What is Equilibrium? Equilibrium Constant, K Equilibrium Expressions Involving Pressures Activity Heterogeneous Equilibria Applications of Equilibrium Constant Solving Equilibrium Problems Le Chatelier’s Principle - very important Equilibria Involving Real Gases Week 9
CHEM 1310 - Sections L and M
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What is Equilibrium?
Equilibrium is the phenomenon that occurs when the rate of the forward reaction equals the rate of the reverse reaction. Week 9
CHEM 1310 - Sections L and M
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What is Equilibrium? At equilibrium, the concentrations of all reactants and products remain constant with time. Example Forward Rxn
Notice how the concentrations of products for the forward and reverse reactions are not necessarily equal at equilibrium! Week 9
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Equilibrium Characteristics What does equilibrium look like in a chemical system? N2O4 colorless
2 NO2 brown Equilibrium
Closed system reaches the same equilibrium concentrations whether the reaction starts with the N2O4 or the NO2! Week 9
CHEM 1310 - Sections L and M
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The Equilibrium Constant
aA + bB Equilibrium Constant
cC + dD K=
[C]c x [D]d [A]a x [B]b
Characteristics Exponents are coefficients from balanced chemical equation. Units for K will vary depending upon coefficients.
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CHEM 1310 - Sections L and M
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The Equilibrium Constant
cC + dD Equilibrium Constant
aA + bB K=
[A]a x [B]b [C]c x [D]d
Characteristics Reversing the reactants and products inverts the equilibrium expression. Thus, Kforward = Week 9
1 Kreverse
CHEM 1310 - Sections L and M
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Law of Mass Action K is constant despite different initial and equilibrium concentrations of reactants and products!
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Manipulation of Equilibrium Eqns Equilibrium equations can be reversed, scaled or combined. Forward: K1
aA + bB
cC + dD
K1 =
Reverse: K2
cC + dD
aA + bB
K2 =
[C]c x [D]d [A]a x [B]b
[A]a x [B]b [C]c x [D]d
By defn: K1 x K2 = 1 Week 9
CHEM 1310 - Sections L and M
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Manipulation of Equilibrium Eqns Equilibrium equations can be reversed, scaled or combined.
PCl3 + Cl2
K
PCl5
K1 =
Scaled: 2 PCl3 + 2 Cl2
K
2 PCl5
When stoichiometry is scaled, the resulting K is raised to the power of the scale factor Week 9
CHEM 1310 - Sections L and M
K2 =
[PCl5] [PCl3] x [Cl2]
[PCl5]
2
2
[PCl3] x [Cl2]
2
Example K2 = (K1)2 12
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Subtraction of Equilibrium Eqns Equilibrium equations can be reversed, scaled or combined. Equation #1
K1
Equation #1
K1
- Equation #2
K2
+ Equation #2
K2
Equation #3
K3 = K1 K2
Equation #3
Subtraction
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K3 = K1 x K2
Addition
CHEM 1310 - Sections L and M
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K and the Reaction Quotient, Q What is the relationship between Q and K? aA + bB
cC + dD
Reaction Quotient Q=
[C]c x [D]d [A]a x [B]b
vs.
Equilibrium Constant K=
Holds whether at equilibrium or not! Week 9
CHEM 1310 - Sections L and M
[C]c x [D]d [A]a x [B]b
Holds at equilibrium only! 14
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Q vs K What is the relationship between Q and K? aA + bB
cC + dD
When Q = K =
[C]c x [D]d [A]a x [B]b
Equilibrium occurs
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CHEM 1310 - Sections L and M
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Q vs K What is the relationship between Q and K? aA + bB
When Q = K =
cC + dD
[C]c x [D]d [A]a x [B]b
WHEN [A] and [B] >>> [C] and [D]
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Q>> [A] and [B]
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Equilibrium occurs
Reverse rxn proceeds
Q>K
CHEM 1310 - Sections L and M
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Q vs K aA + bB
cC + dD
When Q < K
reaction proceeds to the right
When Q = K
equilibrium occurs
When Q > K
reaction proceeds to the left
Thus, knowing K and calculating Q for any given state helps us predict which way a chemical reaction will proceed! Week 9
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Equilibrium Equations for Gases Equilibrium equations for gaseous reactions can be written in terms of concentrations or partial pressures. Why?
Recall…
PV = nRT P=
n V
P=
M (RT)
RT
Pressure is proportional to molar concentration. Week 9
CHEM 1310 - Sections L and M
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Equilibrium and Partial Pressure Equilibrium expressions can be written in terms of the partial pressures of the gases instead of their molar concentrations N2(g) + 3H2(g)
K=
2 PNH
3
P
N2
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2NH3(g)
x
PH3
2
In text, Kp denotes equilibrium constant expressed in terms of partial pressures
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Heterogeneous Equilibria How can we express the equilibrium constant when the reactants and products are in different phases? Si3N4(s) + 4 O2(g)
3 SiO2(s) + 2 N2O(g)
Rule #1. Express gases as partial pressures Rule #2. Express solute in solution as molar conc. Rule #3. Express pure solids/liquids as “1”. Rule #4. Products multiplied in the numerator reactants multiplied in the denominator Week 9
CHEM 1310 - Sections L and M
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Heterogeneous Equilibria How can we express the equilibrium constant when the reactants and products are in different phases? Si3N4(s) + 4 O2(g)
3 SiO2(s) + 2 N2O(g)
Rule #1. Express gases as partial pressures Rule #2. Express solute in solution as molar conc.
K=
x 13
P4
x 1
N2O O2
Rule #3. Express pure solids/liquids as “1”.
P2
Rule #4. Products multiplied in the numerator reactants multiplied in the denominator
P2
K=
N2O
P4
O2
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What To Study and Know… •
Know how to write equilibrium expressions
•
Know how to calculate K and mathematically manipulate K
•
Be able to calculate Q (via conc or partial pressures) and relate Q to K
•
Be able to calculate K for gases in equilibrium
•
Know how to express heterogeneous equilibria
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PRS Question 2 SO2 + O2
2SO3
Calculate K for SO3
K = 7.0 x 1025 SO2 + 0.5 O2
[1]
3.5 x 1025
[3]
1.2 x 10-13
[2]
7.0 x 10-25
[4]
1.4 x 10-26
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CHEM 1310 - Sections L and M
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PRS Question 2 SO2 + O2
2SO3
Calculate K for SO3
K = 7.0 x 1025 SO2 + 0.5 O2
To solve this problem: 1st: Kreverse =
1
Kreverse = 1.4 x 10-26
Kforward 2nd: Molar ratio is half, so take the square root of Kreverse K = 1.2 x 10-13 Week 9
Answer = #3 CHEM 1310 - Sections L and M
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PRS Question Which reaction will tend to proceed farthest toward completion? [1]
H2 + Br2
[2]
2NO
N2 + O2
K = 2.1 x 1030
[3]
2BrCl
Br2 + Cl2
K = 0.145
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2 HBr
CHEM 1310 - Sections L and M
K = 1.4 x 10-21
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PRS Question Which reaction will tend to proceed farthest toward completion? [1]
