This document discusses chemical equilibrium. It defines chemical equilibrium as a state where the rates of the forward and reverse reactions are equal, such that the concentrations of reactants and products no longer change over time. It describes how equilibrium can be altered according to Le Chatelier's principle - if a stress such as a change in concentration, temperature, or pressure is applied to a system at equilibrium, the position of equilibrium will shift to counteract the applied change and re-establish equilibrium. Specific examples are provided of how changing concentration, temperature, and pressure can shift equilibrium in reversible reactions.

1. CHEMICALCHEMICAL
EQUILIBRIUMEQUILIBRIUM
CHEMICALCHEMICAL
EQUILIBRIUMEQUILIBRIUM
Dr. Samina Nazir

2. Slide 2 of 55
Chemical Equilibrium– Reversible reaction
k1
k-1

3. Slide 3 of 55
Homogenous vs. Heterogeneous Reactions
[CH3OH]
[CO][H2]2
Kc =
[NH3]2
[N2][H2]3
Kc =

4. Equilibrium Constant of Pressure
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5. Reaction Quotient (Qc)
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6. 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.
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7. Effect of Change in Concentration on Equilibrium
Slide 7 of 55
+ ve

8. Effect of Change in concentration and temperature
on Equilibrium constant
Slide 8 of 55
Add water
Add conc. HCl
Add CaCl2
Hot water
bathCold
water
bath

9. Effect of Change in Pressure on Equilibrium
• 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.
• Adding water, (Qc > Kc), the reactions shifts to the left as more water is
consumed in making [Co(H2O)6]2+
• Placing [Co(H2O)6]2+
in boiling water, reaction shifts towards forward
direction as this reaction is endothermic.
• Placing [CoCl4]2-
(aq) (blue) in ice bath shifts reaction towards to backward
direction.
• Adding or releasing pressure on gaseous reactions also has the same effect
as concentration.
Adding Catalyst has no effect on equilibrium.

10. Learning Check
Write the equilibrium constant expression for each reaction.
•2SO2(g)+O2(g) 2SO⇌ 3(g)
•N2O(g)+1/2O2(g) 2NO(g)⇌
•Cu(s)+2Ag+
(aq) Cu⇌ +2
(aq)+2Ag(s)
•CaCO3(g) CaCO(s)+CO⇌ 2(g)
•2NaHCO3(s) Na⇌ 2CO3(s)+CO2(g)+H2O(g)
•H+(aq)+OH–(aq)→H2O(l)
•Write the Partial Pressure Equilibrium: C(s)+O2(g)→CO2(g)
•What is the Kc of the following reaction?
2SO2(g)+O2(g) 2SO⇌ 3(g)
SO2(g)=0.2M, O2(g)=0.5M, SO3(g)=0.7M
Also, What is the Kp of this reaction? At room temperature?

11. Answers
Prentice-Hall © 2002 General Chemistry: Chapter 15 Slide 11 of 55