Spontaneity and Equilibrium - SS2 Chemistry Lesson Note
Spontaneity and equilibrium are key concepts in thermodynamics that describe the behaviour of chemical reactions and physical processes. Understanding the factors influencing spontaneity and the conditions for reaching equilibrium is crucial for predicting the direction, extent, and stability of reactions.
Spontaneity of Processes:
Spontaneity refers to the tendency of a process to occur without external intervention. It is influenced by the energy changes and entropy changes associated with the system.
Factors Affecting Spontaneity:
● Enthalpy Change (ΔH): Processes tend to be spontaneous if they involve a decrease in enthalpy, such as exothermic reactions where heat is released. A negative ΔH contributes to the spontaneity of a process.
● Entropy Change (ΔS): Processes tend to be spontaneous if they result in an increase in entropy, such as the mixing of gases or the transition from a solid to a liquid. A positive ΔS contributes to the spontaneity of a process.
● Gibbs Free Energy Change (ΔG): The spontaneity of a process can be determined by the Gibbs free energy change. A negative ΔG indicates a spontaneous process, while a positive ΔG indicates a non-spontaneous process.
Spontaneity and the Gibbs Free Energy Equation:
The relationship between spontaneity and the Gibbs free energy change (ΔG) is given by the equation:
ΔG = ΔH - TΔS
● If ΔG < 0, the process is spontaneous in the forward direction.
● If ΔG > 0, the process is nonspontaneous in the forward direction.
● If ΔG = 0, the system is at equilibrium.
Equilibrium:
Equilibrium refers to a state of balance in a system where the forward and reverse processes occur at equal rates. It is characterised by a constant concentration or partial pressure of reactants and products.
Types of Equilibrium:
● Chemical Equilibrium: Chemical equilibrium occurs in reversible chemical reactions where the concentrations of reactants and products remain constant over time. The forward and reverse reactions occur simultaneously at equal rates.
● Phase Equilibrium: Phase equilibrium refers to the coexistence of multiple phases of a substance, such as liquid-vapour equilibrium (boiling and condensation) or solid-liquid equilibrium (melting and freezing).
Factors Affecting Equilibrium:
● Equilibrium Constant (K): The equilibrium constant expresses the ratio of the concentrations of products to reactants at equilibrium. It is determined by the stoichiometry of the balanced chemical equation and is temperature-dependent.
● Le Chatelier's Principle: According to this principle, when a system at equilibrium is subjected to a change in temperature, pressure, or concentration, the system responds by shifting the equilibrium position to counteract the change and re-establish equilibrium.
Conditions for Reaching Equilibrium:
To reach equilibrium, certain conditions must be met:
● Closed System: The system must be isolated from its surroundings to prevent the exchange of matter or energy.
● Sufficient Time: Sufficient time is required for the forward and reverse reactions to establish a balance.
● Reversible Process: The process must be reversible, allowing the forward and reverse reactions to occur.
Applications of Spontaneity and Equilibrium:
● Chemical Reactions: Understanding spontaneity and equilibrium is vital for predicting the direction and extent of chemical reactions, determining reaction rates, and designing reaction conditions.
● Industrial Processes: Spontaneity and equilibrium considerations are crucial in industrial processes such as manufacturing chemicals, refining petroleum, and producing energy.
● Environmental Systems: Equilibrium concepts are applied to understand natural processes such as the carbon cycle, acid-base equilibria in water, and the equilibrium between air and water pollutants.
Spontaneity and equilibrium are fundamental concepts in thermodynamics that govern the behaviour of chemical reactions and physical processes. Spontaneity is influenced by the enthalpy and entropy changes associated with a system, while equilibrium represents a state of balance in a system. Factors such as enthalpy, entropy, and Gibbs free energy determine the spontaneity of processes, while equilibrium is governed by the equilibrium constant and Le Chatelier's Principle. Understanding the factors influencing spontaneity and the conditions for reaching equilibrium is essential for predicting and controlling the behaviour of chemical reactions and natural processes.