Section 8.3—Reaction Quotient

How can we tell if a reaction has reached equilibrium?

What is the Reaction Quotient?

K Q

Equilibrium Constant Reaction Quotient

Expression is ratio of products to reactants with balanced equation coefficients as powers

Only includes gases and solutions

To solve for K, plug in concentrations at equilibrium

Expression is ratio of products to reactants with balanced equation coefficients as powers

Only includes gases and solutions

To solve for Q, plug in concentrations at any time

The Difference Between K and Q

What exactly is the difference?

2 H2 (g) + O2 (g) 2 H2O (g)][][

][

22

2

22

OH

OHK

2 H2 (g) + O2 (g) 2 H2O (g)][][

][

22

2

22

OH

OHQ

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.

Using Reaction Quotient

[products now]

[reactants now]= Q

[products at equilibrium]

[reactants at equilibrium]=K

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

Let’s Practice

For N2 (g) + O2 (g) 2 NO (g)if [N2] = 0.81 M, [O2] = 0.75 M and [NO] = 0.030 M, is the reaction at equilibrium if K = 0.0025? If not, which

way will it go to reach equilibrium?

]][[

][

22

2

ON

NOQ

Q = 0.0015)75.0)(81.0(

)030.0( 2

MM

MQ

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

Let’s Practice

For N2 (g) + O2 (g) 2 NO (g)if [N2] = 0.81 M, [O2] = 0.75 M and [NO] = 0.030 M, is the reaction at equilibrium if K = 0.0025? If not, which

way will it go to reach equilibrium?