When asked for to find Kc OR to find the final equilibrium concentrations and:
process 1
process 3 using “Q”
process 2
given initial concentrations
OR
given Kc & mole values with
an indication of the
concentration change at
equilibrium
2a
convert Kp to Kc
using
Kp = Kc(RT)Δn
write the Kc
expression
given a mixture of all reactant & product species in the
equilibrium expression
given Kp with mole values
with some indication of the
[change] at equilibrium
or
2b
Commit to using Kp
then do 1 of 2 things
write the Kp expression
Calculate M
ICE Table using
the stoichiometry
Plug into the Kc
expression & solve
for the unknown
write the Kc
expression
Calculate M
use PV = nRT to find
all partial pressures
ICE Table & solve
the Kp expression
OR
ICE Table using
the stoichiometry
Plug into the Kc
expression & solve
When already given
partial pressures, skip
PV=nRT . Go right to an
ICE table and then solve
using the Kp expression
Recall: When Δn = 0, then Kp= Kc you can use process 1
OR
you are unsure you have initial concentrations
OR
you have NO CLUE as to concentration change at
equilibrium
write the Kc or Kp
expression
If given Kp & Δn≠0 it is best to
convert to Kc (probably not on AP)
run a calculation for Qc or Qp and compare the
value to K. This indicates whether the reaction
will run to the right or left. (Working out Q can’t hurt)
This knowledge lets you write an ICE Table
Calculate M (if using Kc)
If using Kp go directly to ICE Table
ICE Table using the knowledge of Q to
add to or subtract from the reactants and
products
Plug into the Kc or Kp expression & solve
You may or may not need to use the
quadratic equation.
NAME ______________________________________
FIGURING OUT AN APPROACH: Q AND K
DIRECTIONS: You do NOT need to solve these problems … rather use 1, 2a, 2b or 3 to identify the approach you would
most likely use to solve the problem. The answers are on the last page.
____1) A mixture of 0.500 mol H2 and 0.500 mol I2 was placed in a 1.00 Liter stainless steel flask at 430°C.
The equilibrium constant Kc for the reaction: H2(g) + I2(g)  2HI(g) is 54.3 at this temperature.
Calculate the concentrations of the three species at equilibrium.
____2) Given: H2(g) + I2(g)  2HI(g) where Kc = 50.5 at 448°C
A 2.00 L container contains at some point, [HI] = 2.0 x 10-2 mol, [H2] = 1.0 x 10-2, [I2] = 3.0 x 10-2,
Calculate the final equilibrium concentrations.
____3) A 1.000 L flask is filled with 1.000 mole of H2(g) and 2.000 mol of I2(g) as initial concentrations
The balanced chemical reaction is H2(g) + I2(g)  2HI(g) where Kc = 50.5 at 448°C
What are the equilibrium concentrations of the three species in moles/L?
____4) Given: N2O4(g)  2 NO2(g)
Kp = 79.5
A 1.0 L reaction vessel contained 2.20 mol N2O4(g) and 3.05 mol of NO2(g) at 400°C
Calculate the final equilibrium concentrations
_____5) Given: 2 CH4(g)  C2H2(g) + 3 H2(g)
A reaction mixture at 1700 °C initially contains [CH4] = 0.115 M. At equilibrium, the mixture contains
[C2H2] = 0.035 M. What is the value of the equilibrium constant?
____6) Given: H2(g) + F2(g)  2HF(g) where Kp = 1.15 x 102
Assume that the reaction for the formation of gaseous hydrogen fluoride from hydrogen and fluorine has
an equilibrium constant of 1.15 x 102 at a specific temperature. In an experiment, 3.000 mol of each of
the 3 substances was added to a 1.500 L flask. Calculate the equilibrium concentration of each species.
____7) Given: PCl5(g)  PCl3(g) + Cl2(g
At a certain temperature, a 1.00 L flask initially contained 0.298 mole of PCl3(g) and 8.70 x 10-3 mole of
PCl5(g). After the system had reached equilibrium, 2.00 x 10-3 mole of Cl2 was found in the flask.
Gaseous phosphorus pentachloride decomposes according to the following reaction. Calculate Kc
____8) Given: I2(g) + Br2(g) 2 IBr(g)
3.0 mol of iodine and 4.0 mol of bromine are placed in a 2.0 L flask at 150°C. The reaction comes to
equilibrium, at which point 3.2 mol of iodine bromide are present. Determine the equilibrium constant
Kc for the reaction.
____9) NO2, a brown gas, and N2O4, a colorless gas, exist in equilibrium 2NO2(g)  N2O4(g)
A closed container at 25°C is charged with NO2 at a partial pressure of 0.56 atm and with N2O4 at a
partial pressure of 0.51 atm. At equilibrium the partial pressure of N2O4 is found to be 0.54 atm.
a) Calculate the partial pressure of NO2 at equilibrium.
b) Calculate the value of Kp
____10) Given: I2(g) + Cl2(g)  2 ICl(g) where Kp = 81.9
A reaction mixture contains PI2 = 0.114 atm and PCl2 = 0.102 atm and PICl = 0.355 atm. Calculate the
final equilibrium partial pressures
____11) Given: H2(g) + F2(g)  2HF(g)
where Kp = 1.15 x 102
Assume that the reaction for the formation of gaseous hydrogen fluoride from hydrogen and fluorine has
an equilibrium constant of 1.15 x 102 at a specific temperature. In an experiment, 3.000 mol of H2 and
6.000 mol of F2 are allowed to react in a 3.000 L flask. Calculate the equilibrium concentration of each
species
____12) Given: CO(g) + H2O(g)  CO2(g) + H2(g) Kc = 5.10 at 700 K
Carbon monoxide reacts with steam to produce carbon dioxide and hydrogen. Calculate the equilibrium
concentrations of each of the 4 species when 1.000 mol of each component is mixed in a 1.000 L flask.
____13) Determine the value of the equilibrium constant for the reaction: A + 2 B ↔ 2C
when 1.0 mol of A, 2.0 mol of B, and 3.0 mol of C are placed in 1.0 L vessel and allowed to come
to equilibrium, the final or equilibrium concentration of C = 1.4 mol/L
____14) For the Haber process: N2(g) + 3 H2(g)↔ 2 NH3(g), Kp = 1.45 x 10-5, at 500°C.
In an equilibrium mixture of the three gases at 500°C, the partial pressure of H2 is 0.928 atm and
that of N2 is 0.432 atm. What is the partial pressure of NH3 in this equilibrium mixture?
____15) A mixture of 0.0.00623 mol H2 and 0.00414 mol I2 and 0.0224 mol HI was placed in a 1.00 Liter stainless
steel flask at 430°C. The equilibrium constant Kc for the reaction H2(g) + I2(g)  2HI(g) is 54.3 at this
temperature. Calculate the concentrations of the three species at equilibrium.
Answers:
1) 1 You are given initial moles, in a 1 L flask …ergo you have concentration. You are given the Kc value … So just create an ICE
table and plug in the values into the equilibrium expression for Kc
2) 3 You are given a Kc, but have no knowledge of the final equilibrium concentrations … or of initial concentrations. In fact, you
are given moles of all three species found in the equilibrium expression. (hint, hint) You are just told at some point the vessel
had certain moles of species. Run a Q … figure out whichway the reaction will run and use this info to construct your ICE table.
3) 1 You are given initial concentrations (you have moles and a 1 Liter flask ….thus moles = M) You can construct an ICE table
which looks like:
H2 + I2 ↔ 2 HI
I
1.000
2.000
0.000
C
-x
-x
+2x
E 1.000-x 2.000-x
2x
The solution in this case will require you to use the quadratic equation. You cannot ignore “x” because Kc is far too large.
(We can ignore “x” only when the equilibrium constant is very small)
4) 2a &/or 3 You are given a Kc, but have no knowledge of the final equilibrium concentrations … or of initial concentrations. You
are just told at some point the vessel had certain moles of species. Run a Q … figure out which way the reaction will
run and use this info to construct your ICE table.
5) 1 You are told that the vessel “initially contains …”
Using this information, you can construct an ICE table, like:
2CH4 ↔ C2H2 + 3H2
I
0.115
0.000
0.000
C -0.070 +0.035 +0.105
E 0.045
0.035
0.105
6) 2a &/or 3 You really should run “Q” … you are given a mole value for each species in the equilibrium expression. You
should find out how the reaction would run before making your ICE table.
With that written, did you recognize that you are given Kp but given moles? You must decide what to do, to align the
Kp and mole values. You can convert Kp to Kc and use the mole values …. OR you can convert the mole values to
partial pressures using PV=nRT. Frankly I feel converting to Kc is easier …. while calculating partial pressures is an
elegant solution showing a great “approach”. It may reinforce that Kp = Kc(RT)Δn is based on PV = nRT … It is your
call
7) 1
8) 1
9) 2b Use the solution system at the bottom of 2b …. write the Kp expression and run an ICE table ….plug and chug from there.
You could use process 3 …for you are given a mixture of species found in the equilibrium expression ….BUT… you are also
given some clue as to the change in equilibrium concentration of N2O4 … Thus, I feel that running Q is a bit of a waste of time,
for it won’t get you further ahead.
10) 3 You don’t know initial concentrations …But you are given mole values for all three species of the equilibrium expression
This is a clue to use Q. You are given a Kp so 2a may raise its head …but really, overall, it’s best to run Q and use the result to
help you figure out what to do on the ICE table.
11) 3 I could be convinced to try 2a …assuming the mol values are initial mol values. The question is a bit vague … Then I could
convert Kp to Kc, and run an ICE table …IF they are assumed to be initial mol values.
12) 3
13) 1
14) 2b …. You have atmospheres …you have Kp … Write the Kp expression … run an ICE Table, plug into the K p expression and
solve for NH3.
15) 3 …. You have a mixture of all three species found in the equilibrium expression…. yuck… You need to figure out which way
the reaction will run … So, run “Q” and compare it to Kc. Use this information to help develop your ICE table. In all
likelihood you will need to use the quadratic equation to solve for “x”