Chapter 19: Equilibrium

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Chapter 19 Equilibrium

• A. Reversible Reactions and Equilibrium
• Reversible reactions reactions that proceed in
  both directions (R ? P and R ? P)
  at the same time
• - when reading to the right, its
  called the forward rxn.
• - when reading to the left, its
  called the reverse rxn.
• Equilibrium when the forward and reverse
  reaction occur at the same rate and
  there is no net-change in the reaction
• - use a double-headed arrow (?)

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• B. Collision Theory and P.E. Diagrams
• Collision theory atoms must gain/absorb enough
  energy to collide with each
  other, thus forming a product in a
  chemical rxn.
• Activation energy the amount of energy needed
  for a reactant to gain/absorb in order to
  start a rxn.
• Activation complex the highest amount of
  energy in a rxn.
• - lasts for a very short time
• - also called the transition state
  because the reaction goes from R ? P

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• Potential energy diagrams shows the change in
  energy of the overall rxn.
• ex.

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• P.E. diagrams (exo vs. endo)
• ex.

Exothermic
Endothermic
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• P.E. diagrams (with and without a catalyst)
• ex.

• Catalysts speed up a reaction by lowering its
• activation energy (red line)
• ?H does not change

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• C. Entropy
• Entropy (?S) the change in disorder in a rxn.
• - the more K.E., the more disorder
• - inc entropy (?S) s lt l lt aq lt g
• - if states of matter are all the
  same, the side with more total moles have
  more entropy (inc)
• ex. C(s) O2(g) ? CO2(g)
• entropy inc. (?S) b/c S ? G
• N2(g) 3H2(g) ? 2NH3(g)
• entropy dec. (?S) b/c 4 moles ? 2 moles

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• D. Equilibrium Constant
• Equilibrium constant (Keq) ratio/fraction of
  the concentration of the
  products over the reactants
• - formula C c x D d
• A a x B b
• when aA bB ? cC dD
• lower case coefficients
• upper case substances
• - solids and liquids cannot have
  a concentration, and are left
  out of the Keq expression

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• ex. Write the Keq expression (set-up only - no
  s) of the following reaction
• 2 CaCO3(s) 2 SO2(g) O2(g) ? 2 CaSO4(s) 2
  CO2(g)
• CO2(g) 2
• Keq -------------------------
• SO2(g) 2 x O2(g) 1
• What is the Keq when
• CO2(g) 1.5 M SO2(g) 0.5 M O2(g)
  0.25 M
• 1.5 M 2
• Keq ----------------------------
• 0.5 M 2 x 0.25 M 1
• Keq 36 (no units conc. cancel out)

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• Gibbs free energy (?G) the free or available
  energy that is able to do work in a
  rxn.
• Spontaneous rxn. the products are favored
• - the rxn. shifts right
• - Keq gt 1
• - heat released (exo) ?G, ?H
• - more disorder ?S
• Non-Spontaneous rxn. the reactants are favored
• - the rxn. shifts left
• - Keq lt 1
• - heat absorbed (endo)
• ?G, ?H
• - less disorder ?S

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• E. Le Chateliers Principle
• Le Chateliers principle when a stress is
  applied to a system at equilibrium,
  the reaction will shift
  (left or right) to adjust to the
  added stress
• - all other reactants and
  products will also adjust
• Stress a factor that affects the equilibrium of
  a given reaction
• ex. concentration of a reactant/product
• temperature/heat (treat the same as conc.)
• pressure (gases only inc. in pressure
  shifts towards the side with less moles)

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• Adding a stress
• Rxn. shifts away (other side) from the stress
• All substances on the other side of the stress
  will now increase
• All substances on the same side of the stress
  will now decrease (not including the stress)
• Removing a stress
• Rxn. shifts towards (same side) as the stress
• All substances on the other side of the stress
  will now decrease
• All substances on the same side of the stress
  will now increase (not including the stress)