1. Predicting Equilibrium

2. OUTCOME QUESTION(S):
C PREDICTING EQUILIBRIUM
Use the value of the reaction quotient, Q to explain how far a system at equilibrium has gone towards completion.
Write solubility product (Ksp) expressions from balanced chemical equations for salts with low solubility and solve related problems.
Vocabulary & Concepts

3. Q Reaction quotient (Q)
A trial KC that allows for the prediction of how a reaction will reach equilibrium:
Q is calculated exactly like Keq
Using the concentrations at that moment
Q is compared to the known constant, Keq
Q indicates which reaction, forward or reverse, must increase – favoured – to reach equilibrium
Similar to: “checking” to see if
pasta is cooked – is it ready, undercooked or overcooked? Q

4. There are three possibilities:
Reactants Products
Kc = 5.0
1. Q = Keq The system is AT equilibrium.
Qc =
[products]
[reactants]
Qc = 5.0
Prediction:
Forward rate = Reverse rate
Concentrations remain constant
Time
Concentration
teqbm
Reactants
Products
“Checking” – it’s done!

5. “Checking” – it’s undercooked!
There are three possibilities:
Reactants Products
Kc = 5.0
2. Q < Keq The system is NOT at equilibrium.
Qc =
[products]
[reactants]
Qc = 1.0
Prediction:
Forward rate favoured (must ↑ )
[Products] must increase
Concentration
“Checking” – it’s undercooked!
Reactants
Products
Time

6. “Checking” – it’s overcooked!
There are three possibilities:
Reactants Products
Kc = 5.0
2. Q > Keq The system is NOT at equilibrium.
Qc =
[reactants]
[products]
Qc = 10.0
Prediction:
Reverse rate favoured (must ↑ )
[Reactants] must increase
Concentration
“Checking” – it’s overcooked!
Reactants
Products
Time

7. 2 HI(g) ↔ H2 (g) + I2 (g) Kc = 3.9 x 10-2
Show the system is not at equilibrium with the following concentrations, and establish what must happen to reach equilibrium.
2 HI(g) ↔ H2 (g) + I2 (g) Kc = 3.9 x 10-2
0.75 M M M
[H2][I2]
[HI]2 Qc =
Q < K
System is NOT at equilibrium.
Prediction:
Forward rate favoured (must ↑ )
[Products] must increase
[0.14][0.10]
[0.75]2 Qc =
Qc = 2.5 x 10-2
Undercooked!

8. Initially 6. 0 mol of SO2, 3. 0 mol of O2 are mixed in a 1
Initially 6.0 mol of SO2, 3.0 mol of O2 are mixed in a 1.0 L reaction container. At equilibrium, 0.50 mol of O2 are found to remain. Kc= 5.6 x 10-3
Is this system at equilbrium?
2 SO3 (g) SO2 (g) + O2 (g) 1
[I]
6.0
3.0
[C]
+ 5.0
- 5.0
- 2.5
[E]
5.0
1.0
0.50
Kc =
[SO2]2[O2]
[SO3]2

9. 2 SO3 (g) 2 SO2 (g) + O2 (g) [E] 5.0 1.0 0.50 [SO2]2[O2] [SO3]2 Qc =
Kc= 5.6 x 10-3
[SO2]2[O2]
[SO3]2 Qc =
Q > K
System is NOT at equilibrium.
Prediction:
Reverse rate favoured (must ↑ )
[Reactants] must increase
[1.0]2[0.50]
[5.0]2 Qc =
Qc = 0.02
Overcooked!

10. CAN YOU / HAVE YOU?
C PREDICTING EQUILIBRIUM
Use the value of the reaction quotient, Q to explain how far a system at equilibrium has gone towards completion.
Write solubility product (Ksp) expressions from balanced chemical equations for salts with low solubility and solve related problems.
Vocabulary & Concepts