Chemical Equilibria

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Chemical Equilibria

• Chapter 16

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Chemical reactions

• Can reverse (most of the time)
• Though might require a good deal of energy
• ? or double arrows reversible rxn
• It swings both ways
• Once fwd/rev rxns occur at equal rates
• equilibrium (no net change seen)

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Equilibrium constant

• aA bB ? cC dD
• Thus,

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Equilibrium constant in action

• Give the equilibrium constant for
• H2(g) I2(s) ?? 2HI(g)

• Initially, 0.0175 M of reactants
• Decrease to 0.0037 M reactants
• What is the concentration of HI formed?

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Solution
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ICE Table

• Initial Change Equilibrium Table
• Great way to explain and show concentration
  changes
• Lets make an ICE table for the previous reaction
  on the board

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Calculate Keq for the reaction
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Problems

• Express Keq for
• CH3OH(g) ? CO(g) 2H2(g)
• Express Keq for
• C3H8(g) 5O2(g) ? 3CO2(g) 4H2O(g)
• Solve Keq for the following
• A(g) ? 2B(g)
• Given Ai 1.00M, Bi 0.00M, and
• Aeq 0.75M

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Caveats to Keq solids

• Solids in reversible rxns are excluded from
  expression since concentration derived from
  constant densities
• S(s) O2(g) ? SO2(g)
• Keq ?

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Caveats to Keq aqueous solns

• Same rule for pure liquids
• Ex NH3(aq) H2O(l) ? NH4(aq) OH-(aq)
• Keq ?

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Caveats to Keq gases Kp

• For gases we can use partial pressures
• Why?
• Hint Relationship between pressure and
  concentration
• See ideal gas law
• Given
• aA bB ? cC dD
• Kp not necessarily equal to Keq

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The relationship between Keq and Kp
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Problem

• Given
• 2NO(g) O2(g) ? 2NO2(g)
• Kp 2.2 x 1012 _at_ 25C
• Find Keq

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Solution
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Determining equilibrium constant

• Given 2SO2(g) O2(g) ? 2SO3(g)
• And SO2i 1.00 M, O2i 1.00 M SO3f
  0.925 M
• What is Keq?

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Solution
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More problem solving

• Sulfuryl chloride (SO2Cl2) dissociates into
  sulfur dioxide and chlorine in the gas phase
• SO2Cl2(g) ? SO2(g) Cl2(g)
• In an experiment, 3.174 g of SO2Cl2 (MW
  134.96g/mol) is placed in a 1.000 L flask and is
  at 100.0C. At equilibrium, the total pressure in
  the flask is 1.30 atm.
• Calculate
• a) The partial pressures of each gas at
  equilibrium.
• b) The Kp at 100.0C for the reaction.

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Solution
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Yet another

• Isopropanol can dissociate into acetone and
  hydrogen
• (CH3)2CHOH(g) ? (CH3)2CO(g) H2(g)
• At 179C, the equilibrium constant is 0.444.
  Calculate the equilibrium partial pressures of
  all three gases if 10.00 g (MW 60.10g/mol) of
  isopropanol are initially placed in a 10.00 L
  vessel.

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Solution
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Another but with a twist

• At 25C, the equilibrium constant for the
  reaction below is 5.9 x 10-13.
• 2NO2(g) ? 2NO(g) O2(g)
• Suppose a container is filled with 0.89 atm of
  NO2
• Calculate the equilibrium partial pressures of
  each gas
• OK to approximate if x ? A0 lt 5
• 5 rule

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Solution
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More on Keq

• Is the rxn product or reactant favored?
• I.e., Will it form more product or reactant?
• If K ? 1, then prod concentration higher than
  reactant concentration ? prod favored
• Makes mathematical sense
• See right
• If K lt 1, then reactant concentration higher than
  prod concentration ? reactant favored
• If K 1, neither favored both equal
  concentration

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Reaction quotient, Q

• When rxn not at equilibrium, use Q
• Where
• aA bB ? cC dD
• Then ?
• Remember, Q used for system when system NOT in
  equilibrium

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Q its benefits

• Answers the question
• Is the system at equilibrium (i.e., does Q K)?
• If not, we can predict which way the reaction
  will continue to proceed
• If Q lt K, rxn still needs to go to prod side to
  achieve equilibrium (i.e., where Q K)
• In other words, insufficient product formed for
  equilibrium conditions
• If Q gt K, rxn has overshot K and needs to go to
  reactant side to achieve equilibrium (i.e., where
  Q K)
• In other words, an excess of product is formed
  for equilibrium conditions

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Work on this

• N2(g) 3H2(g) ? 2NH3(g)
• The equilibrium constant at 400C is Keq0.5.
• Suppose we make a mixture with the following
  concentrations
• NH3 5.0M, N2 3.5M, H2 1.9M
• In which direction will the reaction go?
• a) products b) reactants

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Manipulating Keq

• C(s) ½ O2(g) ? CO(g)
• Kp ?
• Changing it 2C(s) O2(g) ? 2CO(g)
• Kp ?
• How are Kp and Kp related mathematically?

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More on Keq

• Given 2NO2(g) ? N2O4(g)
• Keq ?
• What is Keq when rxn is reversed?
• Therefore, what can we say about Keq fwd and Keq
  rev?

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Even more on Keq

• Remember Hesss Law?
• 1) AB ? C
• 2) BC ? D
• What is the net rxn and its Knet using Hesss
  Law?
• Can one obtain the same values as above not using
  Hesss Law?
• What can we say about Knet using each rxns Keq?

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Disturbing chemical equilibria

• Le Châteliers Principle
• Change one component of the rxn the rxn will
  attempt to rectify it
• Think of it this way
• If something is changed, how can it be undone or
  controlled so that equilibrium is achieved once
  again?

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Temperature variation on equilibrium

• 2NO2(g) ? N2O4(g) heat
• Kp ?
• ?H -57.1 kJ
• _at_ 273 K, Kp 1300, and _at_ 298 K, Kp 170
• Hence, if one raises the temp to 298 K, which way
  will it swing?

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Pressure volume change on equilibrium

• If volume decreased (pressure increased) ? favors
  smaller of molecules
• If volume increased (pressure decreased) ? favors
  larger of molecules
• If reversible rxn has of molecules on each
  side, a volume/pressure change will do nothing

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Question

• What change in equilibrium will be seen when one
  adds a solid and why?

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How about this?

• 2H2S(g) O2(g) ? 2S(s) 2H2O(g) ?H -221.19
  kJ/mol
• If O2(g) is added to the reaction vessel, what
  happens to the amount of S(s)?
• a) It increases b) It decreases c) Nothing
• If the volume of the vessel is cut in half, what
  happens to the ratio of PH2O/PH2S?
• a) It increases b) It decreases c) Nothing
• If the temperature is increased, what happens to
  the equilibrium constant K?
• a) It increases b) It decreases c) Nothing
• If S(s) is added to the reaction, what happens to
  PH2O?
• a) It increases b) It decreases c) Nothing