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Dynamic Equilibria Problem Set
Dynamic Equilibrium
Edward A. Mottel
Department of Chemistry
Rose-Hulman Institute of Technology
04/18/23
Chemical Reactions inDynamic Equilibrium
• Reading Assignment: · Zumdahl Chapter 6.1-6.2
• 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 alwaystwo children in front
of the chairs.
The forces on both sidesof 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).
favorableunfavorable equallyfavorable
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Dynamic Equilibrium Forward rate equals reverse rate.
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.
H2O (l) H2O (g)H2O
H2O
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Law of Mass-ActionMass-Action Expression
Concentration or Pressure of Each Product
Concentration or Pressure of Each ReactantQ =
Reaction Quotientuses the currentconcentrationsor pressures
If the reactants andproducts are in dynamic
equilibrium, the ratio equalsthe Equilibrium Constant
= K
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Mass-Action Expression
• Balance the equation• Write the mass-action expression
· products over reactants
Hydrogen gas reacts with oxygen gas to give steam.
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Mass-Action Expression
Write the mass-action expression
Carbon dioxide gas dissolves in water to giveaqueous carbon dioxide.
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Mass-Action Expression
CO2(g) CO2(aq)
Q[ ]CO2
( )PCO2
=
square bracketsare used to indicate
molar concentrations
What would bethe units of thismass-action expression?
Carbon dioxide gas dissolves in water to giveaqueous carbon dioxide.
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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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Determining If a System is at Equilibrium
The equilibrium constant for CO2 gas dissolved in water is 3.4 x 10-2 M·atm-1
Q[ ]CO2
( )PCO2
= =0.0030 M
2.0 atm= 1.5 x 10-3 M·atm-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
CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO¯(aq)
K = [CH3COOH] [H2O]
[H3O+] [CH3COO¯]
Write the mass-action expression for the reaction ofaqueous 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 doesn’t vary is “constant” and becomes part of the K.
A special subscript for K is added
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CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO¯(aq)CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO¯(aq)
K = [CH3COOH] [H2O]
[H3O+] [CH3COO¯]
[CH3COOH] [H2O]
[H3O+] [CH3COO¯]
Mass-Action Expression Simplifications
What is the molarity of water in pure water?
In this equation which terms can vary?(i.e., which can have different concentrations)
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[CH3COOH]
[H3O+] [CH3COO¯]
Mass-Action Expression Simplifications
CH3COOH(aq) + H2O(l)
[H2O] · K =
H3O+(l) + CH3COO¯(aq)CH3COOH(aq) + H2O(l)
= Ka
Ka is the acid dissociation constant.
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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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Simplified Mass-Action Expression
Fe(OH)3(s) + 3 H+(aq)Fe3+(aq) + 3 H2O(l)
[Fe3+]
[H+]3
= Keq use Keq if youaren’t sure what
to call the constant
Why isn’t water in the mass-action expression?
Why isn’t 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
Ene
rgy
Reactants Products
Reaction CoordinateDiagram
ThermodynamicallyUnstable
2 H2(g) + O2(g)2 H2O(g)
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Ene
rgy
Reactants Products
Reaction CoordinateDiagram
Factors Which May Result in a System Failing to Attain Equilibrium
ThermodynamicallyStable
Large activation barrier
The activation barrier may be overcomeby heating the reactants
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Ene
rgy
Reactants Products
Reaction CoordinateDiagram
Large activation barrier
Factors Which May Result in a System Failing to Attain Equilibrium
The activation barriermay be lowered by
the addition of a catalyst.
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