Equilibrium Expressions

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Equilibrium Expressions
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Chemical Equilibrium

• The state where the concentrations of all
  reactants and products remain constant with time.
• On the molecular level, there is frantic
  activity. Equilibrium is not static, but is a
  highly dynamic situation.

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Chemical Equilibrium(continued)

• Reaction rate dependent on concentration
• Reactant concentrations decrease initially
• Product concentrations increase

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H2O(g) CO(g) gt H2(g) CO2(g)
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The Changes with Time in the Rates of Forward and
Reverse Reactions
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Haber Process N2 3H2 gt 2NH3
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The Law of Mass Action

• For
• jA kB ? lC mD
• K is called the equilibrium constant
• The law of mass action is represented by the
  equilibrium expression

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Equilibrium Expression

• 4NH3(g) 7O2(g) ? 4NO2(g) 6H2O(g)

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Equilibrium Constant, K

• K defines the ratio of product to reactant
  concentrations at equilibrium
• At a given temperature, there is only one value
  for K
• There may be many equilibrium positions (a set of
  concentrations existing at equilibrium)

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Equilibrium Constant, K

• The equilibrium constant for a reaction is the
  reciprocal of that for the reaction written in
  reverse.
• When the equation for a reaction is multiplied by
  n,
• When adding equations, Koverall product of
  Ks of reactions being added.

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N2(g) 3H2(g) 2NH3(g) K3.5x108

• Calculate K for the following reactions
• 2NH3(g) ? N2(g) 3H2(g)
• 3N2(g) 9H2(g) ? 6NH3(g)
• 6NH3(g) ? 3N2(g) 9H2(g)
• N2(g) H2(g) ? NH3(g)

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K, Kc, Kp

• N2(g) 3H2(g) 2NH3(g)

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Kc vs. Kp

• Kp K(RT)?n
• ?n sum of coefficients of gaseous products
  minus sum of coefficients of gaseous reactants.
  (R 0.08206)
• N2(g) 3H2(g) 2NH3(g) K3.5x108
• Kp ?

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Heterogeneous Equilibria

• . . . are equilibria that involve more than one
  phase.
• The position of a heterogeneous equilibrium does
  not depend on the amounts of pure solids or
  liquids present.

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Write the equilibrium expressions

• CaCO3(s) ? CaO(s) CO2(g)
• H2CO3(aq) H2O(l) HCO3-(aq) H3O(aq)
• Ag2SO4(s) 2Ag(aq) SO42-(aq)