Chapter 19: Reaction Rates and Equilibrium

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Chapter 19 Reaction Rates and Equilibrium
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Rate

• The speed of change within an interval of time
• In chemistry, rates of change are usually
  expressed as the amount of reactant changing per
  unit time

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• Visible changes caused by chemical reactions are
  related to changes in the properties of
  individual atoms, ions, or molecules

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Collision Theory

• Atoms, ions, and molecules can react to form
  products when they collide, providing that the
  molecules have enough kinetic energy
• Particles without enough kinetic energy to react
  bounce apart
• http//phet.colorado.edu/simulations/sims.php?sim
  Reactions_and_Rates

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Activation Energy

• The minimum amount of energy that particles must
  have in order to react

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Activated Complex

• Is the arrangement of atoms at the peak of
  activation-energy barrier
• Very unstable- can either reform reactants or
  form products
• Also called transition state

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Activated Complex
Energy of products
Energy of reactants
Activation energy
Net Energy Change
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Factors Affecting Reaction Rates

• Temperature
• Concentration
• Particle Size
• Catalysts
• Inhibitors

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Temperature

• Usually, raising the temperature speeds up
  reactions, while cooling it slows down reactions

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Concentration

• More molecules more collisions increase in
  rate of reaction

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Particle Size

• The smaller the particle size greater surface
  area faster rate of reaction

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Catalysts

• A substance that increases the rate of reaction
  without being used up
• Lower activation energy
• Are neither reactants nor products
• Do NOT affect the amounts of reactants and
  products present at equilibrium
• Ie. Enzymes

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Inhibitors

• A substance that interferes with the action of a
  catalyst

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Reversible Reactions

• The reactions occur simultaneously in both
  directions
• 2SO2 (g) O2 (g) 2SO3 (g)
• http//phet.colorado.edu/simulations/sims.php?sim
  Reversible_Reactions

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Chemical Equilibrium and Equilibrium Position

• A state in which the forward and reverse
  reactions take place at the same rate
• No NET change
• Rates of reaction are equal NOT necessarily
  concentrations
• A B A B

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99
99
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A Favored
B Favored
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Le Châteliers Principle

• A system whose equilibrium has been disturbed,
  makes changes to restore that equilibrium
• This establishes a NEW equilibrium with a NEW
  position of equilibrium

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Le Châteliers Principle

• If a stress is applied to a system in dynamic
  equilibrium, the system changes to relieve that
  stress
• Stresses include
• a. changes in concentration of reactants or
  products
• b. changes in temperature
• c. changes in pressure

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Concentration

• H2CO3 (aq) CO2 (aq) H2O (l)

Add CO2
Remove CO2
Removal of products is often used to increase the
yield of a desired product
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Temperature

• 2SO2 (g) O2 (g) 2SO3 (g) Heat

Add Heat
Cool
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Pressure

• N2 (g) 3 H2(g) 2NH3 (g)

Decrease Pressure
Increase Pressure
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Example

• N2 (g) 3 H2 (g) 2NH3 (g) 92 kJ
• Addition of heat
• Increase in pressure
• Addition of catalyst
• Removal of heat

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Le Châteliers Principle

• http//www.mhhe.com/physsci/chemistry/essentialche
  mistry/flash/lechv17.swf

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Equilibrium Constants (Keq)

• The ratio of product concentrations to reactant
  concentrations
• aA bB cC dD
• Keq Cc x Dd
• Aa x Bb

Lower case letters represent moles at equilibrium
capital letters concentrations in mol/L
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Equilibrium Constants (Keq)

• Temperature dependent
• Show whether products or reactants are favored
• Keq gt 1 products favored at equilibrium
• Keq lt 1 reactants favored at equilibrium

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Example

• Analysis of an equilibrium mixture in a 1 L flask
  at 300oC gives the following results
• Hydrogen 0.15 mol
• Nitrogen 0.25 mol
• Ammonia 0.10 mol
• Calculate Keq and determine which is favored
• N2 (g) 3 H2(g) 2NH3 (g)

Keq Cc x Dd Aa x Bb
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Free Energy

• Is the energy available to do work
• Is not always used efficiently
• NO process can be made 100 efficient
• Energy can only be obtained if the reaction
  actually occurs

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Spontaneous Reactions

• Are reactions that occur naturally and that favor
  the formation of the products at the specified
  conditions
• ALL spontaneous reactions release free energy

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Nonspontaneous Reactions

• Reactions that do NOT favor the formation of
  products at the specified conditions
• Do not give substantial amounts of products at
  equilibrium
• H2CO3 (aq) CO2 (g) H2O (l)
• lt1 gt99

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• Term spontaneous and nonspontaneous do NOT refer
  to how fast the reactants form products

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• Some reactions that are nonspontaneous at one set
  of conditions MAY be spontaneous at other
  conditions
• Changing Temperature
• Changing Pressure
• Coupled Reactions

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Coupled Reactions

• Sometimes a nonspontaneous reaction can be made
  to occur by coupling it with a spontaneous
  reaction that releases free energy

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Enthalpy

• Some processes may be spontaneous even if the
  absorb heat (endothermic)

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Entropy

• The disorder of a system

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Law of disorder

• Processes move in the direction of maximum
  possible disorder or randomness
• More disorder lower energy

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Gas
Liquid
Solid
Increasing Entropy
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Entropy increases when a substance is divided
into parts
Increasing Entropy
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Entropy tends to increase in chemical reactions
where the total number of molecules increases
Increasing Entropy
2 H2O2
2 H2O O2
2 Molecules
3 Molecules
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Entropy tends to increase when temperature
increases
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Heat, Entropy, and Free Energy

• The size and direction of heat (enthalpy) and
  entropy changes together determine whether a
  reaction is spontaneous

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How Changes in Heat Affect Reaction Spontaneity How Changes in Heat Affect Reaction Spontaneity How Changes in Heat Affect Reaction Spontaneity

Heat Change Entropy Spontaneous Reaction
Decreases Increases (more disorder Yes
(Exothermic) in products than reactants)  
     
Increases Increases Only if unfavorable heat change
(Endothermic)   is off set by favorable entropy change
     
Decreases Decreases (less disorder Only in unfavorable entropy change
(Exothermic) in products than reactants is offset by favorable heat change
     
Increases Decreases No
(Endothermic)    
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Entropy Calculations (S)

• Entropy is a QUANTITATIVE measure of the disorder
  of a system
• Units J/Kmol

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Standard Entropy (S0)

• 25oC 298 K
• Pressure (for gasses) 101.3 kPa

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Standard Entropy Changes

• DS0 S0 (products) S0 (reactants)

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Example

• 2NO O2 ? 2NO2
• What is the standard state of entropy for this
  reaction at STP
• S0 for NO 210.6 J/Kmol
• S0 for O2 205.0 J/Kmol
• S0 for NO2 240.5 J/Kmol
• DS0 S0 (products) S0 (reactants)

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Gibbs Free Energy Change DG

• The maximum amount of energy that can be coupled
  to another process to do useful work
• DG DH TDS
• DH change in Enthalpy
• DS change in Entropy
• T temperature in Kelvins

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Gibbs Free Energy Change DG

• DG is negative in spontaneous processes (energy
  is lost)
• DG is positive in nonspontaneous processes
  (energy is gained)
• DG 0 for elemental substances
• A reaction that is nonspontaneous under one set
  of conditions may be spontaneous under another
  set of conditions

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Gibbs Free Energy Change DG

• DG0 DH0 TDS0
• DG0 DG0 (products) DG0 (reactants)

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Example

• N2 (g) 3H2 (g) ? 2 NH3 (g)
• DG 16.64 kJ/mol
• S0 N2 191.5 kJ/mol
• S0 H2 130.6 kJ/mol
• S0 NH3 192.5 kJ/mol
• Calculate DH0
• DG0 DH0 TDS0
• DS0 S0 (products) S0 (reactants)

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Rate Laws

• The rate of a reaction depends in part on the
  concentrations of the reactants

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Rate Laws

• Expresses the rate of reaction to the
  concentration of the reactants

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Rate Laws

• A ? B
• Rate DA k x A
• Dt
• Specific rate constant (k) for a reaction
• A molar concentration
• A? B quickly, k value will be large
• A ? B slowly, k value will be small

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First Order Reaction

• The reaction rate is directly proportional to the
  concentration of only one reactant
• A? B

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First Order Reaction
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Double-Displacement Reactions

• aA bB ? cC dD
• Rate k AaBb
• If BOTH a and b 1 than the reaction is 1st
  order A and1st order B 2nd order overall
• The overall order is the sum of the exponents for
  an individual reaction (for simple one step
  reactions)

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Example