Chem. 1B – 9/8 Lecture
correction on slide #8
Announcements I
• Lab Announcements (see p. 9 of syllabus) for
Wed. and Thurs. labs
– Quiz I (on review topics, lab I relevant topics, and
equilibrium lecture topics)
– Review topics (nomenclature and aqueous reactions
including net ionic equations) – besides text pages
given previously, also see p. 13 – p. 24 + p. 221 –
223 in lab manual
– Pre-lab questions (p. 30 – p. 37)
• This week is last to add (normally)
• SacCT – set up
Announcements II
• Mastering Chemistry
– Some (43 students) not yet signed in as of Monday
– first assignment due next Tuesday (9/15)
• Last lecture follow up
– state symbols – include in all reactions
• Today’s Lecture – note: can solve problems for K
(14.6) or for unknown concentrations (14.8)
– Equilibrium Problems: STARTING AT EQUILIBRIUM
– Equilibrium Problems: STARTING AT INITIAL
CONDITIONS
– Reaction Quotient and Reaction Direction (if time)
Chem 1B - Equilibrium
Equilibrium Problems – AT EQUILIBRIUM
• In this case the equilibrium equation is used
with concentrations (or pressures) given AT
EQUILIBRIUM
• These types of problems are very important for
environmental chemistry, but underemphasized
in text
• For example, an atmospheric chemist measured
high NO in air near fresh lava. He wondered if it
came from the N2(g) + O2(g) ↔ 2NO(g)
reaction. If KP(T = 1000 K) = 7 x 10-9, calculate
PNO in equilibrium with N2 and O2 in air.
Chem 1B - Equilibrium
Equilibrium Problems – AT EQUILIBRIUM
• 2nd Example Problem:
A rich chemist wants to measure KC for the
reaction: N2O4 (g) ↔ 2NO2 (g)
He puts N2O4 in a container at the temperature he
wants to measure KC. He measures [NO2] and
[N2O4] (using an expensive mass spectrometer)
until the concentrations stop changing. He finds
[NO2] = 0.0311 M and [N2O4] = 0.000170 M.
What is KC?
Chem 1B - Equilibrium
Equilibrium Problems – FROM INITIAL CONDITIONS
• In this case initial concentration(s) or
pressure(s) are given (typically of reactants)
• The reaction then proceeds to equilibrium
• The student calculates K (equilibrium
information needed) or the equilibrium
concentration of a reactant or product
• An important part of working out this problem is
to make an ICE table
• ICE stands for initial change equilibrium
• The ICE table accounts for rxn stoichiometry
Chem 1B - Equilibrium
Equilibrium Problems – Starting from initial conditions
• To understand how an ICE table works, let’s start
with a reaction that goes 100% to completion
• Example: 2.00 mol/L H2 + 2.00 mol/L O2 going to
H2O in a container at 200°C. (Note: for gases we
also could use atm in place of mol/L)
reaction
initial conc.
change
completion
2H2(g)
+
O2(g) → 2H2O(g)
2.00 mol/L
2.00 mol/L
0
-2.00 mol/L -1.00 mol/L +2.00 mol/L
0 mol/L
1.00 mol/L
2.00 mol/L
mol/L O2 lost = (2.00 H2 mol /L)(1 mol O2/2 mol H2)
limiting reagent
remember: for every 2 mol H2 we
use 1 mol O2
Chem 1B - Equilibrium
Equilibrium Problems – Starting from initial conditions
• Now, let’s look at the same type problem (bad
example in this case) which goes to some
equilibrium state where [H2(g)] ≠ 0
• Since we don’t know the final conc., we must
change the table, but keep the stoichiometry
reaction
I = initial conc.
C = change
E = equil
2H2(g)
+
O2(g) →
2.00 mol/L
2.00 mol/L
-2x
-x
2.00 – 2x
2.00 – x
2H2O(g)
0
+2x
2x
• In this case, we would need to know K or a final
concentration to solve this problem
Chem 1B - Equilibrium
Equilibrium Problems – Starting from initial conditions
• Next Example Problem: The rich chemist lost his
research grant and had his mass spectrometer
repossessed. He still has a UV-Visible
spectrometer to measure [NO2] (it’s a brown gas
– while N2O4 is invisible).
– Can he still calculate K?
– Yes, but we need to define the experiment more
carefully
– Initially, the chemist puts 0.0100 mol N2O4 into a 5.00
L container and sets T. He measures [NO2]. When
the concentration stops increasing, he finds [NO2] =
0.00281 M. What is K?
Chem 1B - Equilibrium
Equilibrium Problems – Starting from initial conditions
• Similar Example Problem: In the following
reaction, the concentration of I2 can be
measured (it is purple in color)
H2 (g) + I2(g) ↔ 2HI(g)
– A reaction starts with 0.100 mol of H2 and 0.100 mol
I2 in a 1.00 L flask. As the reaction proceeds, I is
measured. At equilibrium (when I2(g) doesn’t
change), [I2 (g)] is found to be 0.015 M. Calculate KC
Chem 1B - Equilibrium
Equilibrium Problems – Starting from initial conditions
• Q. Can the now not rich chemist measure K
without any chemical measurements?
• A. Actually, he can. He could put N2O4 into a
flask and measure the initial pressure (PMeasured
= PN2O40). As the reaction occurs, because 1
mol N2O4 = 2 mol NO2, the pressure increases:
N2O4 (g) ↔
I PN2O40
C -x
E PN2O40 – x
2NO2 (g)
0
+2x
2x
P0 = PN2O40
Pequil = PN2O4 + PNO2 =
PN2O40 + x or
DP = Pequil – P0 = x and
KP = (2x)2/(PN2O40 - x)
Chem 1B - Equilibrium
Equilibrium Problems – Starting from initial conditions
• Determination of Equilibrium Concentrations
– This is usually more difficult than determining K or
determining equilibrium concentrations when given
concentrations AT EQUILIBRIUM
– Sometimes we can get an algebraic expression for the
answer, but it is difficult to solve (e.g. a third order
polynomial requires a cubic equation)
Chem 1B - Equilibrium
Equilibrium Problems – Starting from initial conditions
• Example Problem 1
– At a certain temperature, KC = 0.38 for N2O4 (g) ↔
2NO2 (g)
– If a 10.0 L container initially has 0.100 mol of N2O4,
what is the equilibrium concentration of NO2?
Chem 1B - Equilibrium
Equilibrium Problems – Starting from initial conditions
• Problem 1 required the quadratic – Is this
needed always?
• No. Depends on K value and stoichiometry
• Example Problem 2:
A 10.0 L flask is filled with 0.0020 mol NO2 (g) and
it is expected to decompose as (ignoring the N2O4
formation reaction previously mentioned):
2NO2(g) ↔ 2NO (g) + O2(g)
With KC = 4.5 x 10-16
Calculate the equilibrium concentration of each gas
Chem 1B - Equilibrium
Equilibrium Problems – Overview
Does problem as to calculate K or an
unknown concentration at equilibrium?
K
Unknown conc.
Are concentrations of
all species given at
equilibrium?
Are concentrations of
all but 1 species given
at equilibrium?
Yes
Yes
No ICE
table
needed
No
ICE table
needed along
with given
equil. conc.
No ICE
table
needed
No
ICE table
needed
Chem 1B – Equilibrium
The Reaction Quotient and Reaction Direction
• For a given “system” (e.g. closed flask containing
chemicals), the system can either be AT EQUILIBRIUM
or under some other conditions (e.g. initial conditions)
• The equilibrium equation and constant only applies to
equilibrium conditions
• A second quantity, the REACTION QUOTIENT = Q, can
be calculated under any conditions (also QC and QP)
• For generic reaction: aA + bB ↔ cC + dD
• Q = equilibrium constant and for above reaction,

C  D 
Q
Aa Bb
c
d
note: for this reaction, [C] = conc.
C (but not necessarily at
equilibrium conditions)
Chem 1B – Equilibrium
The Reaction Quotient and Reaction Direction
• When Q > K, we are too heavy on products, so reaction
would proceed toward reactants (loss of C and D and
gain of A and B)
• When Q < K (e.g. initial conditions if A and B are mixed
and Q = 0), reaction proceeds toward products
Chem 1B - Equilibrium
The Reaction Quotient and Reaction Direction
• Example: An air resource board employee is studying the
effects of car exhaust pipe length on pollution
concentrations
• Air leaving the engine has both NO and NO2 (NO2 is a
worse pollutant)
• In the exhaust pipe, the reaction can continue toward
equilibrium: 2NO (g) + O2(g) ↔ 2NO2 (g) with KP = 4.2
x 108
• The gas partial pressures are measured just leaving the
engine (start of exhaust pipe) and found to be: PNO =
1.0 x 10-4 atm, PO2 = 0.030 atm, and PNO2 = 2.2 x 10-7
atm.
• In which direction will this reaction proceed?