Chemical Equilibrium

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Chemical Equilibrium

• Chapter 14
• 14.1-14.5

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Equilibrium

• Equilibrium is a state in which there are no
  observable changes as time goes by.
• Chemical equilibrium is achieved when
• 1.) the rates of the forward and reverse
  reactions are equal and
• 2.) the concentrations of the reactants and
  products remain constant

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Equilibrium

• There are two types of equilibrium Physical and
  Chemical.
• Physical Equilibrium
• H20 (l) ? H20 (g)
• Chemical Equilibrium
• N2O4 (g) ? 2NO2

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Physical Equilibrium
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Chemical Equilibrium
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Chemical Equilibrium
N2O4 (g) ? 2NO2 (g)
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Law of Mass Action

• Law of Mass Action- For a reversible reaction at
  equilibrium and constant temperature, a certain
  ratio of reactant and product concentrations has
  a constant value (K).
• The Equilibrium Constant (K)- A number equal to
  the ratio of the equilibrium concentrations of
  products to the equilibrium concentrations of
  reactants each raised to the power of its
  stoichiometric coefficient.

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Law of Mass Action

• For the general reaction

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Equilibrium Constant

• N2O4 (g) ? 2NO2 (g)

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Chemical Equilibrium

• Chemical equilibrium is defined by K.
• The magnitude of K will tell us if the
  equilibrium reaction favors the reactants or the
  products.
• If K 1..favors products
• If K 1..favors reactants

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Equilibrium Constant Expressions

• Equilibrium constants can be expressed using Kc
  or Kp.
• Kc uses the concentration of reactants and
  products to calculate the eq. constant.
• Kp uses the pressure of the gaseous reactants and
  products to calculate the eq. constant.

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Equilibrium Constant Expressions

• Equilibrium Constant Equations

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Homogeneous Equilibrium

• Homogeneous Equilibrium- applies to reactions in
  which all reacting species are in the same phase.
  
• N2O4 (g) ? 2NO2 (g)

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Equilibrium Constant Expressions

• Relationship between Kc and Kp

Kp Kc(RT)Dn
Dn moles of gaseous products moles of gaseous
reactants
(c d) (a b)
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Equilibrium Constant Calculations

• The equilibrium concentrations for the reaction
  between carbon monoxide and molecular chlorine to
  form COCl2 (g) at 740C are CO 0.012 M, Cl2
  0.054 M, and COCl2 0.14 M. Calculate the
  equilibrium constants Kc and Kp.

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Kc
Kp Kc(RT)Dn
Dn 1 2 -1
R 0.0821
T 273 74 347 K
Kp 220 x (0.0821 x 347)-1 7.7
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Equilibrium Constant Calculations

• The equilibrium constant Kp for the reaction is
  158 at 1000K. What is the equilibrium pressure
  of O2 if the PNO 0.400 atm and PNO 0.270 atm?

347 atm
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Heterogeneous Equilibrium

• Heterogeneous Equilibrium- results from a
  reversible reaction involving reactants and
  products that are in different phases.
• Can include liquids, gases and solids as either
  reactants or products.
• Equilibrium expression is the same as that for a
  homogeneous equilibrium.
• Omit pure liquids and solids from the equilibrium
  constant expressions.

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Heterogeneous Equilibrium Constant
CaCO3 constant CaO constant
The concentration of solids and pure liquids are
not included in the expression for the
equilibrium constant.
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Heterogeneous Equilibrium Constant
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Equilibrium Constant Calculations

• Consider the following equilibrium at 295 K
• The partial pressure of each gas is 0.265 atm.
  Calculate Kp and Kc for the reaction.

0.265 x 0.265 0.0702
Kp Kc(RT)Dn
Kc Kp(RT)-Dn
Dn 2 0 2
T 295 K
Kc 0.0702 x (0.0821 x 295)-2 1.20 x 10-4
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Multiple Equilibria

• Multiple Equilibria- Product molecules of one
  equilibrium constant are involved in a second
  equilibrium process.

Kc
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Writing Equilibrium Constant Expressions

• The concentrations of the reacting species in the
  condensed phase are expressed in M. In the
  gaseous phase, the concentrations can be
  expressed in M or in atm.
• The concentrations of pure solids, pure liquids
  and solvents do not appear in the equilibrium
  constant expressions.
• The equilibrium constant is a dimensionless
  quantity.
• In quoting a value for the equilibrium constant,
  you must specify the balanced equation and the
  temperature.
• If a reaction can be expressed as a sum of two or
  more reactions, the equilibrium constant for the
  overall reaction is given by the product of the
  equilibrium constants of the individual reactions.

14.2
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What does the Equilibrium Constant tell us?

• We can
• Predict the direction in which a reaction mixture
  will proceed to reach equilibrium
• Calculate the concentration of reactants and
  products once equilibrium has been reached

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Predicting the Direction of a Reaction

• The Kc for hydrogen iodide in the following
  equation is 53.4 at 430ºC. Suppose we add 0.243
  mol H2, 0.146 mol I2 and 1.98 mol HI to a 1.00L
  container at 430ºC. Will there be a net reaction
  to form more H2 and I2 or HI?
• H2 (g) I2 (g) ? 2HI (g)

Kc 111
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Reaction Quotient

• The reaction quotient (Qc) is calculated by
  substituting the initial concentrations of the
  reactants and products into the equilibrium
  constant (Kc) expression.
• IF
• Qc gt Kc system proceeds from right to left to
  reach equilibrium
• Qc Kc the system is at equilibrium
• Qc lt Kc system proceeds from left to right to
  reach equilibrium

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Reaction Quotient
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Calculating Equilibrium Concentrations

• If we know the equilibrium constant for a
  reaction and the initial concentrations, we can
  calculate the reactant concentrations at
  equilibrium.
• ICE method
• Reactants Products
• Initial (M)
• Change (M)
• Equilibrium (M)

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Calculating Equilibrium Concentrations

• At 1280ºC the equilibrium constant (Kc) for the
  reaction is 1.1 x 10-3. If the initial
  concentrations are Br2 0.063 M and Br
  0.012 M, calculate the concentrations of these
  species at equilibrium.

Let x be the change in concentration of Br2
0.012
0.063
Initial (M)
-x
2x
Change (M)
0.063 - x
0.012 2x
Equilibrium (M)
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Calculating Equilibrium Concentrations
4x2 0.048x 0.000144 0.0000693 0.0011x
4x2 0.0491x 0.0000747 0
x -0.0105
ax2 bx c 0
x -0.00178
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Calculating Equilibrium Concentrations
At equilibrium, Br 0.012 2x -0.009 M
or 0.00844 M
At equilibrium, Br2 0.063 x 0.0648 M
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Calculating Equilibrium Concentrations

• Express the equilibrium concentrations of all
  species in terms of the initial concentrations
  and a single unknown x, which represents the
  change in concentration.
• Write the equilibrium constant expression in
  terms of the equilibrium concentrations. Knowing
  the value of the equilibrium constant, solve for
  x.
• Having solved for x, calculate the equilibrium
  concentrations of all species.

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Factors that Affect Chemical Equilibrium

• Chemical Equilibrium represents a balance between
  forward and reverse reactions.
• Changes in the following will alter the direction
  of a reaction
• Concentration
• Pressure
• Volume
• Temperature

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Le Châtliers Principle

• Le Châtliers Principle- if an external stress is
  applied to a system at equilibrium, the system
  adjusts in such a way that the stress is
  partially offset as the system reaches a new
  equilibrium position.
• Stress???

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Changes in Concentration

• Increase in concentration of reactants causes the
  equilibrium to shift to the ________.
• Increase in concentration of products causes the
  equilibrium to shift to the ________.

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Changes in Concentration

• Change Shift in Equilibrium
• Increase in Products left
• Decrease in Products right
• Increase in Reactants right
• Decrease in Reactants left

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Changes in Concentration

• FeSCN2(aq) ? Fe3(aq) SCN-(aq)
• a.) Solution at equilibrium
• b.) Increase in SCN-(aq)
• c.) Increase in Fe3(aq)
• d.) Increase in FeSCN2(aq)

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Changes in Volume and Pressure

• Changes in pressure primarily only concern gases.
  
• Concentration of gases are greatly affected by
  pressure changes and volume changes according to
  the ideal gas law.
• PV nRT
• P (n/V)RT

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Changes in Pressure and Volume

• Change Shift in Equilibrium
• Increase in Pressure Side with fewest moles
• Decrease in Pressure Side with most moles
• Increase in Volume Side with most moles
• Decrease in Volume Side with fewest moles

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Changes in Pressure and Volume
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Changes in Temperature

• Equilibrium position vs. Equilibrium constant
• A temperature increase favors an endothermic
  reaction and a temperature decrease favors and
  exothermic reaction.
• Change Endo. Rx Exo. Rx
• Increase T K decreases K increases
• Decrease T K increases K decreases

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Changes in Temperature

• Consider N2O4(g) ? 2NO2(g)
• The forward reaction absorbs heat endothermic
• heat N2O4(g) ? 2NO2(g)
• So the reverse reaction releases heat exothermic
• 2NO2(g) ? N2O4(g) heat
• Changes in temperature??

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Effect of a catalyst

• How would the presence of a catalyst affect the
  equilibrium position of a reaction?