Transcript Chapter 3 Chemical Equilibrium Atkins: Chapters 9,10,11 Silberberg: Chapters 17, 18 & 19 Equilibrium applies to the extent of a reaction, the concentration of product.

Chapter 3
Chemical Equilibrium
Atkins: Chapters 9,10,11
Silberberg: Chapters 17, 18 & 19
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.
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)
N2O4 (g)
2NO2 (g)
equilibrium
equilibrium
equilibrium
Start with NO2
Start with N2O4
Start with NO2 & N2O4
constant
Law of Mass Action
N2O4 (g)
2NO2 (g)
K=
[NO2]2
[N2O4]
aA + bB
= 4.63 x 10-3
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).
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:
Significance of the Magnitude of K
K=
[C]c[D]d
aA + bB
[A]a[B]b
any number greater than 10
cC + dD
Equilibrium Will
K >> 1
Lie to the right
Favor products
K << 1
Lie to the left
Favor reactants
any number less than 0.1
Homogenous equilibrium applies to reactions in which all
reacting species are in the same phase.
K using Partial Pressures
gas activities as partial pressures,
= moles of gaseous products - moles of gaseous reactants
concentration equilibrium constant
except when
Heterogenous equilibrium applies to reactions in which
reactants and products are in different phases.
pure solids
Equilibrium pressure - independent of the amount of either solid.
P
PCO 2 = Kp
Multiple Equilibria
A+B
C+D
K1
C+D
E+F
K2
A+B
E+F
K12
PcPd
K1 =
PAPB
PEPF
K2 =
PCPD
PEPF
K12 =
PAPB
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.
Form of K and the Equilibrium Expression
1. 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.
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
Calculating Equilibrium Concentrations
1. Express the equilibrium concentrations of all species in terms
of the initial concentrations and a single unknown x, which
represents the change in concentration.
2. 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.
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
Free Energy Versus Extent of Reaction
DG0 < 0
DG0 > 0
DG0 = - RT lnK
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)
2NH3 (g)
Equilibrium
shifts left to
offset stress
Add
NH3
Le Châtelier’s Principle
Changes in Concentration (continued)
aA + bB
cC + dD
Change
Shifts the Equilibrium
Increase concentration of product(s)
Decrease concentration of product(s)
Increase concentration of reactant(s)
Decrease concentration of reactant(s)
left
right
right
left
Le Châtelier’s Principle
Changes in Volume and Pressure
general ideal gas reaction,
in terms of mole fractions.
Reaction shifts forward – toward products
Reaction shifts in reverse – toward reactants
No net change
Le Châtelier’s Principle
Changes in Volume and Pressure: Summary
A (g) + B (g)
Change
Increase pressure
Decrease pressure
Increase volume
Decrease volume
C (g)
Shifts the Equilibrium
Side with fewest moles of gas
Side with most moles of gas
Side with most moles of gas
Side with fewest moles of gas
Changes in Temperature
endothermic
colorless
red-brown
exothermic
colder
hotter
Quantifying Changes in Temperature
DHo and DSo are approximately constant between T1 and T2
van’t Hoff equation
Le Châtelier’s Principle
Changes in Temperature: Qualitative Summary
Change
Exothermic
Endothermic
Increase temperature
K decreases
K increases
Decrease temperature
K increases
K decreases
Le Châtelier’s Principle Summary
Change
Shift Equilibrium
Change Equilibrium
Constant
Concentration
yes
no
Pressure
yes
no
Volume
yes
no
Temperature
yes
yes
Life at High Altitudes and Hemoglobin Production
Hb (aq) + O2 (aq)
HbO2 (aq)
[HbO 2 ]
K
[Hb ]PO2