1. Section 8.3—Reaction Quotient
How can we tell if a reaction has reached equilibrium?

2. What is the Reaction Quotient?K Q
Equilibrium Constant
Reaction Quotient
Expression is ratio of products to reactants with balanced equation coefficients as powers
Expression is ratio of products to reactants with balanced equation coefficients as powers
Only includes gases and solutions
Only includes gases and solutions
To solve for K, plug in concentrations at equilibrium
To solve for Q, plug in concentrations at any time

3. The Difference Between K and Q
What exactly is the difference?
2 H2 (g) + O2 (g)  2 H2O (g)
2 H2 (g) + O2 (g)  2 H2O (g)
The expressions for K and Q are the same.
To solve for “K”, plug in concentrations at equilibrium only.
To solve for “Q”, plug in concentrations right now.

4. Using Reaction Quotient
[products now]
[products at equilibrium] = Q K =
[reactants now]
[reactants at equilibrium]
Q = K
[now] = [equilibrium]
System is at equilibrium
Q > K
[Products now] too large
[Reactants now] too small
System will make more reactants to reach equilibrium
Q < K
[Products now] too small
[Reactants now] too large
System will make more products to reach equilibrium

5. Let’s Practice For N2 (g) + O2 (g)  2 NO (g)
if [N2] = 0.81 M, [O2] = 0.75 M and [NO] = M, is the reaction at equilibrium if K = ? If not, which way will it go to reach equilibrium?

6. Let’s Practice For N2 (g) + O2 (g)  2 NO (g)
if [N2] = 0.81 M, [O2] = 0.75 M and [NO] = M, is the reaction at equilibrium if K = ? If not, which way will it go to reach equilibrium?
Q =
Q < K
Reaction is not at equilibrium
More products will need to be made (and also thereby reducing reactants) to have Q = K
Reaction will go to the right to reach equilibrium