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Gas Phase Equilibrium Chemical Equilibrium Equilibrium Constant Keq Equilibrium constant expression Relationship between Kp and Kc Heterogeneous Equilibria Meaning of Keq Calculations of Kc
Solving Equilibrium Problems
USING AN EQUILIBRIUM CONSTANT Information obtained from Kc 1. Predict direction of a reaction Q ⇒ reaction quotien 2. Obtaining equilibrium concentrations of reactants and
products.
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Equilibrium Equilibrium is achieved when opposing processes
proceed at equal rates.
equilibria previously discussed
Eg.
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Chemical Equilibrium In chemical equilibria, forward and reverse reactions occur at
equal rates.
N2O4 (g) 2NO2(g)
colorless brown
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Achieving equilibrium Initial State: reactants only Initial state: products only
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Equilibrium Constant Equilibrium point of any reaction is characterized by
a single number.
Example: 2NO 2A N2O4 B
In general: jA + kB pR + qS
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EXPRESSION for Keq
What is the equilibrium constant expression for the Haber process?
N2(g) + 3H2(g) 2NH3(g)
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Equilibrium Constant The ratio of equilibrium concentrations will be constant:
Keq is the equilibrium constant for the reaction.
It is a NUMBER.
Keq (the number) DOES NOT depend on concentration
It’s a function of temperature only.
Notation:
Keq = Kc concentration […] is expressed in M (mol/L)
concentration is expressed using partial pressures in atm Keq = Kp
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When given Keq how do I know if it is Kc or Kp?
Use context of problem.
• If concentrations are given in M,
then Keq = Kc
• If concentrations are given in P,
then Keq = Kp
where Δn = nprod − nreact
Relationship between Kp and Kc
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Calculating Kp and Kc
Equilibrium concentrations of NH3, N2, and H2 were determined at 472°C.
[H2] = 0.1207M [N2] = 0.0402M [NH3] = 0.00272M
Find Kc and Kp.
2NH3(g) N2(g) + 3H2(g)
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Heterogeneous Equilibria involves reactants and products in more than one
phase
3Fe(s) + 4H2O(g) Fe3O4(s)+4H2(g)
€
Keq =Fe3O4[ ] H2[ ]4
Fe[ ]3 H2O[ ]4What is [Fe]? [Fe3O4]?
Activities of pure solids and liquids = 1.
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What is the equilibrium constant expression for these reactions?
AgCl(s)
C6H6(g) + 3H2(g)
Ag+(aq) + Cl−(aq)
C6H12(g).
CaCO3(s) CaO(s) + CO2(g)
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Meaning of Keq Reactants Products
Is Keq large or small???
K ≈ 1
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Problem
Which one of the following has the greatest tendency to proceed as written?
1. 4NH3(g) + 3O2(g) ⇔ 2N2(g) + 6H2O(g) Kp = 1×1022
2. N2(g) + O2(g) ⇔ 2NO(g) Kp = 5 ×10-31
3. 2HF(g) ⇔ F2(g) + H2(g) Kp = 1×10-13
4. 2NOCl(g) ⇔ 2NO(g) + Cl2(g) Kp = 4.7×10-4
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Is Keq large or small?
Cu+2(aq) + 4NH3(aq) → Cu(NH3)4+2(aq)
Ni+2(aq) + 6NH3(aq) → Ni(NH3)6+2(aq)
PbI2(s) → Pb+2(aq) + 2I-(aq)
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More Relationships
What is Keq if the reaction is doubled??
H2(g) + I2(g) C 2HI(g) Keq =
What is Keq for the reverse reaction?
2H2(g) + 2 I2(g) 4HI(g) B
Keq =
Keq =
H2(g) + I2(g) 2HI(g) A
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At a certain temperature, Kc for the following reaction is 16.
H2(g) + I2(g)
½ H2(g) + ½ I2(g)
2HI(g)
HI(g)
At the same temperature, what is Kc for this reaction?
1. 1/16
2. 4
3. 1/4
4. 16
5. there is not enough information to answer this question.
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General Approach to Equilibrium Constant Problems
1. Write the balanced reaction. 2. Write the general form for Keq. 3. Set up a data table:
(may need algebraic unknowns) -initial conditions -changes in concentrations -equilibrium concentrations
4. Substitute equilibrium concentrations into the expression for Keq and solve.
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Calculating Keq 1.00 mole of SO2(g) and 1.00 mole of O2(g) are added to a 1.00 L container and react until equilibrium is achieved. At equilibrium, the container has 0.919 moles of SO3(g).
Find Kc at 1000K.
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Calculating Keq
3Fe(s) + 4H2O(g) initial
change
final
Fe3O4(s) + 4H2(g)
A one L container holds 224g of Fe and 5.00 mole of H2O(l). It is heated to 1000K and reaches equilibrium. 56g of Fe are left unreacted. What is Kc at 1000K?
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Predicting the direction of a reaction
Note: the concentrations used are NOT equilibrium concentrations.
Reaction quotient ⇒ Q
aA + bB cC + dD
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Q =C[ ]c D[ ]d
A[ ]a B[ ]b
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Predicting the Direction of a Reaction
• When Q < Kc reaction moves to right (produces product)
• When Q > Kc reaction moves to left (produces reactant)
When Q = Kc system IS at equilibrium
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2HI(g)
If we put 0.1 mole of HI in a 1L container, what will happen? 1 reaction shifts to right → 2 reaction shifts to left ← 3 no change occurs
€
Kc =H2[ ] I2[ ]HI[ ]2
=1.25 ×10−3
If the initial concentrations of all three gases in the vessel are 0.1 mol/L, what will happen?
H2(g) + I2(g)
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Using Keq to obtain equilibrium concentrations of reactants and products.
Initially [IBr] = [I2] = [Br2] = 0.0500 M
What are the final concentrations of reactants and products? Kc = 2.50 × 10-3
2 IBr(g) Br2(g) + I2(g)
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Haber process Industrial process used to make ammonia
N2(g) + 3H2(g) 2NH3(g) + heat
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Factors that affect equilibrium What happens to a system at equilibrium when it is disturbed by changing
concentration
volume
pressure
temperature
Answer: Le Chatelier’s Principle
If a system at equilibrium is disturbed, the system will shift its equilibrium position to minimize the effect of the disturbance.
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Le Chatelier’s Principle To illustrate this principle use the following chemical
system in a closed 1L container:
ΔHrxn = -92.4kJ/mol T = 700°C Initial conditions:
[N2] = 3.00 M [H2] = 4.50 M [NH3] = 0 At equilibrium:
[N2] = 2.00M [H2] = 1.50 M [NH3] = 1.41 M What is Kc?
N2(g) + 3H2(g) 2NH3(g) + heat
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Use LeChatelier’s principle to answer the following questions
1. What will happen to [NH3] if 3.0 moles of H2 are added to the system at equilibrium? Increase the amount of a reactant or product
2. What will happen if the volume decreases from 1.0L to 0.5L? Increase P by decreasing volume
4. What will happen if 5.0 moles of inert gas is added to the system? Increase pressure by adding an inert gas.
6. What will happen if the temperature is increased to 1000K?
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1. What will happen to [NH3] if 3.0 moles of H2 are added to the system at equilibrium?
System is no longer at equilibrium.
Which direction will it go to get to equilibrium?
N2(g) + 3H2(g) 2NH3(g) + heat
Using LeChatelier’s principle:
Reactant or product added to a mixture at equilibrium will cause reaction to shift in the direction that consumes part of the added material.
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2. Which direction will the reaction proceed if the volume is decreased to 0.5L?
LeChatelier’s principle:
Since pressure is increased by disturbance, reaction will shift in direction that will reduce pressure
N2(g) + 3H2(g)
4 moles
2NH3(g)
2 moles
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3. What happens when 5.0 moles of an inert gas are added?
Inert gas: gas that does not participate in the reaction. How does inert gas effect concentrations of reactants and products?
LeChatelier’s principle:
N2(g) + 3H2(g)
4 moles
2NH3(g)
2 moles
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Examples
FeSCN+2(aq) Yellow colorless red
N2O2(g) colorless
Fe+3(aq) + SCN-(aq) Change concentration
Change pressure 2NO2(g) brown
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changing concentration (or V so that [ ] changes) puts a stress on the system.
Stresses do not change Keq!
Q changes; system shifts to re-establish equilibrium
Q→K
Summary
WHAT IF TEMPERATURE CHANGES? • Keq changes • change depends on whether the reaction is
exothermic or endothermic. ↑ ↑
-ΔH +ΔH
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4. What happens when the temperature is increased to 1000K?
Treat heat like a product (exothermic) or reactant (endothermic).
ΔHrxn = −92.4 kJ/mol
N2(g) + 3H2(g) 2NH3(g) + heat
Le Chatelier’s Principle: heat is added to a system, the reaction will shift in the direction that absorbs heat Reaction is exothermic (heat is a product) so adding heat will cause reaction to shift to left.
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4. What happens when the temperature is decreased?
heat + Co[H2O]62+ (aq) + 4Cl−(aq)
CoCl42− (aq)+ 6H2O()
pink blue
Treat heat like a product (exothermic) or reactant (endothermic).
Example: endothermic reaction
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Summarize N2(g) + 3H2(g) 2NH3(g) + heat
Problem: rate of reaction increases as T increases, BUT equilibrium constant decreases at higher T.
To produce ammonia:
Do we want high or low temperature?
Do we want high or low pressure?
Liquefy ammonia as process proceeds. WHY?