Chem 12 Notes
2.8 Notes –Equilibrium Calculations (ICE Tables)
A chemical system can be thought of as being either:
1. At Equilibrium
or
2. Not At Equilibrium (Initial)
A system which is not at equilibrium will move spontaneously to a position of
being at equilibrium.
Type 1 Calculations – System is AT EQUILIBRIUM
 If the system is at equilibrium, there are no changes in concentrations.
 Keq can be easily calculated by “plugging” in the values for equilibrium
concentrations into the Keq expression.
 Whenever a question says something like " ...the equilibrium
concentrations of the following are..." or something to that effect, it is a
Type 1 Calculation.
Type 1 - Example:
Given the equilibrium system:
PCl5(g)
PCl3(g) + Cl2(g)
The system is analyzed at a certain temperature and the equilibrium
concentrations are
as follows:
[PCl5] = 0.32 M, [PCl3] = 0.40 M and the [Cl2] = 0.40 M.
Calculate the Keq for this reaction:
** Notice that there are no units given in the answer. Even though Keq technically
has units, they are fairly meaningless and they are just dropped. So don't
include any units when you state the value for Keq .
Type 1 - Example 2:
At 200°C, the Keq for the reaction: N2(g) + 3H2(g)
2NH3(g) is 625
If the [N2] = 0.030 M, and the [NH3] = 0.12 M, at equilibrium, calculate the
equilibrium [H2].
Do Worksheet 2. 8: Type 1 Calculations
Type 2 Calculations – System has MOVED to Equilibrium (ICE
Problems).
Given initial concentrations of all species and equilibrium concentration of one
species. You are asked to calculate the equilibrium concentrations of all species or
the Keq.
We set up an ICE table:
I=
C=
E=
Type 2 – Example:
Given the reaction:
N2(g) + 3H2(g)
2NH3(g)
Some H2 and N2 are added to a container so that initially, the [N2] = 0.32 M
and [H2] = 0.66 M.
At a certain temperature and pressure, the equilibrium [H2] is found to be 0.30
M.
a) Find the equilibrium [N2] and [NH3].
b) Calculate Keq at this temperature and pressure.
Type 2 – Example 2:
Consider the equilibrium system:
A
+ 3B
2C
0.20 moles of A and 0.60 moles of B are placed in a 2.0 L container. When
equilibrium is reached, the [A] is found to be 0.08 M. Calculate the
equilibrium [B] and the equilibrium [C]:
Do Hebden Q’s pg. 71- #47-49
Trial Keq Calculations
When we are given the initial concentrations of species (before equilibrium is
established), we can calculate a Trial Keq. We can then compare the Trial Keq to
the actual Keq value and determine which way the system will shift to reach
equilibrium.
Recall how we write the equilibrium expression:
Keq = ____________________
If Trial Keq < Keq the reaction will shift to the right. (a higher
[products] is needed .)
If Trial Keq > Keq the reaction will shift to the left. (a lower [products]
and a higher [reactants] is needed.)
If Trial Keq = Keq , the reaction will not shift at all. (It is already at
equilibrium).
Example 1:
At a particular temperature, for the reaction:
H2 +
I2
2HI
The Keq = 55.6
If the initial [H2] = 0.200 M and [I2] = 0.200 M, calculate the trial Keq. Which
way will the reaction shift?
Example 2:
CO(g)
+
H2O(g)
CO2(g) +
H2(g)
The value of Keq = 10.0 at a temperature of 600 °C.
A reaction mixture is analyzed and found to contain 0.80M CO, 0.050M H2O,
0.50M CO2 and 0.40M H2.
Determine which direction (left, right or not at all) the reaction will have to
shift in order to reach equilibrium.
Do Hebden Questions: pg. 71 # 50, 51, 54