Equilibrium

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Equilibrium

• state of balance
• condition in which opposing forces exactly
  balance/equal each other
• need 2-way or reversible situation
• need a closed system

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Dynamic Equilibrium

• macroscopic level
• looks like nothing is happening
• microscopic level
• lots going on

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3 Kinds of Equilibria

• phase equilibrium physical
• solution equilibrium physical
• chemical equilibrium - chemical

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Phase Equilibrium

• phase changes are reversible processes
• H2O(l) ? H2O(g)
• begin end with same substance
• only phase is different

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Examples - Phase Equilibrium

• water water vapor in sealed
• container
• ice cubes water in
  insulated container
• perfume in partially full, sealed flask

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Solution Equilibrium Solids

• saturated solution dynamic equilibrium
• NaCl(s) ? NaCl(aq)
• dissolving solidification occur at equal rates

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Solution Equilibrium Gases
CO2 in water CO2(g) ? CO2(aq) favored by high
pressure low temperature
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Reversible Reactions

• N2(g) 3H2(g) ? 2NH3(g)
• forward rxn
• N2 H2 consumed NH3 produced
• 2NH3(g) ? N2(g) 3H2(g)
• reverse rxn
• NH3 consumed N2 H2 produced

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Reversible Reactions 1 Equation

• N2(g) 3H2(g) ? 2NH3(g)
• forward reaction reactants on L
• read left to right
• reverse reaction reactants on R
• read in reverse right to left
• reaction runs in both directions all the time

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Why is this point significant?
Concentration
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Reaction Rate

• depends on concentration of reactants
• as concentration reactants ?,
• rate forward reaction ?
• as concentration product ?,
• rate reverse reaction ?

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Chemical Equilibrium

• state in which forward reverse rxns balance
  each other
• Rateforward rxn Ratereverse rxn
• does this mean concentrations reactants/products
  are equal?

NO!
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Chemical Equilibrium

• Rateforward rxn Ratereverse rxn
• at equilibrium concentrations all species are
  constant
• stop changing
• rarely ever equal

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Reversible Reactions vs. Reactions that Go to
Completion

• If goal is to maximize product yield
• easier in reaction that goes to completion
• use up all reactants
• left with only product
• Reversible reactions are different
• look at ?conc/?time picture again

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Original Equilibrium Point
Concentration
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Reversible Reactions

• once reach equilibrium, dont produce any more
  product
• bad news if product is what youre selling

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• can you change the equilibrium concentrations?
• if so how can it be done?
• for example, how can you maximize product?
• What you would really like to see

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lots of product created as fast as possible
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• equilibrium can be changed or affected by
• anything that affects forward and reverse
  reactions differently

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What factors affect rate of rxn?

• concentration/pressure (gases only)
• temperature
• presence of catalyst

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Catalyst

• same effect on both forward reverse reactions
• equilibrium reached more quickly, but
  equilibrium point not shifted
• equilibrium concentrations are same with or
  without catalyst

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Concentration, Pressure, Temperature

• changes in concentration, pressure, temperature
  affect forward reverse reactions differently
• composition of equilibrium mixture will shift to
  accommodate these changes

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LeChateliers Principle

• If system at equilibrium is subjected to
  stress, the system will act to reduce stress
• stress change in concentration, pressure,
• or temperature
• system tries to undo stress

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System

• only 2 possible actions
• 1. shift to right form more product
• forward reaction speeds up more than reverse
  reaction
• 2. shift to left form more reactant
• reverse reaction speeds up more than forward
  reaction

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A B ? C D (at equilibrium)

• If ? concentration A, how will system react?
• wants to get rid of excess A
• Use logic
• If you ? A the system wants to ? A
• must use A up, so forward reaction speeds up
• How does new equilibrium mixture compare to
  original equilibrium mixture?
• concentrations will be different but still
  constant

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A B ? C D
increase ?
decrease ?
decrease ?
increase ?
increase ?
decrease ?
decrease ?
increase ?
decrease ?
decrease ?
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Changes in Temp

• exothermic reaction A B ? C D
  heat
• If ? temperature, system shifts to consume heat
• shifts to left
• endothermic reaction A B heat ? C
  D
• If ? temperature, system shifts to consume heat
• shifts to right

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Changes in Pressure

• N2(g) 3H2(g) ? 2NH3(g)
• If ? pressure
• system shifts to side with fewer moles of gas
• left side 4 moles of gas right side 2 moles
• ? pressure causes shift to right
• If ? pressure
• system shifts to side with more moles of gas
• ? pressure causes shift to left

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H2(g) I2(g) ? 2HI(g)

• this system has 2 moles gas on left 2 moles gas
  on right
• systems with equal moles gas on each side cannot
  respond to pressure changes so
• NO shift occurs