Chemical Equilibrium

1
Chemical Equilibrium

• Chapter 15 in Brown LeMay

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Definition

• Occurs when opposing reactions are occurring at
  the same time
• Weve had physical equilibria before
• dissolving
• vapor pressure
• We will use partial pressure for gases
• We will use molarity for solutes

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Why partial pressure?

• A(g) ? B(g)
• A(g) ? B(g) rate kfA
• A(g) ? B(g) rate krB
• For ideal gas equation, what is the concentration
  of the gas?
• n/v P/RT

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At Equilibrium

• Rate of forward rate of reverse
• kfPA/RT krPB/RT
• R and T are constant
• Kf/kr PB/PA
• This is called the equilibrium constant

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The constant

• Use this as an example
• N2(g) 3H2(g) ? 2NH3(g)
• This is called the Haber Process. Why?
• Equilibrium conditions can be reached from either
  direction
• Equilibrium expression uses coefficients as
  exponents, products over reactants
• Do it for above reaction

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Good Website

• http//jersey.uoregon.edu/vlab/Thermodynamics/

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Constant cont

• It is the value specific for that reaction under
  those conditions, no matter what you start with
• animation
• Keq greater than 1 products favored
• Keq less than 1 reactants favored
• Temperature dependent defined for one
  temperature
• Keq for a reaction is 1/Keq for the reverse
• If reaction is multiplied by 2, Keq is squared,
  etc.
• Dimensionless
• Try some problems

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Heterogeneous Equilibria

• Not just gases or solutions
• Solids, liquids included in reaction
• CaCO3(s) ? CaO (s) CO2(g)
• Pure solids, liquids, and solvents are not
  included in the equilibrium expression because
  their concentration is usually in excess and does
  not change significantly in the reaction.

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Calculating Keq

• Easy when all concentrations are known just
  substitute
• Try PE

10
RICE Problems

• RICE is a table approach to solving equilibria
  problems
• Reaction, Initial conc,Change in conc,
  Equilibrium conc
• Used when there are unknowns in a problem
• Try one together bottom of page 589
• PE

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Q Problems

• Used when concentrations of reactants and
  products are given (or calculated by RICE) and
  Keq is given
• Question asked is Has equilibrium been reached?
• Called Q or reaction quotient
• Calculated using Keq expression
• If Q lt Keq, reaction must go to right
• If Q gt Keq, reaction must go to left

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X problems

• Used when amounts of reactants or products at
  equilibrium are unknown
• You may or may not need to use RICE
• Use x for unknown amounts
• We will not have to use the quadratic equation
• Try PE