CHEMICAL
EQUILIBRIUM
Dr. Samina Nazir
Chemical Equilibrium– Reversible reaction
k1
k-1
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Homogenous vs. Heterogeneous Reactions
Kc =
[CH3OH]
[CO][H2
]2
Kc =
[NH3]2
[N2][H2]3
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Equilibrium Constant of Pressure
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Prentice-Hall © 2002
General Chemistry: Chapter 15
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Reaction Quotient (Qc)
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Prentice-Hall © 2002
General Chemistry: Chapter 15
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Altering Equilibrium Conditions: Le
Châtellier’s Principle
• Reversible reaction proceed in both directions
• A reaction is at equilibrium if the reaction quotient ( Q ) is equal
to the equilibrium constant ( K ).
• If a system at equilibrium is subjected to a perturbance or stress
(temperature, pressure, or concentration) the position of
equilibrium changes.
• Since this stress affects the concentrations of the reactants and the
products, the value of Q will no longer equal the value of K .
• To re-establish equilibrium, the system will either shift toward
the products (if (Q≤K) or the reactants (if (Q≥K) until Q
returns to the same value as K . This process is described by Le
Chatelier's principle.
Prentice-Hall © 2002
General Chemistry: Chapter 15
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Effect of Change in Concentration on Equilibrium
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+ ve
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Effect of Change in concentration and temperature
on Equilibrium constant
Add conc. HCl
Add water
Cold
water
bath
Hot water
bath
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Effect of Change in Pressure on Equilibrium
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Adding CaCl2 or NaCl or conc. HCl results in concentration of reactant (Cl-)
to increase, (Qc < Kc), the reactions shifts to the right to consume increased
amount of Cl-, more [CoCl4]2-(aq) (blue) is produced.
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Adding water, (Qc > Kc), the reactions shifts to the left as more water is
consumed in making [Co(H2O)6]2+
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Placing [Co(H2O)6]2+ in boiling water, reaction shifts towards forward
direction as this reaction is endothermic.
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Placing [CoCl4]2-(aq) (blue) in ice bath shifts reaction towards to backward
direction.
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Adding or releasing pressure on gaseous reactions also has the same effect
as concentration.
Adding Catalyst has no effect on equilibrium.
Learning Check
Write the equilibrium constant expression for each reaction.
• 2SO2(g)+O2(g)⇌2SO3(g)
• N2O(g)+1/2O2(g)⇌2NO(g)
• Cu(s)+2Ag+(aq)⇌Cu+2(aq)+2Ag(s)
• CaCO3(g)⇌CaCO(s)+CO2(g)
• 2NaHCO3(s)⇌Na2CO3(s)+CO2(g)+H2O(g)
• H+(aq)+OH–(aq)→H2O(l)
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Write the Partial Pressure Equilibrium: C(s)+O2(g)→CO2(g)
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What is the Kc of the following reaction?
2SO2(g)+O2(g)⇌2SO3(g)
SO2(g)=0.2M, O2(g)=0.5M, SO3(g)=0.7M
Also, What is the Kp of this reaction? At room temperature?
Answers
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Kc=[SO3]2/[O2][SO2]2
Kc=[NO]2/[O2]0.5[N2O]
Kc=[Cu+2]/[Ag+]2
Kc=[CO2]/[CaCO3]
Kc=[H2O][CO2]
Kc=1/[H+][OH−]
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Kp=PCO2/PO2
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Kc: 24.5, Kp: 1 atm
Kc = [SO3]2 /[SO2] 2 [O2] = (0.7)2 /(0.2) 2 (0.5) = 0.49/0.02 = 24.5
Kp = Kc (RT)∆n = 24.5 (0.0820575 atm/mol.K × 298K)-1 = 24.5 (24.60) -1
= 24.5 × 0.04065 = 1
Prentice-Hall © 2002
General Chemistry: Chapter 15
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