Chemistry of Solutions

• Size of K is determined by thermodynamic factors
• Reaction does not occur to any significant extent when the value of K is very small
• Equilibrium lies to the right when the value of K is much larger than 1
• Equilibrium Constant helps in determining whether a given set of concentrations represent an equilibrium condition
• Reaction Quotient, Q


  1. Used to determine the direction of movement toward equilibrium when all of the initial concentrations are nonzero

  2. Obtained by applying the law of mass action

  3. Use initial concentrations instead of equilibrium concentrations
• Tendency for a reaction to occur is given by the magnitude of K
• Size of K and Time Required to Reach Equilibrium are not directly related
• Reaction goes essentially to completion when the value of K is much larger than 1
• When the value of K is very small, the system at equilibrium will consist mostly of reactants
• Equilibrium position that will be achieved from a given set of initial concentrations can be calculated if equilibrium constant and initial concentrations are known
• When the value of K is much larger than 1, at equilibrium, the reaction system will consist mostly of products
• Equilibrium position lies far to the left when the value of K is very small
• Time required to reach equilibrium depends on the rate of the reaction
• Equilibrium Constant helps in predicting features of reactions, such as determining the tendency of a reaction to occur
• If pure solids or pure liquids are involved in a chemical reaction, their concentrations are not considered
• Summary of results for Heterogeneous Equilibria

  • If pure solids or pure liquids are involved in a chemical reaction, their concentrations are not included in the equilibrium expression for the reaction
  • Does not apply to solutions or gases
• Equilibrium Position versus Equilibrium Constant

  Equilibrium position refers to each set of equilibrium concentrations. There can be an infinite number of positions for a reaction. It depends on initial concentrations. Equilibrium constant is one constant for a particular system at a particular temperature. It remains unchanged and depends on the ratio of concentrations
• Types of Equilibria

  • Homogeneous equilibria: Involve reactants and products that are in one phase
  • Heterogeneous equilibria: Involve reactants and products that exist in more than one phase
• Equilibrium Expressions involving Pressures

  Consider the ideal gas equation PV = nRT (or) P= nRT/V. C represents the molar concentration of a gas where C = n/V or C equals the number of moles n of gas per unit volume V
• Water is a liquid, as a result it has a constant concentration; which explains why it is omitted from the equilibrium constant expression
• Square brackets indicate the concentrations of the chemical species at equilibrium
• A, B, C, and D are chemical species, and j, k, l, and m are the respective coefficients
• If reactants are favored, the equilibrium position of the reaction lies far to the left
• Concentrations of the reactants and products in a given chemical equation remain unchanged because the system is at chemical equilibrium and the forward and reverse reactions are too slow
• Chemical Equilibrium is a state where the concentrations of all reactants and products remain constant with time and is attained by reactions that take place in a closed environment; it may favor either products or reactants
• K is the equilibrium constant
• Visible changes cannot be detected in reactions that have achieved chemical equilibrium
• The law of mass action is represented by the equilibrium expression K = [C]^l[D]^m / [A]^j[B]^k
• Factors determining equilibrium position

  • Initial concentrations
  • Relative energies of reactants and products
  • Relative degree of organization of reactants and products
• Consider the following reaction: jA + kB ⇌ lC + mD
• Frantic activity takes place on a molecular level; equilibrium is not static but is a highly dynamic situation