1. Chapter 3 Chemical Equilibrium Atkins: Chapters 9,10,11
Silberberg: Chapters 17, 18 & 19

2. At equilibrium: rateforward = ratereverse
Equilibrium applies to the extent of a reaction, the concentration of product that has appeared after an unlimited time, or once no further change occurs.
At equilibrium: rateforward = ratereverse
A system at equilibrium is dynamic on the molecular level; no further net change is observed because changes in one direction are balanced by changes in the other.
Kinetics applies to the speed of a reaction, the concentration of product that appears (or of reactant that disappears) per unit time.

3. Reversible reactions can proceed in either the forward or reverse direction.
Equilibrium is a state in which there are no observable changes as time goes by.
Chemical equilibrium is achieved when:
the rates of the forward and reverse reactions are equal and
the concentrations of the reactants and products remain constant
Physical equilibrium
H2O (l)
H2O (g)
Chemical equilibrium
N2O4 (g)
2NO2 (g)

4. N2O4 (g) 2NO2 (g) equilibrium equilibrium equilibrium Start with NO2
Start with NO2 & N2O4

5. constant

6. Law of Mass Action N2O4 (g) 2NO2 (g) [NO2]2 K = = 4.63 x 10-3 [N2O4]
aA + bB cC + dD
ai is called the activity of component i.
Law of mass action: for a reversible reaction at equilibrium and at a constant temperature, a certain ratio of reactant and product concentrations has a constant value K (called the equilibrium constant).

7. Activity
The activity (a) of a species in a reaction is generally a complex function of the pressures and concentrations of all the components present in the reaction mixture.
Ideal gases:
Solutes in dilute solution:
Pure solids and liquids:

8. Significance of the Magnitude of K
[C]c[D]d
[A]a[B]b
aA + bB cC + dD
any number greater than 10
Equilibrium Will
K >> 1
Lie to the right
Favor products
K << 1
Lie to the left
Favor reactants
any number less than 0.1

9. Homogenous equilibrium applies to reactions in which all reacting species are in the same phase.

10. K using Partial Pressures
gas activities as partial pressures,
= moles of gaseous products - moles of gaseous reactants
concentration equilibrium constant
except when

13. Heterogenous equilibrium applies to reactions in which reactants and products are in different phases.
pure solids

14. Equilibrium pressure - independent of the amount of either solid.
PCO 2
= Kp

17. Multiple Equilibria
K1 =
PcPd
PAPB
A + B C + D K1
K2 =
PEPF
PCPD
C + D E + F K2
K12 =
PEPF
PAPB
A + B E + F
K12
If a reaction can be expressed as the sum of two or more reactions, the equilibrium constant for the overall reaction is given by the product of the equilibrium constants of the individual reactions.

18. Form of K and the Equilibrium Expression
When the equation for a reversible reaction is written in the opposite direction, the equilibrium constant becomes the reciprocal of the original equilibrium constant.
2. The value of K also depends upon how the equilibrium equation is balanced.

21. Predicting the Direction of a Reaction
The reaction quotient (Qc ) is calculated by substituting the initial concentrations of the reactants and products into the equilibrium constant (Kc) expression. IF
Qc > Kc system proceeds from right to left to reach equilibrium
Qc = Kc the system is at equilibrium
Qc < Kc system proceeds from left to right to reach equilibrium

24. Calculating Equilibrium Concentrations
Express the equilibrium concentrations of all species in terms of the initial concentrations and a single unknown x, which represents the change in concentration.
Write the equilibrium constant expression in terms of the equilibrium concentrations. Knowing the value of the equilibrium constant, solve for x.
3. Having solved for x, calculate the equilibrium concentrations of all species.

31. Gibbs Free Energy and Chemical Equilibrium
DG = DG0 + RT lnQ
R is the gas constant (8.314 J/K•mol)
T is the absolute temperature (K)
Q is the reaction quotient
At Equilibrium
DG = 0
Q = K
0 = DG0 + RT lnK
DG0 = - RT lnK

32. Free Energy Versus Extent of Reaction
DG0 < 0
DG0 > 0

33. DG0 = - RT lnK

37. 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
N2 (g) + 3H2 (g) NH3 (g)
Add
NH3
Equilibrium shifts left to offset stress

38. Le Châtelier’s Principle
Changes in Concentration (continued)
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

40. Le Châtelier’s Principle
Changes in Volume and Pressure
general ideal gas reaction,
in terms of mole fractions.

41. Reaction shifts forward – toward products
Reaction shifts in reverse – toward reactants
No net change

42. Le Châtelier’s Principle
Changes in Volume and Pressure: Summary
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

46. Changes in Temperature
endothermic
colorless
red-brown
exothermic
colder
hotter

47. Quantifying Changes in Temperature
DHo and DSo are approximately constant between T1 and T2
van’t Hoff equation

50. Le Châtelier’s Principle
Changes in Temperature: Qualitative Summary
Change
Exothermic
Endothermic
Increase temperature
K decreases
K increases
Decrease temperature
K increases
K decreases

51. Le Châtelier’s Principle Summary
Change Equilibrium
Constant
Change
Shift Equilibrium
Concentration
yes no
Pressure
yes no
Volume
yes no
Temperature
yes
yes

52. Life at High Altitudes and Hemoglobin Production
Hb (aq) + O2 (aq) HbO2 (aq)