chemical equilibrium...expression for equilibrium constant consider the following equilibrium...
TRANSCRIPT
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Chemical Equilibrium
• What is equilibrium?
• Expressions for equilibrium constants, Keq;
• Calculating Keq using equilibrium concentrations;
• Factors that affect equilibrium;
• Le Chatelier’s Principle
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What is Equilibrium?
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This is not Equilibrium?
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Chemical Equilibrium
• Consider the following reactions:
CaCO3(s) + CO2(aq) + H2O(l) Ca2+(aq) + 2HCO3
-(aq) ..(1)
and
Ca2+(aq) + 2HCO3-(aq) CaCO3(s) + CO2(aq) + H2O(l) ..(2)
Reaction (2) is the reverse of reaction (1).
At equilibrium the two opposing reactions occur at the
same rate.
Concentrations of chemical species do not change once
equilibrium is established.
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Expression for Equilibrium Constant
Consider the following equilibrium system:
wA + xB ⇄ yC + zD
Keq =
• The numerical value of Keq is calculated using the
concentrations of reactants and products that exist at
equilibrium.
xw
z
[B][A]
[D][C]y
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Expression and Value of Equilibrium Constant for a Reaction
• The expression for K depends on the equation
• The value of K applies to that equation; it does
not depend on how the reaction occurs;
• Concentrations used to calculate the value of K
are those measured at equilibrium.
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Expressions for Equilibrium Constants
Examples:
N2(g) + 3H2(g) ⇄ 2NH3(g); Keq =
PCl5(g)⇄ PCl3(g) + Cl2(g); Keq =
CH4(g) + H2(g)⇄ CO(g) + 3H2(g);
Keq =
3
22
2
3
]][H[N
][NH
][PCl
]][Cl[PCl
5
23
O]][H[CH
][CO][H
24
3
2
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Relationships between chemical equations and
the expressions of equilibrium constants
• The expression of equilibrium constant depends on how the
equilibrium equation is written. For example, for the
following equilibrium:
H2(g) + I2(g)⇄ 2 HI(g);
• For the reverse reaction:
2HI(g)⇄ H2(g) + I2(g);
]][I[H
[HI]
22
2
eq K
eq2
22eq 1/
[HI]
]][I[H ' KK
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Homogeneous & Heterogeneous Equilibria
Homogeneous equilibria:
CH4(g) + H2O(g) ⇄ CO(g) + 3H2(g);
CO(g) + H2O(g) ⇄ CO2(g) + H2(g);
Heterogeneous equilibria:
CaCO3(s) ⇄ CaO(s) + CO2(g);
HF(aq) + H2O(l) ⇄ H3O+(aq) + F-(aq);
PbCl2(s) ⇄ Pb2+(aq) + 2 Cl-(aq);
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Equilibrium Constant Expressions for
Heterogeneous System
Examples:
CaCO3(s)⇄ CaO(s) + CO2(g);
K = [CO2] or K = PCO2;
HF(aq) + H2O(l)⇄ H3O+(aq) + F-(aq);
[HF]
]F][O[H
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3eq
K
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PbCl2(s) ⇄ Pb2+
(aq) + 2Cl-(aq);
K = [Pb2+][Cl-]2
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Le Chatelier’s Principle states that:
When a system at equilibrium is stressed, the equilibrium will shift in the direction that will relieve the stress.
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What are “stresses” to an equilibrium?
• change in pressure
• change in concentration
• change in temperature
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Changes in PRESSURE
• only affect gases
RULE: If the pressure on a system increases, the shift will be towards the side of the eqn. with the LOWER # of moles of gas
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Changes in PRESSURE
•How do you figure out the number of moles of gas?
•Add up the coefficients in the balanced eqn.
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EXAMPLE 1:
3 H2 (g) + N2 (g) 2 NH3 (g)
If P increases… shift to RIGHT side
because there are 4 moles of gas on left side, only 2 moles of gas on right side.
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EXAMPLE 2:
H2 (g) + I2 (g) 2 HI (g)
If P increases… there will be NO SHIFT
because there are 2 moles of gas on the left side & 2 moles of gas on the right side.
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Changes in CONCENTRATION
RULE:
If the [concentration ] of substance on one side of eqn. increases, equilibrium will shift towards the other side.
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EXAMPLE 3:3 H2 (g) + N2 (g) 2 NH3 (g)
If [N2] increases…
shift towards RIGHT side.
If [NH3] increases…
shift towards LEFT side.
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Changes in CONCENTRATION
RULE:
If the [concentration] of substance on one side of eqn. decreases, equilibrium will shift towards that side.
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EXAMPLE 4:
3 H2 (g) + N2 (g) 2 NH3 (g)
If [H2] decreases…
shift towards LEFT side.
If [NH3] is removed…
shift towards RIGHT side.
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Changes in CONCENTRATION
RULE:
If the [concentration] of substance on one side of eqn. increases, substances on same side of eqn. will decrease.
Substances on other side will increase.
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In other words…
Same side of eqn. = opposite direction
Opposite side of eqn. = same direction
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EXAMPLE 5:
4 HCl (g) + O2 (g) 2 H2O (g) + 2 Cl2 (g)
If [O2] decreases… shift towards LEFTside.
[HCl] increases (same side as O2, so opposite direction)
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EXAMPLE 5:4 HCl (g) + O2 (g) 2 H2O (g) + 2 Cl2 (g)
If [O2] decreases…
[H2O] decreases (opposite side from O2, so same direction)
[Cl2] decreases
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EXAMPLE 5: still!
4 HCl (g) + O2 (g) 2 H2O (g) + 2 Cl2 (g)
If [H2O] increases…
shift towards LEFT side
[HCl] increases
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EXAMPLE 5:
4 HCl (g) + O2 (g) 2 H2O (g) + 2 Cl2 (g)
If [H2O] increases…
[O2] increases
[Cl2] decreases
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Changes in TEMPERATURE
RULE:
The word “heat” or a # of J, kJ, or calshould be treated as another reactant or product.
Follow same rules as with concentration.
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If heat is added to start the rxn. & the temp. increases…
Heat is located on the left side of the eqn.
It is an endothermic rxn.
The Keq value increases.
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If heat is given off by the rxn. & the temp. increases…
Heat is located on the right side of the eqn.
It is an exothermic rxn.
The Keq value decreases.
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EXAMPLE 6:
2 H2O (g) 2 H2 (g) + O2 (g) + 16 kcal
(exothermic)
If T increases…
shift towards LEFT side
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Changes in Keq value
RULE:
Only changes in temperature affect the Keqvalue.
Changes in pressure and/or concentration do NOT affect the Keq value.