygen molecule increases the affinity of the remaining heme groups for oxygen
  • The oxygen-hemoglobin equilibrium is sensitive to changes in pH, carbon dioxide concentration, and temperature, allowing for efficient oxygen delivery to tissues under varying physiological conditions
• Phase transitions are exploited in many industrial processes, such as distillation, crystallization, and extraction
  • Distillation is a method for separating mixtures of liquids based on their different boiling points and vapor pressures
  • Crystallization is a technique for purifying solid compounds by controlling their solubility and nucleation kinetics
  • Extraction is a process for separating a desired component from a mixture using a solvent that preferentially dissolves the component based on its partition coefficient between the two phases
• Understanding chemical equilibrium and phase transitions is essential for designing and optimizing processes in fields as diverse as materials science, chemical engineering, environmental science, and biotechnology