# Unit 2: Chemical Equilibrium

Unit 2 Objectives

By the end of this unit, you will be able to:

• Describe the concept of dynamic equilibrium
• Calculate equilibrium constants from data set and modify equilibrium constants (mathematically) when reactions are altered
• Relate Kc and Kp
• Calculate equilibrium concentrations from initial conditions use ICE tables (simple, quadratic and assumption)
• Predict the direction of a reaction from a reaction quotient and equilibrium constant
• Describe LeChatelier's Principle
• Predict direction of change of reaction based upon changes in reaction conditions (temperature, addn of reactants or products, volume change)

• 2.1: The Concept of Dynamic Equilibrium
At equilibrium, the forward and reverse reactions of a system proceed at equal rates. Chemical equilibrium is a dynamic process consisting of forward and reverse reactions that proceed at equal rates. At equilibrium, the composition of the system no longer changes with time. The composition of an equilibrium mixture is independent of the direction from which equilibrium is approached.
• 2.2: The Equilibrium Constant (K)
The law of mass action describes a system at equilibrium in terms of the concentrations of the products and the reactants. For a system involving one or more gases, either the molar concentrations of the gases or their partial pressures can be used.
• 2.3: Expressing the Equilibrium Constant in Terms of Pressure
An equilibrated system that contains products and reactants in a single phase is a homogeneous equilibrium; a system whose reactants, products, or both are in more than one phase is a heterogeneous equilibrium.
• 2.4: Interpreting and Working with Equilibrium Constants
Various methods can be used to solve the two fundamental types of equilibrium problems: (1) those in which we calculate the concentrations of reactants and products at equilibrium and (2) those in which we use the equilibrium constant and the initial concentrations of reactants to determine the composition of the equilibrium mixture. When an equilibrium constant is calculated from equilibrium concentrations, concentrations or partial pressures are use into the equilibrium constant expression.
• 2.5: Heterogenous Equilibria - Reactions Involving Solids and Liquids
When the products and reactants of an equilibrium reaction form a single phase, whether gas or liquid, the system is a homogeneous equilibrium. In such situations, the concentrations of the reactants and products can vary over a wide range. In contrast, a system whose reactants, products, or both are in more than one phase is a heterogeneous equilibrium, such as the reaction of a gas with a solid or liquid.
• 2.6: The Reaction Quotient- Predicting the Direction of Change
The reaction Quotient has the same form as the equilibrium constant expression, but it is derived from concentrations obtained at any time. When a reaction system is at equilibrium, Q=K . Graphs derived by plotting a few equilibrium concentrations for a system at a given temperature and pressure can be used to predict the direction in which a reaction will proceed. Points that do not lie on the line or curve are nonequilibrium states.
• 2.7: Le Châtelier’s Principle- How a System at Equilibrium Responds to Disturbances
Systems at equilibrium can be disturbed by changes to temperature, concentration, and, in some cases, volume and pressure; volume and pressure changes will disturb equilibrium if the number of moles of gas is different on the reactant and product sides of the reaction. The system's response to these disturbances is described by Le Châtelier's principle: The system will respond in a way that counteracts the disturbance. Adding a catalyst affects the reaction rates but does not alter equilibrium.