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							Section 17.1
Reaction Rates and Equilibrium
Objectives
1. To understand the collision model of chemical
    reactions
2. To understand activation energy
3. To understand how a catalyst speeds up a chemical
   reaction
4. To explore reactions with reactants or products in
   different phases
5. To learn how equilibrium is established
6. To learn about the characteristics of chemical
   equilibrium
Section 17.2
Characteristics of Equilibrium
Objectives
1. To understand the law of chemical equilibrium
2. To learn to calculate values for the equilibrium constant
3. To understand how the presence of solids or liquids
   affects the equilibrium expression
Section 17.2
Characteristics of Equilibrium
A. The Equilibrium Constant: An Introduction
• Law of chemical equilibrium
   – For a reaction of the type
     aA + bB  cC + dD
   – Equilibrium expression




• Each set of equilibrium concentrations is called an
  equilibrium position.
Section 17.2
Characteristics of Equilibrium

B. Heterogeneous Equilibria
• Heterogeneous equilibria – an equilibrium system where
  the products and reactants are not all in the same state
Section 17.1
Reaction Rates and Equilibrium
A. How Chemical Reactions Occur

• Collision model – molecules must collide in order for a
  reaction to occur
   – Rate depends on concentrations of reactants and
     temperature.
Section 17.1
Reaction Rates and Equilibrium

B. Conditions That Affect Reaction Rates

• Concentration – increases rate because more molecules
  lead to more collisions
• Temperature – increases rate
   – Why?
Section 17.1
Reaction Rates and Equilibrium

B. Conditions That Affect Reaction Rates
• Activation energy – minimum energy required for a
  reaction to occur
Section 17.1
Reaction Rates and Equilibrium

B. Conditions That Affect Reaction Rates

• Catalyst – a substance that speeds up a reaction without
  being consumed
   – Enzyme – catalyst in a biological system
Section 17.1
Reaction Rates and Equilibrium

C. Heterogeneous Reactions
• Homogeneous reaction – all reactants and products are in
  one phase
   – Gas
   – Solution
• Heterogeneous reaction – reactants in two phases
Section 17.1
Reaction Rates and Equilibrium

C. Heterogeneous Reactions
Section 17.1
Reaction Rates and Equilibrium

D. The Equilibrium Condition
• Equilibrium – the exact balancing of two processes, one
  of which is the opposite of the other
Section 17.1
Reaction Rates and Equilibrium

D. The Equilibrium Condition
• Chemical equilibrium – a dynamic state where the
  concentrations of all reactants and products remain
  constant
 Section 17.1
 Reaction Rates and Equilibrium

 E. Chemical Equilibrium: A Dynamic Condition




Equal numbers     The reaction       The reaction    Although time continues to
of moles of H2O   begins to occur,   continues as    pass, the numbers of
and CO are        and some           time passes     reactant and product
mixed in a        products (H2 and   and more        molecules are the same as
closed            CO2) are           reactants are   in (c). No further changes
container.        formed.            changed to      are seen as time continues
                                     products.       to pass. The system has
                                                     reached equilibrium.
Section 17.1
Reaction Rates and Equilibrium

E. Chemical Equilibrium: A Dynamic Condition
• Why does equilibrium occur?
Section 17.3
Application of Equilibria
Objectives
1. To learn to predict the changes that occur when a
   system at equilibrium is disturbed
2. To learn to calculate equilibrium concentrations
3. To learn to calculate the solubility product of a salt
4. To learn to calculate solubility from the solubility product
Section 17.3
Application of Equilibria
A. Le Chatelier’s Principle
• Le Chatelier’s Principle – when a change is imposed on
  a system at equilibrium the position of the equilibrium
  shifts in a direction that tends to reduce the effect of that
  change
Section 17.3
Application of Equilibria
A. Le Chatelier’s Principle
• Effect of a Change in Concentration
Section 17.3
Application of Equilibria
A. Le Chatelier’s Principle
• Effect of a Change in Concentration
   – When a reactant or product is added the system shifts
     away from that added component.
   – If a reactant or product is removed, the system shifts
     toward the removed component.
Section 17.3
Application of Equilibria
A. Le Chatelier’s Principle
• Effect of a Change in Volume




          The system is          The piston is pushed in,
          initially at           decreasing the volume and
          equilibrium.           increasing the pressure. The
                                 system shifts in the direction
                                 that consumes CO2 molecules,
                                 lowering the pressure again.
Section 17.3
Application of Equilibria
A. Le Chatelier’s Principle
• Effect of a Change in Volume
   – Decreasing the volume




   – The system shifts in the direction that gives the fewest
     number of gas molecules.
Section 17.3
Application of Equilibria
A. Le Chatelier’s Principle
• Effect of a Change in Volume
   – Increasing the volume
      – The system shifts in the direction that increases its
        pressure.
Section 17.3
Application of Equilibria
A. Le Chatelier’s Principle
• Effect of a Change in Temperature
   – The value of K changes with temperature. We can
     use this to predict the direction of this change.
   – Exothermic reaction – produces heat (heat is a
     product)
      • Adding energy shifts the equilibrium to the left (away
        from the heat term).
   – Endothermic reaction – absorbs energy (heat is a
     reactant)
      • Adding energy shifts the equilibrium to the right (away
        from the heat term).
Section 17.3
Application of Equilibria
B. Applications Involving the Equilibrium Constant
The Meaning of K
   – K > 1  the equilibrium position is far to the right
   – K < 1  the equilibrium position is far to the left
Section 17.3
Application of Equilibria
B. Applications Involving the Equilibrium Constant
• The value of K for a system can be calculated from a
  known set of equilibrium concentrations.
• Unknown equilibrium concentrations can be calculated if
  the value of K and the remaining equilibrium
  concentrations are known.
Section 17.3
Application of Equilibria
C. Solubility Equilibria
• The equilibrium conditions also applies to a saturated
  solution containing excess solid, MX(s).
   – Ksp = [M+][X] = solubility product constant
   – The value of the Ksp can be calculated from the
     measured solubility of MX(s).