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Please Pick Up

• Dynamic Equilibria Problem Set

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Dynamic Equilibrium

• Edward A. Mottel
• Department of Chemistry
• Rose-Hulman Institute of Technology

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Chemical Reactions inDynamic Equilibrium

• Reading Assignment
• Chang Chapter 14.1-14.2
• Processes which do not proceed to completion, but
  which appear to stop on a macroscopic scale.
• The Law of Mass Action and the equilibrium
  constant are introduced as means to interpret
  this process.

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Dynamic versus Static Equilibrium
There are always two children in front of the
chairs.
The forces on both sides of the teeter-totter are
the same.
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Dynamic Equilibrium

• There is both a forward and reverse process.
• The process can be described with an equilibrium
  arrow (actually two arrows).

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Dynamic Equilibrium
H2O

• Forward rate equals reverse rate.

H2O

• Not all reactions are 100 complete.
• Note it does not mean that
• reactants and products are equally stable.
• reactants and products have the same or equal
  concentrations.

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Law of Mass-ActionMass-Action Expression
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Mass-Action Expression
Hydrogen gas reacts with oxygen gas to give steam.

• Balance the equation
• Write the mass-action expression
• products over reactants

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Mass-Action Expression
Hydrogen gas reacts with oxygen gas to give steam.
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Mass-Action Expression
Carbon dioxide gas dissolves in water to
give aqueous carbon dioxide.
Write the mass-action expression
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Mass-Action Expression
Carbon dioxide gas dissolves in water to
give aqueous carbon dioxide.
What would be the units of this mass-action
expression?
square brackets are used to indicate molar
concentrations
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The Reaction Quotient

• The pressure of CO2 gas is 2.0 atm
• The concentration of CO2 is 0.0030 M

What is the numeric value of the reaction
quotient?
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The Reaction Quotient

• The pressure of CO2 gas is 2.0 atm
• The concentration of CO2 is 0.0030 M

What is the numeric value of the reaction
quotient?
1.5 x 10-3 Matm-1
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Determining If a System is at Equilibrium

• The equilibrium constant for CO2 gas dissolved in
  water is 3.4 x 10-2 Matm-1

Is the reaction at equilibrium?
What must occur to attain equilibrium?
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Equilibrium Constant

• K Q when the system is at equilibrium

• Special equilibrium constants
• K or Keq no special conditions
• Kp partial pressures
• Kc molarity concentrations

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Mass-Action Expression
Write the mass-action expression for the reaction
of aqueous acetic acid with water to give
hydronium ion and acetate ion.
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Mass-Action Expression Simplifications

• If the concentration of a reactant or product
  does not or cannot vary, the mass-action
  expression can usually be simplified.
• The term which doesnt vary is constant and
  becomes part of the K.
• A special subscript for K is added

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Mass-Action Expression Simplifications
In this equation which terms can vary? (i.e.,
which can have different concentrations)
What is the molarity of water in pure water?
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Mass-Action Expression Simplifications
Ka is the acid dissociation constant.
CH3COOH(aq) H2O(l)
H2O K
Ka
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Mass-Action Expression Simplifications

• solvent
• insoluble or sparingly soluble solid
• insoluble or sparingly soluble liquid

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Write the Simplified Mass-Action Expression for

• Aqueous iron(III) ion reacts with water to give
  insoluble iron(III) hydroxide and aqueous
  hydrogen ions.
• Write a balanced and annotated equation
• Write the mass-action expression
• Simplify the mass-action expression

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Write the Simplified Mass-Action Expression for

• Aqueous iron(III) ion reacts with water to give
  insoluble iron(III) hydroxide and aqueous
  hydrogen ions.

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Write the Simplified Mass-Action Expression for

• Simplify the expression
• Which terms can vary?

Khyd
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Simplified Mass-Action Expression
Why isnt water in the mass-action expression?
Why isnt Fe(OH)3 in the MAE
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Factors Which May Result in a System Failing to
Attain Equilibrium

• Reaction coordinate diagram
• Thermodynamic stability
• Kinetic stability
• activation energy
• catalyst

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Factors Which May Result in a System Failing to
Attain Equilibrium
Reaction Coordinate Diagram
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Factors Which May Result in a System Failing to
Attain Equilibrium
Reaction Coordinate Diagram
The activation barrier may be overcome by heating
the reactants
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Factors Which May Result in a System Failing to
Attain Equilibrium
Reaction Coordinate Diagram
Large activation barrier
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