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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 processesphase changes are reversible processes• H2O(l) H2O(g)
• H2O(l) H2O(s)
same substance on both sides only its phase is different
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Examples - Phase Equilibrium
• water & water vapor in sealed bottle• perfume in partially full, sealed flask• ice cubes & water in insulated container• dry ice & CO2(g) in a closed aquarium
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Solution Equilibrium: Solids
• saturated solution = dynamic equilibrium
• dissolving & solidification occur at equal rates
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Solid in Liquid
• NaCl(s) NaCl(aq)
• Favored a little bit by higher temperature
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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 ReactionsReversible Reactions
• N2(g) + 3H2(g) 2NH3(g) • forwardforward:
–N2 & H2 consumed; NH3 produced
• 2NH3(g) N2(g) + 3H2(g) • reversereverse:
–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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Time
Conc
entra
tion
NH3
H2
N2
N2(g) + 3H2(g) 2NH3(g)Why is this point Why is this point significant?significant?
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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
• RateRateforwardforward rxnrxn = Rate = Ratereversereverse rxnrxn
• does this mean concentrations reactants/products are equal?
NO!NO!
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Chemical Equilibrium
• Rateforward rxn = Ratereverse rxn
• at equilibrium: concentrations all species are constantconstant– 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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Time
Conc
entra
tion
NH3
H2
N2
N2(g) + 3H2(g) 2NH3(g)OriginalEquilibrium Point
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Reversible Reactions
• once reach equilibrium, don’t produce any more product– bad news if product is what you’re selling
• can you change the equilibrium concentrations?
if so how can it be done? for example, how can you maximize product?
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What you would really like to see…
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lots of product created as fast as possible
New equilibrium point
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• equilibrium can be changed or affected by:– any factor 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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LeChatelier’s 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 only 2 possible actions2 possible actions• shift to shift to rightright & form more product
– forward reaction speeds upforward reaction speeds up more than reverse reaction
• shift to shift to leftleft & form more reactant– reverse reaction speeds upreverse reaction speeds up more than
forward reaction
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A + B C + D (at equilibrium)
• If ↑ concentration A, how will system react?
• How does new equilibrium mixture compare to original equilibrium mixture?
• Use logic:– If you ↑ [A]: the system wants to ↓ [A]
• must use A up, so forward reaction speeds up
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A + B C + D
DEC DEC ______INC left[B]
INC ______DEC DEC right[C]
______DEC INC INC left[D]
INC INC DEC ______right[A]
[D][C][B][A]Equil. Shift
Stress
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Changes in Temp• exothermic reaction: A + B C + D + heat
– If ↑ temperature, system shifts to consumeconsume heat so shifts to left
• endothermic reaction: A + B + heat C + D– If ↑ temperature, system shifts to consume heat so
shifts to right
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Changes in PressureN2(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