1. Michael Grab, “Gravity Glue”
Reaction Quotient, Q
Michael Grab, “Gravity Glue”
Honors Chemistry

2. More about equilibrium:
We’ve been looking at systems that reach equilibrium!
What if a reaction has not yet reached equilibrium?
How can you predict the direction in which the reaction must proceed to reach equilibrium?
We use the reaction quotient!

3. Reaction Quotient (Q):
For a general reaction:
aA (aq) + bB (aq) ↔ cC (aq) + dD (aq)
the reaction quotient (Q) is:
𝑄= [𝐶] 𝑐 [𝐷] 𝑑 [𝐴] 𝑎 [𝐵] 𝑏
What does this look like?
Keq!
Q is used same way as Keq but uses actual concentrations while the reaction is proceeding, not when it has reached equilibrium.

4. The Relationship Between the Equilibrium Constant:
If Qc > Kc: There are too many products. Reaction moves left (from products to reactants)
If Qc < Kc: There are too many reactants. Reaction moves right (from reactants to products)
What if Qc = Kc ?
Then system is at equilibrium!

5. Think mathematically….

6. Example:
Calculate the Qc for the following reaction if these concentrations are present: [N2] = 0.10 M, [H2] = M and [NH3] = 0.20 M.
Is the system at equilibrium if Kc of the reaction is 0.40?
N2 (g) + 3H2 (g) ↔ 2NH3 (g)

7. Example continued… N2 (g) + 3H2 (g) ↔ 2NH3 (g) Calculate Qc:
Q= [N H 3 ] 2 N 2 [ H 2 ] 2 = [0.20 ] [0.30 ] 3
Q = 14.8
2. Compare Qc to Kc:
14.8 > 0.4
Qc > Kc (reaction shifts right to left (P to R)