chemical equilibrium the study of reactions that occur in both directions
TRANSCRIPT
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Chemical Equilibrium
The study of reactions that occur in both directions.
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So far with reactions….
Looked at reactions that go to completion
Used stoichiometry for calculation of many quantities
Looked at spontaneity and rates of reactions
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Now…
Reactions can be reversible– They reach a state of equilibrium
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Examples
Vapor pressure– Evaporation begins– Over time, the
system undergoes evaporation and condensation at the same rate
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Examples
Dissolving and Crystallization– A system could
have an equal amount of precipitate (changes states at an equal rate)
System of iron (II) dichromateat equilibrium
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Examples
NO2 (g) + NO2 (g) N2O4 (g)
– NO2 = dark brown, N2O4 = colorless
– Ultimately ends up somewhere in between
Equlibrium Change in Action
NO2
N2O4
Equilibrium!!
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Equilibrium Defined
Concentration of products and reactants remain constant over time– The reaction is reversible (can go both
directions)– The rate of forward reaction equals the
rate of the reverse reaction– Dynamic!! (looks the same when taking a
snapshot, but constantly moving back and forth)
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Demo, then Graph of Equilibrium
Time
[R]
[P]
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Explaining the Graph
Time
[R]
[P]
What’s happening to the products?
What’s happening to the reactants?
Which one is favored?
Different reactions have different equilibria…
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On a molecular level…
Where does the following reach equilibrium?
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Equilibrium Expressions
Idea by Guldberg and Waage (1864)– Called the Law of Mass Action
– Given a reaction:
aA + bB cC + dD
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Equilibrium Expressions
aA + bB cC + dD
=[C]c [D]d
[A]a [B]b
Equilibriumconstant
MUST useconcentrations ofproducts over reactants
coefficients of balancedequation becomeexponents
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Equilibrium Expressions
Works only for GASES and IONS– No pure solids or liquids included
Example– C3H8 (g) + O2 (g) CO2 (g) + H2O (g)
=[C]c [D]d
[A]a [B]b
=[CO2]3 [H2O]4
[C3H8] [O2]5
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Equilibrium Expressions
What if it has solids or liquids?– Called heterogeneous equilibrium– The concentration of solids and liquids is
assumed to always remain constant, so they are not included…
– Example: Ca (s) + O2 (g) CaO (s)
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Equilibrium Expressions
More examples
– H2 (g) + I2 (s) 2 HI (g)
– CuSO45 H2O (s) CuSO4 (s) + 5 H2O (g)
– N2 (g) + 3 H2 (g) 2 NH3 (g)
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Values of K Equilibrium constant, K, is found by:
[products]
[reactants]
– If K = 1…..• equal ratio of products and reactants
– If K > 1 ….• reaction favors products
– If K < 1 ….• reaction favors reactants
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What changes K?
1. Change the temperature.– Equilibrium is temperature dependent.
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What changes K?
2. Change the reaction– Look at the original reaction and see the change
that was made to it.• Whatever you do to the reaction, you do to the
power of K
– Example• 2 NO(g) + O2 (g) 2 NO2 (g) K= 4.67 x 1013
• Change the coefficients• NO(g) + 1/2 O2 (g) NO2 (g)
• What is the change?• K’ = K1/2 = (4.67 x 1013)1/2 = 6.83 x 106
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What changes K?
2. Change the reaction– Look at the original reaction and see the change
that was made to it.• Whatever you do to the reaction, you do to the
power of K
– Example• 2 NO(g) + O2 (g) 2 NO2 (g) K= 4.67 x 1013
• Reverse the reaction• 2 NO2 (g) 2 NO (g) + O2 (g)
• What is the change?• K’ = 1/K = 1/(4.67 x 1013) = 2.14 x 10-14
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Equilibrium Constant
Can also find from reaction mechanism– OVERALL:
• N2O(g) + 3/2 O2 (g) 2 NO2 (g) Kc = ??
– Steps:• N2O(g) + 1/2 O2 (g) 2 NO (g) Kc1 = 1.7 x 10-13
• 2 NO(g) + O2 (g) 2 NO2 (g) Kc2 = 4.67 x 1013
– To find Kc?
• Kc = Kc1 x Kc2
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Equilibrium including Gases
Look at partial pressures instead of molarities– aA (g) + bB (g) cC (g) + dD (g)
=(PC)c (PD)d
(PA)a (PB)b
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Equilibrium including Gases
If it says Kp, you must use pressures
– Given moles and L, must use temperature and PV = nRT to get pressures of each species
K or Kc is still molarity
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Practice Problems
Write equilibrium expressions for:– a) 2 O3(g) 3 O2(g)
– b) 2 NO(g) + Cl2(g) 2 NOCl(g)
– c) BaSO4(s) Ba+2(aq) + SO4-2(aq)
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Practice Problems
Changing K by changing the equation– Original equation:
2 NO (g) + O2 (g) 2 NO2 (g) Keq = 0.00103
• NO (g) + ½ O2 (g) NO2
• 2 NO2 (g) 2 NO (g) + O2 (g)