Ch 13

• Chemical Equilibrium

  • State where the concentrations of all reactants and products remain constant with time
  • Attained by reactions that take place in a closed environment
  • May favor either products or reactants
  • If products are favored, the equilibrium position of the reaction lies far to the right 
• Factors Determining Equilibrium Position of a Reaction

  • Initial concentrations
  • Relative energies of reactants and products 
  • Relative degree of organization of reactants and products 
• Law of Mass Action

  • Consider the following reaction :
  • A, B, C, and D are chemical species, and j, k, l, and mare the respective coefficients
  • The law of mass action is represented by the following equilibrium expression
  • K is the equilibrium constant
• Equilibrium constant 

  •One constant for a particular system at a particular temperature 

  •Remains unchanged

  •Depends on the ratio of concentrations 

  vs

  Equilibrium position

  •Refers to each set of equilibrium concentrations

  •There can be infinite number of positions for a reaction

  •Depends on initial concentrations
• • Homogeneous equilibria: Involve reactants and products that are in one phase
  • Heterogeneous equilibria: Involve reactants and products that exist in more than one phase
• K > 1

  At equilibrium, the reaction system will consist of mostly products 

  • Equilibrium lies to the right
  • Reaction goes essentially to completion
• K < 1

  • The system at equilibrium will consist mostly of reactants 
  • Equilibrium position lies far to the left
  • Reaction does not occur to any significant extent
• Size of K and Time Required to Reach Equilibrium 

  • Not directly related
  • Time required depends on the rate of the reaction 
  • Determined by the size of the activation energy 
  • Size of Kis determined by thermodynamic factors
  • Example -Energy difference between products and reactants 
• Reaction Quotient, Q

  • Used to determine the direction of movement toward equilibrium when all of the initial concentrations are nonzero
  • Obtained by applying the law of mass action 
  • Use initial concentrations instead of equilibrium concentrations
• Q < K

  System has too much reactant

  • Shift reaction to the right
• Q = K

  System is at equilibrium
• Q > K

  System has too much product

  Shirt reaction to the left
• Le Châtelier’s Principle

  • If a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change.
  • Helps in the qualitative prediction of the effects of changes in concentration, pressure, and temperature on a system at equilibrium
• Addition of an inert gas DOES NOT affect the concentrations or partial pressures of the reactants or products.
• Volume is reduced = Total number of gaseous molecules is reduced,

  Equilibrium position shifts toward the side of the reaction that involves smaller number of gaseous molecules in the balanced equation 
• Increase of Temperature

  • Treat energy as a reactant (endothermic process) or as a product (exothermic process)
  • Predict the direction of shift in the same way as when an actual reactant or product is added or removed

  ex: Exothermic = eq. shifts to the reactants

  Endothermic = eq. shifts to the product