Principles of Reactivity: Chemical Equilibria

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Principles of ReactivityChemical Equilibria
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• Chemical reactions are reversible
• In a closed system, an equilibrium state will be
  reached between reactants and products
• Outside forces can affect the equilibrium

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Ca2(aq) 2HCO3- (aq) CaCO3(s) CO2(g)
H2O(l)

• Initially the product concentration is zero
• The reactants react at some rate
• As the reaction proceeds, the forward reaction
  rate decreases
• Products begin to react to reform reactants
• Reverse reaction rate increases as products are
  being formed
• Eventually the rate of the forward reaction
  equals the rate of the reverse reaction
• Dynamic equilibrium is
• established

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• Some links http//www.chm.davidson.edu/ChemistryAp
  plets/equilibria/BasicConcepts.html
• http//www.chm.davidson.edu/ronutt/che115/EquKin/E
  quKin.htm

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Reaction Quotient and Equilibrium Constant

• At equilibrium, concentration of reactants and
  products are related
• At equilibrium, ratio of products to reactants is
  constant
• HI2 a constant
• 
  H2I2

H2(g) I2 (g) 2HI(2)
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Equilibrium Constant

• ICE table
• Equation H2(g) I2 (g)
  2H2(g)
• Initial conc (M) 0.0175
  0.0175 0
• Change in conc (M) -0.0138
  -0.0138 0.0276
• as rxn proceeds to
• equilibrium
• Equilibrium conc (M) 0.0037
  0.0037 0.0276
• HI2 (0.0276)2 56
• H2I2
  (0.0037)(0.0037)

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Reaction Quotient and Equilibrium Constant

• aA bB cC dD
• Under any reaction conditions
• Reaction quotient Q product
  concentration CcDd
• 
  reactant concentration AaBb
• At equilibrium
• Reaction quotient equilibrium constant Keq
  CcDd
• 
  AaBb
• When reaction has reached equilibrium, the
  reaction quotient has a constant value, known as
  the equilibrium constant, Keq
• Law of Mass Action
• Expresses relative concentrations of reactants at
  equilibrium in terms of an equilibrium constant

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Equilibrium Constant Expression

• aA bB cC dD
• Keq CcDd
• AaBb
• Product concentration in numerator
• Reactant concentration in denominator
• Concentration raised to power of its
  stoichiometric coefficient in balanced equation
• K depends on particular reaction and temperature
• K has no units
• Value of K is indicative of whether reaction is
  product or reactant favored
• Can calculate reactant or product concentration
  at equilibrium

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• Homogeneous equilibria equilibrium conditions
  where all reactants and products are in the same
  state
• Heterogeneous equilibria equilibrium conditions
  where reactants and products are in more than one
  state

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Writing Equilibrium Constant Expressions

• Concentrations of any solid or liquid reactants
  and products are not included in the equilibrium
  expression
• S(s) O2(g) SO2(g) Keq SO2
• O2
• NH3(aq) H2O(l) NH4(g) OH-(aq)
  KeqNH4OH
• NH3
• Equilibrium constant expression written with
  reactant and product concentration in moles/L
• Keq sometimes written as Kc

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Graph pg 664 butane-isobutane equilibrium

• There are an infinite number of possible sets of
  equilibrium concentrations of reactants to
  products
• A straight line produced
• Slope K
• Sets of concentrations of reactants and products
  that do not lie along equilibrium line are not at
  equilibrium.

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Reactions Involving Gases

• Equilibrium constant expression can be written in
  terms of partial pressures of reactants and
  products
• H2(s) I2(g) 2HI(g) Kp P2 HI
• PH2PI2
• sometimes Kc and Kp are the same but not
  usually

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Meaning of the Equilibrium Constant, K

• Kgtgt1 Reaction is product favored
• Kltlt 1Reaction is reactant favored

reactants products

• Do exercise 16.2 pg 663

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Meaning of the Reaction Quotient, Q

• Under any reaction conditions
• Reaction quotient Q product concentration
  CcDd
• 
  reactant concentration AaBb
• Compare Q and K
• Q lt K, the system not at equilibrium and some
  reactants will be converted to products
• Q gt K, the system is not at equilibrium and some
  products will be converted to reactants
• Q K, the system is at equilibrium
• Example and exercise 16.3, 16.4 pg 665-666

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Determining an Equilibrium Constant

• Substitute concentrations of reactants and
  products
• at equilibrium in equilibrium expression
• But, we usually do not know all of the
  concentrations of reactants and products at
  equilibrium
• Know equilibrium concentration of only one
  reactant or product
• Remaining ones need to be determined using
  stoichiometry and the balanced equation
• Set up an ICE table to help in organization of
  data

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Determining an Equilibrium Constant

• Equation (_at_ 1000K) 2SO2(g)
  O2 (g) 2SO3(g)
• Initial conc (M) 1.00
  1.00 0
• Change in conc (M) -0.925
  -0.925/2 0.925
• Equilibrium conc (M) 1.00-0.925
  1.00- 0.925/2 0.925
• Measured values 0.075
  0.537
• Mol SO3 produced mol SO2 consumed
• 22 ratio from balanced equation
• Mol SO3 produced 0.5 mol O2 consumed
• 21 ratio from balanced equation
• K SO32 (0.925)2
  2.8 x102 _at_ 1000K
• SO22O2 (0.075)2(0.537)
• Example and exercises 16.3, 16.4, 16.5 pg 667-669

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Calculating equilibrium concentrations from Keq

• Look at example 16.5 pg 668
• Calculations involving the quadratic equation
  (ax2 bx c 0)
• x -b v b2 -4ac
• 2a
• Equation (K 1.20) PCl5(g)
  PCl3 (g) Cl2(g)
• Initial conc (M) 1.60
  0 0
• Change in conc (M) 1.60 - x
  x x
• Equilibrium conc (M) 1.60 - x
  x x
  
• K 1.20 PCl3Cl2 x2
  x2 1.20x -1.92 0
• PCl5
  1.60 x
• solve for x x 0.91 and -2.11 Cant
  have negative value, x 0.91
• PCl5 1.60 - 0.91 0.69M
• PCl3 Cl2 0.91M

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Calculating equilibrium concentrations from Keq

• Is there anything less complicated that still
  gives us a reasonably accurate answer?
• Need to understand meaning of Keq
• When Keq is large
• conc of products high, reactants mostly consumed
  value of x large
• When Keq is small
• conc of reactants high, products low value of x
  small
• A B C
• Keq BC x2
  if Keq and A0 are known, need
• A A0 x
  to find C, B
• When K is very small, x ltlt A0 , therefore A0
  x A0
• Therefore Keq x2
• A0

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Calculating equilibrium concentrations from Keq

• When can we use this approximation technique?
• If 100Keq lt A0 you can use this
  technique
• See example 16.6 pg 670
• Do exercise 16.7 pg 671

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More about Keq and balanced equations

• When the stoichiometric coefficients of a
  balanced equation are multipled by some factor,
  the equilibrium constant for the new equation is
  the old equilibrium constant raised to the power
  of the multiplication factor
• C(s) ½ O2(g) CO(g) Keq1
  CO/O2 ½ 4.6x 1023
• 2C(s) O2(g) 2CO(g) Keq2
  CO2/O2 2.1x 1047
• Keq2 CO2/O2 CO/O2 ½ 2 Keq12
• Keq2 (Keq1)2

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More about Keq and balanced equations

• The equilibrium constants for a reaction and its
  reverse are the reciprocals of each other
• HCO2H(aq) H2O (l) HCO2-(aq)
  H3O(aq)
• Keqf HCO2-H3O Keqr HCO2H
• HCO2H
  HCO2-H3O
• 1.8x 10-4 5.6x 103
• Keqr 1/Keqf

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More about Keq and balanced equations

• When two or more chemical equations are added to
  produce a net equation, the equilibrium constant
  for the net equation is the product of the
  equilibrium constants for the added equations
• AgCl(s) Ag(aq)
  Cl-(aq) Keq1 1.8 x
  10-10
• Ag(aq) 2NH3 (aq) Ag(NH3) (aq)
  Keq2 1.6 x 107
• AgCl(s) 2NH3 (aq) Ag(NH3)
  (aq) Cl-(aq)
• Keqnet Keq1 Keq2 AgCl- Ag(NH3)
  Ag(NH3) Cl-
• 
  AgNH32 NH32
• Keqnet Keq1 Keq2 2.9 x 10-3
• Examples and exercises 16.7,16.8, 16.9 pg.
  673-674

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Disturbing a Chemical EquilibriumLe Chateliers
Principle

• Le Chateliers Principle
• If a stress (disturbance) is applied to a system
  at equilibrium, the system will tend to adjust so
  that the stress is reduced
• (the equilibrium will shift to right or left)
  Change in temperature
• What stresses (disturbances)
• Change in concentration of one of the reactants
  or products
• Change in volume, pressure for a system of gases
• Change in temperature
• Be familiar with table 16.2 pg 675

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Disturbing a Chemical EquilibriumLe Chateliers
Principle

• Change in temperature
• endothermic
• N2(g) O2(g) 2NO(g) ?Hrxn
  180.5kJ
• Keq T
• 4.5 x 10-31 298
• 6.7 x 10-10 900
• 1.7 x 10-3 2300
• exothermic
• 2NO2(g) N2O4(g)
  ?Hrxn -57.2kJ
• Keq T
• 1300 273
• 170 298

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Effect of Temperature Change

• Think of heat as a reactant or a product
• endothermic
• equilibrium shifts to the right
• exothermic
• equilibrium shifts to the left
• New equilibrium will be established with new
  equilibrium concentrations of rectants and
  products and a new Keq

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Effect of addition or removal of a reactant or
product

• If conc. of reactant or product is changed at a
  given T, equilibrium will eventually be
  re-established
• New equilibrium concentrations of reactants and
  products will be different
• But ratio of products to reactants (Keq) will be
  the same
• CH3CH2 CH2 CH3 (g)
  CH3CH(CH3)CH3 (g)
• If add more butane reaction will
  proceed more in the forward
• direction than the reverse direction
  until equilibrium has been re-
• established
• Example 16.8, exercise 16.10 pg 678

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CH4(g) H2O(g) CO(g) 3H2(g)

• In what direction will the equilibrium shift if
• Add water
• Add CO
• Remove hydrogen
• Add methane
• Remove methane
• Remove CO
• Remove water
• Add hydrogen

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Effect of Volume Change for a gas phase equilibria

• What happens to concentration or pressure of a
  gas as volume changes?
• If pressure is increased (decrease in volume) the
  system will shift in direction that produces
  fewer particles
• If pressure is decreased (increase in volume) the
  system will shift in direction that produces the
  greater number of particles
• 2NO2(g) N2O4(g)
• Exercise 16.12

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In what direction will the equilibrium shift if

• PCl5(g) heat PCl3 (g) Cl2(g)
• Add chlorine
• Increase pressure
• Remove heat
• Remove PCl3
• 2NH3(g) CO2(g) CO(NH2)2 (g) H2O(g)
• Add CO2
• Remove CO(NH2)2
• Increase T
• Decrease P
• Add water