Transcript Chapter 14

Chapter 14
EQUILIBRIUM
SUROVIEC
SPRING 2014
I. Equilibrium
 So far we have mostly
considered just forward
reactions
aA + bB  cC + dD
 But all reactions can
move in either direction
A. State of equilibrium
 Rate of equilibrium
 Equilibrium is dynamic
B. Positions of equilibrium
II. Equilibrium Constant
 Omit concentrations of
pure solids and liquids
because those
concentrations will not
change
 Look at elementary
steps:
A. What does K mean?
1.
Large value of K means
product favored
2. Small value of K means
reactant favored
III. Reaction Quotient
Example
Given the K of this reaction to be 170, when the conditions are changed what is the Q?
Given the new Q, what is the direction of the reaction to re-establish equilibrium?
IV. Calculations with K
 We need the equilibrium
concentrations to determine K,
be we usually know initial
concentrations, we can
therefore get there from the
balanced chemical equations!!!
 Butane interconverts to
isobutane at 25oC with a K =
2.5. If 0.017 moel of butane is
in 50 mL, what is the
concentration of both species at
equilibrium?
Example
PCl5 (g)
PCl3 (g) + Cl2 (g)
K is 1.2 at 25oC. If you place 0.300 M PCl5 (g) in a
flask what are the equilibrium concentrations of
PCl3 (g), PCl3 (g) and Cl2 (g)?
Example
H2 (g) + Br2(g)
2HBr (g)
K is 2.18 × 106 at 730oC. If you place 0.267 M HBr (g) in a
flask and allow it to come to equilibrium, what are the
equilibrium concentrations of H2 (g), Br2 (g) and HBr (g)?
V. Manipulating Equilibrium Expressions
N2(g) + 3H2 (g)
2NH3 (g)
NH 3 ]
[
8
K1 =
=
3.5
´
10
3
[ N 2 ][ H 2 ]
2
2N2(g) + 6H2 (g)
NH 3 ]
[
K2 =
2
6
[ N2 ] [ H2 ]
4
4NH3 (g)
ìï [ NH ] 2 üï
8 2
17
2
3
=í
=
3.5´10
=1.2
´10
=
K
ý
(
)
1
3
ïî [ N 2 ] [ H 2 ] ïþ
2
Rules for Manipulating
 When the stoichiometric coefficients in a balanced
reaction are changed by a factor of n:
Knew  K
n
old
• When a chemical reaction is reversed in
direction:
1
K new 
K old
• When several balanced reactions (each with its own
equilibrium constant) are added to obtain a net balanced
equation:
Knet  K1  K2  K3 
Ex
Calculation Knet for the following reaction:
Fe (s) + H2O (g)
FeO (s) + H2 (g)
Given the following information:
H2O (g) + CO (g)
H2 (g) + CO2 (g) K1 = 1.6
FeO (s) + CO (g)
Fe (g) + CO2 (g) K2 = 0.67
VI. LeChatelier’s Principle
 Any change in any of the factors that determine the
equilibrium conditions of a system will cause the
system to change in such a manner to counteract the
effect of the change
 Systems always want to be at equilibrium and will
work to get back there.
Example
2SO2 (g) + O2 (g)
2SO3(g)
Disturbances:
1.Add more SO2(g)
2.Take away O2 (g)
3.Decrease the volume by ½
ΔHrxn = -197 kJ