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