Chapter 7 - Reversible Reactions and Chemical Equilibrium

Chapter 7.1: Recognizing Equilibrium

  • Equilibrium: products and reactants are occurring simultaneously at the same rate. water in a closed jar has the number of evaporating molecules and number of condensing molecules equal
  • Chemical processes that reach equilibrium:
    • Homogeneous equilibrium: the equilibrium reactions in which the reactants and products are in the same phase
    • Heterogeneous equilibrium: the equilibrium of reactions in which the reactants and products are in different phases

Chapter 7.2: Thermodynamics and Equilibrium

  • Favorable change: change that has a natural tendency to happen under certain conditions
  • Enthalpy: Exothermic: when products have less enthalpy than reactants, it releases energy
  • Endothermic: when reaction absorbs energy 
  • Temperature: changes with the direction
  • Entropy S: a tendency towards randomness or disorder in a system
  • The second law of thermodynamics: total entropy of the universe is constantly increasing must add together changes in the entropy of the system, ∆Ssys, and changes in the entropy of the surroundings, ∆Ssur
  • Free energy: available energy; useful work obtained from reaction
    • Also called gibbs free energy G
  • G = ∆H − TS
    • Change in free energy
    • Change in enthalpy
    • Change in entropy kelvin temperature
  • G
    • Negative: forward reaction is favorable
    • Zero: reaction at equilibrium
    • Positive: reaction favorable in reverse direction only

Chapter 7.3 The equilibrium constant

  • Law of chemical equilibriumAt equilibrium, there is a constant ratio between the concentrations of the products and reactants in any change
  • equilibrium constant keq or kc: forward rate constant divided by the reverse rate constant
    • kf / kr = keq
    • kc: uses concentrated values when concentration is at equilibrium
  • ICE table: table is used to record the initial, change, and equilibrium values of the reacting species,
  • K > 1, products are favoured. The equilibrium lies far to the right. Reactions where K is greater than 1010 are usually regarded as going to completion
  • K ≈ 1, there are approximately equal concentrations of reactants and products at equilibrium
  • K < 1, reactants are favoured. The equilibrium lies far to the left. Reactions in which K is smaller than 10−10 are usually regarded as not taking place at all

7.4 Predicting the Direction of a Reaction

  • Reaction quotient, Qc: expression that is identical to equilibrium constant expression, but concentration not necessarily at equilibrium
  • Qc = ( [R]c[S]d ) / ( [P]a[Q]b )
    • If Qc equal to kc then it is Qc close to equilibrium
  • Le Châtelier’s principle: a dynamic equilibrium tends to respond so as to relieve the effect of any change in the conditions that affect the equilibrium
    • Common ion effect: common ion involves adding ion to a solution in which theion is already present, the equilibrium shifts away from the added ion
    • applies Le Châtelier’s principle to ions in an aqueous solution
    • Endothermic change (∆H > 0):
    • An increase in temperature shifts the equilibrium to the right, forming more products.
      • Kc increases.
    • A decrease in temperature shifts the equilibrium to the left, forming more reactants.
      • Kc decreases.
    • Exothermic change (∆H < 0):
    • An increase in temperature shifts the equilibrium to the left, forming more reactants.
      • Kc decreases.
    • A decrease in temperature shifts the equilibrium to the right, forming more products.
      • Kc increases.
    • Reducing the volume of an equilibrium mixture of gas (constant temperature), causes sift a shift in equilibrium in direction with few gas molecules
    • Catalyst does not affect the position of equilibrium, only affects the time

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