1. Predicting if a Reaction is in Equilibrium
Trial Keq
Lesson 9

2. 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

3. 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.

4. 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
If Kt = Keq
If Kt > Keq
Shifts right
equilibrium
Shifts left Kt Kt Kt
Keq 5 20 35

5. 1. 10. 0 moles of NH3, 15. 0 moles of N2, and 10
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) ⇄ N2(g) H2(g) Keq = 10
2.0 M M 2.0 M
[N2][H2]3
Kt =
= (3)(2)3
= 6
(2)2
[NH3]2
Kt < Keq
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
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 mL container. Is the system in equilibrium and how will it shift if it is not?
2NH3(g) ⇄ N2(g) H2(g) Keq = 10
9.12 x 10-5 M x 10-5 M 5.32 x 10-2 M
[N2][H2]3
Kt =
= (1.124 x 10-3 )(5.32 x 10-2 )3 =
(9.12 x 10-5 )2
[NH3]2
Kt > Keq
Not in equilibrium
Shifts left!

7. 4. If 4. 00 moles of CO, 4. 00 moles H2O, 6. 00 moles CO2, and 6
4. If 4.00 moles of CO, 4.00 moles H2O, 6.00 moles CO2, and 6.00 moles H2 are placed in a L container at 670 oC.
CO(g) H2O(g) ⇄ CO2(g) H2(g) Keq = 1.0
2.00 M M M M
Is the system at equilibrium?
+x x x x
(3)(3)
Kt = =
(2)(2)
x x x x
Not in equilibrium
Shifts left!
[CO2][O2]
Keq =
[CO][H2O]
Calculate all equilibrium concentrations.

8. (3 - x)2 =
(2 + x)2
3 - x =
2 + x
3 - x = x
1 = 2x
x = M
[CO2] = [H2] = = M
[CO] = [H2O] = = M

9. Size of the Keq

10. Big Keq
products
Keq =
reactants
Keq =

11. reactants Little Keq Keq = Keq = 0.1
products
Keq =
reactants
Keq = 0.1
Note that the keq cannot be a negative number!

12. Keq = 1
products
Keq =
reactants