Chemical Equilibrium

Chemical Equilibrium Rate of forward Rx = Rate of reverse Rx

As a system approaches equilibrium, both the forward and reverse reactions are occurring.

A System at Equilibrium

Once equilibrium is achieved…

the amount of each reactant and product remains constant

Equilibrium

In a system at equilibrium

both the forward and reverse reactions are being carried out

equation written with a double arrow

N2O4 (g) 2 NO2 (g)

Equilibrium

Which arrow is larger if…..

1) [Reactants] > [Products]

Would the theoretical yield by high or low?

2) [Reactants] < [Products]

Would the theoretical yield by high or low?

Law of Mass Action

Rate of any Chemical Reaction is proportional to:

1) Product of the masses of reacting substances

AND

2) Mass of each substance raised to the power of the coefficient in a chemical equation

Mathematical Relationships in Equilibrium

Forward reaction:N2O4 (g) 2 NO2 (g)

Rate law:Rate = kf [N2O4]

Mathematical Relationships in Equilibrium

Reverse reaction:2 NO2 (g) N2O4 (g)

Rate law:Rate = kr [NO2]2

Mathematical Relationships in Equilibrium

At Equilibrium:

Ratef = Ratertherefore

kf [N2O4] = kr [NO2]2

kfkr

[NO2]2

[N2O4]=

Equilibrium Equation

Ratio of the rate constants is a constant at that temperature

expression becomes

Keq =kfkr

[NO2]2

[N2O4]=

Equilibrium Expression and Constant

For any reaction:

aA + bB cC + dD

Kc = [C]c[D]d

[A]a[B]b

Equilibrium expression for this reaction

Equilibrium Constant

What Does the Value of K Mean?

• If K >> 1

• reaction is product-favored;

• product predominates at equilibrium.

• If K << 1• reaction is reactant-favored;• reactant predominates at

equilibrium.