Predicting if a Reaction is in Equilibrium
Trial Keq
Lesson 9
The Keq is a constant- a number that does not change
Changing the volume, pressure, or any concentration, does not change the Keq.
Only temperature changes the Keq
Increasing a [Reactant] shifts right and maintains the Keq
Increasing the temperature of an endothermic equilibrium shifts right and increases the Keq
Ktrial
How can you tell if a system is in equilibrium or not?
Calculate a trial Keq.
Use initial concentrations in the equilibrium expression- evaluate.
How can you tell if a system is in equilibrium or not?
Calculate a trial Keq. Put initial concentrations into the equilibrium expression and evaluate.
If Kt
<
Keq
Shifts right
Kt
If Kt
=
Keq
If Kt
>
equilibrium
Shifts left
Kt
Kt
Keq
5
20
35
Keq
1.
10.0 moles of NH3, 15.0 moles of N2, and 10.0 moles of H2 are put in a 5.0 L container.
Is the system in equilibrium and how will it shift if it is not?
⇄
2NH3(g)
2.0 M
N2(g)
+
3H2(g)
3.0 M
Kt
=
[N2][H2]3
[NH3]2
Kt < Keq
Keq = 10
2.0 M
=
(3)(2)3
(2)2
Not in equilibrium
Shifts right!
= 6
2.
4.56 x 10-5 moles of NH3, 5.62 x 10-4 moles of N2, and 2.66 x 10-2 moles of H2 are put in a
500.0 mL container. Is the system in equilibrium and how will it shift if it is not?
⇄
2NH3(g)
9.12 x 10-5 M
Kt
N2(g)
+
3H2(g)
1.124 x 10-5 M
=
[N2][H2]3
[NH3]2
Kt > Keq
=
Keq = 10
5.32 x 10-2 M
(1.124 x 10-3 )(5.32 x 10-2 )3
(9.12 x 10-5 )2
Not in equilibrium
Shifts left!
= 20.3
4.
If 4.00 moles of CO, 4.00 moles H2O, 6.00 moles CO2, and 6.00 moles H2 are placed in a
2.00 L container at 670 oC.
CO(g)
+
2.00 M
+x
H2O(g)
⇄
2.00 M
+x
2.00 + x
2.00 + x
CO2(g)
3.00 M
-x
3.00 - x
+
H2(g)
3.00 M
-x
Keq = 1.0
Is the system at equilibrium?
Kt
=
(3)(3)
(2)(2)
= 2.25
3.00 - x
Not in equilibrium
Keq =
[CO2][O2]
Shifts left!
[CO][H2O]
Calculate all equilibrium concentrations.
(3 - x)2
=
2
(2 + x)
3 - x
2 +x
=
1.0
1.0
3 - x = 2 + x
1
=
2x
x = 0.50 M
[CO2]
=
[H2]
=
3.00 - 0.50 = 2.50 M
[CO]
=
[H2O]
=
2.00 + 0.50 = 2.50 M
Size of the Keq
Big Keq
products
Keq
=
reactants
Keq
=
10
Little Keq
products
Keq
=
reactants
Keq
=
0.1
Note that the keq cannot be a negative number!
Keq
=
1
products
Keq
=
reactants