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 − T∆S
  • 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 equilibrium: At 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