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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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