1. Chapter 15 Chemical Equilibrium
Lecture Presentation
Chapter 15 Chemical Equilibrium
James F. Kirby
Quinnipiac University
Hamden, CT

15. The equilibrium constant Kp for the reaction
is 158 at 1000K. What is the equilibrium pressure of O2 if the PNO = atm and PNO = atm?
2NO2 (g) NO (g) + O2 (g) 2
Kp = 2
PNO PO
PNO PO 2
= Kp
PNO PO 2
= 158 x (0.400)2/(0.270)2
= 347 atm

18. Homogeneous Equilibrium
CH3COOH (aq) + H2O (l) CH3COO- (aq) + H3O+ (aq)
[CH3COO-][H3O+]
[CH3COOH][H2O]
Kc = ‘
[H2O] = constant
[CH3COO-][H3O+]
[CH3COOH] =
Kc [H2O] ‘
Kc =

19. Heterogenous equilibrium applies to reactions in which reactants and products are in different phases.
CaCO3 (s) CaO (s) + CO2 (g)
Kc = ‘
[CaO][CO2]
[CaCO3]
[CaCO3] = constant
[CaO] = constant
Kc x ‘
[CaCO3]
[CaO]
Kc = [CO2] =
Kp = PCO 2
PCO 2
does not depend on the amount of CaCO3 or CaO
The concentrations of pure solids, pure liquids and solvent do not appear in the equilibrium constant expressions.

45. Factors that affect chemical equilibrium

47. Le Châtelier’s Principle
If an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset as the system reaches a new equilibrium position.
Changes in Concentration
Remove
Add
Add
Remove
aA + bB cC + dD
Change
Shifts the Equilibrium
Increase concentration of product(s)
left
Decrease concentration of product(s)
right
Increase concentration of reactant(s)
right
Decrease concentration of reactant(s)
left

48. Le Châtelier’s Principle
Changes in Volume and Pressure
A (g) + B (g) C (g)
Change
Shifts the Equilibrium
Increase pressure
Side with fewest moles of gas
Decrease pressure
Side with most moles of gas
Increase volume
Side with most moles of gas
Decrease volume
Side with fewest moles of gas
Change pressure will not affect the concentration of reacting species in condensed phase, e.g. liquid and solid.
In general, the increase of pressure favors the net reaction that decreases the total number of moles of gases and decrease of pressure favors the net reaction that increases the total number of moles of gases

49. Le Châtelier’s Principle
Changes in Temperature
Change
Exothermic Rx
Endothermic Rx
Increase temperature
K decreases
K increases
Decrease temperature
K increases
K decreases
Increase temperature favors an endothermic reaction, and the temperature decrease favors an exothermic reaction.

50. Effect of Pressure Change
Changes in the partial pressure of substances that are part of the equilibrium are handled in the same way as adding or removing a substance.
(Changing the partial pressures of gaseous reactants and products by changing volume)
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51. The following reaction is at equilibrium: COCl2(g)  CO(g) + Cl2(g)
a. Predict the direction of reaction when chlorine gas is added to the reaction mixture.
b. Predict the direction of reaction when carbon monoxide gas is removed from the mixture.
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52. Note: forward = right = .
COCl2(g)  CO(g) + Cl2(g)
When we add Cl2, the reaction will shift in the reverse direction to use it.
Note: reverse = left = .
When we remove CO, the reaction will shift in the forward direction to produce it.
Note: forward = right = .
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53. Effect of Temperature Change
Temperature has profound effect on most of the reactions. Reaction rates usually increase with an increase in temperature.
Equilibrium constant vary with temperature.
Changing the temperature changes the value of the equilibrium constant.
Changing the temperature can also cause a shift in the equilibrium.
The direction of each of these changes depends on the sign of DHo.
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54. For an endothermic reaction, DHo > 0 (positive), we consider that heat is a reactant.
For an exothermic reaction, DHo < 0 (negative), we consider that heat is a product.
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55. Le Châtelier’s Principle
Change Equilibrium
Constant
Change
Shift Equilibrium
Concentration
yes no
Pressure
yes no
Volume
yes no
Temperature
yes
yes
Catalyst no no