Reaction Rates and Equilibrium

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Chapter 19

• Reaction Rates and Equilibrium

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Reaction Rates and Equilibrium

• Activation energy the minimum amount of energy
  that particles must have in order to react
• An activated complex is the arrangement of atoms
  at the peak of the activation-energy barrier.
  Activation complex is sometimes called the
  transition state because a very unstable
  activated complex is likely to re-form reactants
  as it is to form products.

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Factors that affect reaction rates

• Temperature raising the temperature speeds up
  reactions, while lowering the temperature slows
  down reactions.
• Concentration increasing the concentration of
  reactants in a fixed volume increases the
  collision frequency, therefore the reaction rate.
  
• Particle Size An increase in surface area
  increases the amount of the reactant exposed for
  reaction, which further increases the collision
  frequency and the reaction rate.

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Catalyst

• Catalyst is a substance that increases the rate
  of a reaction without being used up itself during
  the reaction. Catalyst lowers the activation
  energy barrier therefore more reactants can form
  products within a given time. Catalysts are not
  consumed during the reaction.

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Chemical Equilibrium

• Chemical equilibrium is a state at which the
  forward and reverse reactions take place at the
  same rate.
• Rate at which the products are formed from the
  reactants equals the rate at which the reactants
  are formed from the products

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Equilibrium Constant

• (Keq) or Equilibrium constant is the ratio of
  product concentrations to the reactant
  concentrations at equilibrium, with each
  concentration raised to a power equal to the
  number of moles of that substance in the balanced
  chemical equation.
• aA bB ?? cC dD
• Keq (C c D d) / (A a B b)
• Do 2H2O ?? 2H2 O2
• Calculate the Keq for 2Mg O2 ?? 2MgO if the
  concentrations are 0.01M for Mg, 0.01 for O2, and
  2M for MgO

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Le Chateliers Principle

• Le Chateliers Principle states that if a stress
  is applied to a system in dynamic equilibrium,
  the system changes to relieve the stress.
• Stresses are Concentration, Temperature, and
  Pressure

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Concentration

• Concentration changing concentration, of any
  reactant or product in a system at equilibrium
  disturbs the equilibrium.
• Adding a product always pushes a reversible
  reaction in the direction of reactants
• Removing a product always pulls a reversible
  reaction in the direction of products
• When a reactant is added to a system at
  equilibrium, the reaction shifts in the direction
  of the formation of products.
• When a reactant is removed, the reaction shifts
  in the direction of formation of reactants.
• N2 3H2 ?? 2NH3

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Temperature

• Temperature increasing the temperature causes
  the equilibrium position of a reaction to shift
  in the direction that absorbs heat
• 2SO2 O2 ?? 2SO3 heat
• Adding more heat pushes the equilibrium position
  to the left, product yield decreases
• Removing heat pulls the equilibrium to the right,
  product yield increases

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Volume and Pressure Changes

• Increasing pressure of a gaseous equilibrium
  mixture causes the system to shift in the
  direction that reduces the number of moles of gas
• Decreasing pressure causes a shift in the
  direction that produces more gas molecules
• N2 3H2 ?? 2NH3

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Entropy and Enthalpy

• Entropy a measure of the disorder of the system
• Increasing entropy
• solid ? liquid ? gas
• Increase temperature
• Entropy is decreasing in which of the following
  air escaping from a tire, a liquid cooling, snow
  melting, salt dissolving in water
• Enthalpy the heat content of a system at
  constant pressure

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Spontaneous vs. nonspontaneous

• Spontaneous reactions reactions that occur
  naturally and that favor the formation of
  products at the specified conditions
• Nonspontaneous reactions reactions that do not
  favor the formation of products at the specified
  conditions
• Two factors that determine if a reaction is
  spontaneous or nonspontaneous are enthalpy and
  entropy

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Spontaneous reaction?

• Exothermic (heat-releasing) increase in entropy
  spontaneous reaction
• Endothermic (heat-absorbing) decrease in
  entropy nonspontaneous
• Endothermic increase in entropy spontaneous
  only if unfavorable heat change is offset by
  favorable entropy change
• Exothermic decrease in entropy spontaneous
  only if unfavorable entropy change is offset by
  favorable heat change

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Standard Change in Entropy

• The standard entropy is the entropy of a
  substance in its stable state at 25 degree
  Celsius and 1 atm.
• Standard entropies change S0 (products) S0
  (reactants)
• Find the standard change in entropy for 2NO (g)
  O2 (g) ? 2NO2
• NO (g) S0 210.6 J/Kmol
• O2 (g) S0 205.0 J/Kmol
• NO2 (g) S0 240.5 J/Kmol
• (-145.2)

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• THE END