# Chemical Equilibrium

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• Published : April 25, 2013

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Chapter 15 Chemical Equilibrium

Equilibrium

The Concept of Equilibrium
N2O4 (g) 2 NO2 (g)

Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate.

Equilibrium

The Concept of Equilibrium
• As a system approaches equilibrium, both the forward and reverse reactions are occurring. • At equilibrium, the forward and reverse reactions are proceeding at the same rate. Equilibrium

A System at Equilibrium
Once equilibrium is achieved, the amount of each reactant and product remains constant.

Equilibrium

Depicting Equilibrium
In a system at equilibrium, both the forward and reverse reactions are being carried out; as a result, we write its equation with a double arrow N2O4 (g) 2 NO2 (g)

Equilibrium

The Equilibrium Constant

Equilibrium

N 2 O 4 (g)

2NO 2 (g)

• Remember both forward and reverse reactions are elementary reactions

Equilibrium

The Equilibrium Constant
• Forward reaction:
N2O4 (g) → 2 NO2 (g)

• Rate law:
Rate = kf [N2O4]

Equilibrium

The Equilibrium Constant
• Reverse reaction:
2 NO2 (g) → N2O4 (g)

• Rate law:
Rate = kr [NO2]2

Equilibrium

The Equilibrium Constant
• Therefore, at equilibrium Ratef = Rater kf [N2O4] = kr [NO2]2 • Rewriting this, it becomes

kf kr

[NO2]2 = [N2O4]

Equilibrium

The Equilibrium Constant
The ratio of the rate constants is a constant at that temperature, and the expression becomes

kf Keq = kr

[NO2]2 = [N2O4]
Equilibrium

We learn following things from these reactions about equilibrium: 1. When a mixture of reactants and products is formed in which the concentration no longer change with time, it indicates that an equilibrium is reached. 2. For an equilibrium to occur neither reactant or product can escape. 3. At equilibrium the ratio of concentrations remains constant. Equilibrium

The Equilibrium Constant
• To generalize this expression, consider the elementary reaction

aA + bB

cC + dD

• The law of mass action states that if the system is at equilibrium at a given temperature, then the following ratio is a constant

[C]c[D]d Kc = [A]a[B]b

The equilibrium constant for this reaction would be Kc i.e. the constant when molarity of the solution is considered. This relationship is called equilibriumconstant expression. Equilibrium

• The Law of Mass Action expresses the relationship between the concentrations of the reactants and products present at equilibrium.

[C]c[D]d Kc = [A]a[B]b
Equilibrium

The equilibrium constant expression depends only on the stoichiometry of the reaction, not on its mechanism. • And remember that the reactants go into the denominator. • Kc is independent of the initial concentration of the reactants and products, but on the concentrations at the equilibrium. Equilibrium

• The equilibrium constant is written without a unit.

Equilibrium

What Are the Equilibrium Expressions for These Equilibria?

Equilibrium

Kc is independent of the initial concentration of the reactants:

N2O4 (g) → 2 NO2 (g) As you can see, the ratio of [NO2]2 to [N2O4] remains constant at this temperature no matter what the initial concentrations of NO2 and N2O4 are Calculate the Kc for yourself.

Equilibrium

The Equilibrium Constant
Because pressure is proportional to concentration for gases in a closed system, the equilibrium expression can also be written

(PC)c (PD)d Kp = (PA)a (PB)b
Equilibrium

• The numerical value of Kc is different than the numerical value of Kp. We must indicate the subscript c or p

Equilibrium

Relationship between Kc and Kp
• From the ideal gas law we know that

PV = nRT
• Rearranging it, we get

n P= RT V
Equilibrium

n is nothing but molarity ( moles / liter) V
So for substance A we can write P=

nA V
P RT

RT

P = [A] RT or [A] =

Equilibrium

Relationship between Kc and Kp
aA + bB cC + dD

[C]c[D]d Kc = [A]a[B]b

PA [A]= RT

(PC)c...