Intersection 9: Equilibrium
10/31/06
Reading: 14.1-14.2 (p671-679); 14.3-14.5 (p682694); 14.6 Changing Concentrations of Reactants
or Product (p694-696); Changing Temperature
(p698-701.)
Gateway Chemistry 130/125/126
Section 600
6 – 8 PM Thursday Nov. 30th
A Gateway to Scientific Ethics
Scientists Employ the Scientific Method to Explore the Physical World.
• What are the ethical considerations associated with being a scientist?
• How do they relate to the use of the Scientific Method?
• Are there ethical considerations beyond the Scientific Method?
We will employ a case-study approach to examining these questions
including a prominent case of fraud from Lucent Technologies as well
as the classic Millikan-Ehrenhaft debate.
Dinner Provided
Please RSVP by email to Prof Banaszak Holl
(mbanasza@umich.edu)
Gateway evenings are optional
and will not affect your course grade
MSF
• Thank you!
• What we can’t change:
– Exam time
– Class lecture time
• Will be you prepared for your next chemistry class:
– Chem 130 concepts
– No answer keys
• How we hope to improve the course
– Grading rubrics for remaining projects (ex. Proposal)
– Try to preview topics and do more practice/homework
problems in lecture
– Give GSIs some more time for problem solving in
studio
Outline
•
•
•
•
Equilibrium defined
Equilibrium constant
ICE
Shifting equilibrium
– Equilibrium Law
– LeChâtelier’s principle
• Practice problems
A
Reactions that Don’t Go to
Completion
• Generally, we assume that reactions “go to
completion”…as much of the reactants are used
up as possible; there may be a limiting reagent and
thus reactants may be left over
• We assume that the reaction can only go in the
forward direction.
• A rate of (forward) reaction can be measured.
A
What's a rate of reaction?
For a simple reactions A B, rate = k[A].
Most reactions slow
down as they
proceed and as the
concentration/s of
the reactant/s
decreases (the rate
approaches zero.)
A
Problem 1
2L of a 0.1M solution of magnesium chloride
and 1 L of a 0.5 M solution of silver nitrate
are mixed together.
– Write out the balanced net ionic equation
– What is the limiting reagent?
– How many grams of silver chloride will you
make?
A
A New Scenario: Equilibrium
A reaction in equilibrium is going forward and
backward at the same rate.
All reactants and products are present and
actively interchanging
A
Forward and Reverse Rates
Reactions in equilibrium have both a
significant forward and reverse rate of
reaction. A  B Forward reaction: A → B
Reverse reaction: B → A
When equilibrium
is reached, the rates
of the forward and
reverse reaction are
equal AND are
NOT equal to 0.
A
An Equilibrium Model
http://www.chm.davidson.edu/ronutt/che115/
EquKin/EquKin.htm
2A  B
B  2A
When has the reaction reached equilibrium? (How can you tell?)
Is the forward reaction (2A  B) still taking place?
Is the reverse reaction (B  2A) still taking place?
M
Equilibrium Constant
• A  B Forward Rate = kfwd [A]
• B  A Reverse Rate = krev[B]
At equilibrium we have the following equality:
kfwd[A] = krev[B]
forward rate = reverse rate
Rearranging this equation yields:
Keq = kfwd/krev = [B]/[A]
Keq is the equilibrium constant
M
What does Keq tell you?
• A↔B
Keq =
• For low values of Keq, do you expect there
to be a higher concentration of products or
reactants at equilibrium?
Rate vs. Tim e
Concentration vs tim e
rate forw ard
B
2
Rate [M]/s
Concentration [M]
A
1.5
1
0.5
0
0
10
20
Tim e (seconds)
30
0.25
0.2
0.15
rate backw ard
0.1
0.05
0
0
10
20
Tim e (seconds)
30
M
Keq for more complex reactions
aA + xX ↔ bB + yY
Keq
[B]b[Y]y
=
[A]a[X]x
M
Writing Equilibrium Constants
*First, balance the equation.
1) NO(g) + Cl2(g) ↔ NOCl(g)
2) H2(g) +
I2(g) ↔
HI(g)
3)# CaCO3(s) ↔ CaO(s) +
# In
CO2(g)
any equilibrium expression, the concentration of a
pure liquid (e.g water) or pure solid is considered a
constant (1).
M
Equilibrium Constant Family
Keq -a generic equilibrium constant
Kc -an equilibrium constant calculated using
equilibrium concentrations in M (mol/L)
Kp - associated with gaseous equilibria; found
using equilibrium pressures (atm)
Pressure is directly proportional to
concentration (PV = nRT or P = (n/V)RT).
A
Determining Keq
How would you determine the equilibrium constant?
2NO2 (g)
↔
N2O4 (g) ΔH = -24.02 KJ/mol
(red-brown)
(colorless)
Suppose that 0.55 moles of NO2 are placed in an empty 5.00 L
flask which is subsequently heated to 407 K. By measuring the
intensity of the color of red-brown NO2, it can be determined
that its concentration at equilibrium is 0.10 mol/L. What is the
expression for Keq? What is the value of Keq at 407 K?
Keq = [N2O4]/[NO2]2
A
Put the Reaction on ICE
↔ N2O4 (g)
2NO2 (g)
Initial
0.55 moles/5 L
Concentration (M)
Change in
- 2x
Concentration (M)
Equilibrium
Concentration (M)
0.10 mol/L
0.11 mol/L – 2x = 0.10 mol/L
x = 0.005 mol/L
0 moles/L
+x
0.005 mol/L
Keq = [N2O4]/[NO2]2 = 0.5
A
Temperature
Does temperature matter?
2NO2 (g)
(red-brown)
↔
Keq(407K) = 0.5
N2O4 (g) ΔH = -24.02 KJ/mol
(colorless)
M
ICE applied
In the future, you can look up the equilibrium constants in Table
14.1 p685 as well as the Appendices) *
What if you want to know equilibrium
concentrations?
H2 (g) + I2 (g) ↔ 2 HI(g) Keq = 2.5 x 101
Two moles of hydrogen and 2 moles of iodine are
added to a 4 L container; what are the
concentrations of all reactants and products at
equilibrium?
M
Keq = 2.5 x 101
Initial
Change
Equilibrium
Keq = 25 =
↔ 2 HI(g)
H2(g)
I2(g)
2mol / 4L
2 mol / 4L
0
-x
-x
+2x
0.5 - x
+2x
0.5 - x
[HI]2 =
[2x]2
[H2][I2]
[0.5-x][0.5-x]
x = 0.8, 0.4
M
Problem 2: Making Ammonia
Desired for fertilizing (belief that without
ammonia, wouldn’t be able to feed world.)
N2(g) + 3 H2(g) ↔ 2 NH3(g) Kc = 3.5 x108 (25oC)
– What is the Kc if the reaction were written for the
production of 1 mole of ammonia?
1/2 N2(g) + 3/2 H2(g) ↔
NH3(g)
– If 10 moles of nitrogen and 10 moles of hydrogen are
placed in a 1 L flask, how many moles of ammonia can
you make? How many moles of starting material
would be left over?
A
Disturbing Equilibrium
• Sir Isaac Newton claimed that a ball at rest
would remain at rest unless disturbed. You
might be tempted to apply this logic to
equilibrium and get: A reaction at
equilibrium will remain at equilibrium
unless disturbed; consequently, the reaction
will shift so as to come back to equilibrium.
A
Can the Equilibrium Constant be
Changed?
2NO2 (g)
↔ N2O4 (g) ΔH = -24.02 KJ/mol
(red-brown)
(colorless)
A
Evaluating Changes in Equilibrium
Method 1: LeChâtelier's Principle
if a system at equilibrium is disturbed or
stressed by a change in temperature,
pressure or concentration of one of the
components, it will shift its equilibrium so
as to oppose the stress.
How does LeChâtelier's Principle explain
the demonstration that you just saw?
A
Can Equilibrium be Changed?
Use LeChâtelier's Principle to predict what you will see:
Fe(NO3)3 + KSCN ↔ Fe(SCN)+2 + KNO3
red
DH < 0
M
Evaluating Changes in Equilibrium
Method 2: Equilibrium Law (Q)
aA + xX ↔ bB + yY
• Keq is used at equilibrium to represent the ratio of reactants
to products for a give reaction.
Keq = [B]b[Y]y
[A]a[X]x
• Q, the reaction quotient, is used for this ratio under any
conditions at any point in time, not just equilibrium.
Q = [B]b[Y]y
[A]a[X]x
• At equilibrium, Keq and Q are EQUAL.
• According to the equilibrium law, the system will proceed
to bring Q and Keq equal to each other.
M
Q vs. Keq
• In general, how would the reaction proceed
to result in a decreased Q?
• What if an increased Q were the desired
result?
M
Applying the Equilibrium Law
• What is the equilibrium expression for this
reaction?
H2O(l) + C6H5CO2H (aq) ↔ C6H5CO2-(aq) + H3O+(aq)
• Keq was determined to be 6.42x10-5 at 25oC.
• At equilibrium is this reaction product
favored or reactant favored?
M
H2O(l) + C6H5CO2H ↔ C6H5CO2- + H3O+(aq)
2.00 moles of C6H5CO2-, 1.00 mole of H3O+ and
3.00 moles of C6H5CO2H are placed in 1 liter of
water.
What is the value of Q under these conditions?
Compare Q with Keq (6.42 x10-5). Will the reaction
proceed to form more C6H5CO2-(aq) and H3O+ (aq)
or more C6H5CO2H(aq) or not change at all?
M
LeChâtelier trumps Q
One instance where Le Châtelier's principle provides
us with information that the equilibrium law
cannot is in the case of changing temperature.
Suppose we have the following reaction,
CaCO3(s) ↔ CaO(s) + CO2(g)
ΔH > 0
What happens if you increase the temperature?
M
Q trumps LeChâtelier
CaCO3(s) ↔ CaO(s) + CO2(g)
ΔH > 0
Using each method, explain what will happen to the
concentration of CO2 if solid lime (CaCO3) is
added to the system?
A
Problem 3a
S2(g) and C(graphite) when placed together in a closed
system form an equilibrium with CS2(g).
C(graphite) + S2(g) ↔ CS2(g)
Suppose that the equilibrium constant for this
reaction is 4.0.
Draw a qualitative graph that shows how the
concentration of each gas changes with time if the
system initially consists of pure S2(g) and graphite.
A
Problem 3b
C(graphite) + S2(g) ↔ CS2(g)
Draw a picture representing the molecules
present under initial conditions and when
the reaction reaches equilibrium.
Will the amount of graphite in the system
be the same, more, or less at equilibrium
than it was initially? Why?
A
Problem 3c
C(graphite) + S2(g) ↔ CS2(g)
Draw a second graph showing what happens
if the system initially contains pure CS2(g)
and graphite.
Draw a picture representing the molecules
present under initial conditions and when
the reaction reaches equilibrium.
Will the amount of graphite have changed in
this scenario? If so, how?
M
Problem 4
At room temperature, the equilibrium constant for
the reaction:
2NO(g) ↔ N2(g) + O2(g) is 1.4 x1030
Is this reaction product-favored or reactant-favored?
In the atmosphere at room temperature, the
concentration of N2 is 0.33 mol/L, and the
concentration of O2 is about 25% of that value.
Calculate the equilibrium concentration of NO in
the atmosphere.
M
Problem 5
CO(g) + H2O(g) ↔ CO2(g) + H2(g)
Kc = 4.00 at 500 K.
A mixture of 1.00 mol CO and 1.00 mol H2O is
allowed to come to equilibrium in a flask of
volume 0.5 L at 500K,
Calculate the final concentrations of all four species:
CO, H2O, CO2 and H2
What would be the equilibrium concentrations if an
additional 1.00 mol of each CO and H2O were
added?
A
Equilibrium Representation
(Friday 11/10)
Your group will create a visual representation of a
dynamic equilibrium. The medium is completely
up to you (animation, skit, artwork, song, etc.),
and creativity is encouraged.
– The representative system that is in a stable dynamic
equilibrium.
– A stress to the system and how it would respond
according to Le Châtelier's principle.
You will tell the class the system what species
(chemical or otherwise) that are present etc., but
the class will have to infer the stress placed on the
system by its response to that stress.