Chemical Equilibrium

Physical Equilibrium

AND

• Describe physical and chemical equilibrium • Describe the conditions needed for equilibrium.

• Write the equilibrium law given the chemical equation for the equilibrium system.

Kinetics applies to the speed of a reaction.

Equilibrium applies to the extent of a reaction.

2 Na (aq) + Cl2 (aq) 2 NaCl (aq)

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

Very few reactions actual proceed to completion.

Many reactions proceed in both directions “Reversible”

CLOSED CONTAINER:

1. More molecules become vapour, more molecules collide with surface and condense.

2. As evaporation increases - condensation increases until there is a balance.

Balance is (dynamic) equilibrium liquid-vapour equilibrium

Dynamic equilibrium exists when the rate of the forward process is equal to the rate of the reverse process.

Equilibrium will be reached only with:1. Sealed container 2. Constant condition

This environment is known as a closed system.

H2O (l) H2O (g)

Saturated solution:

A saturated solution is in equilibrium between the undissolved solute and the dissolved solute.

Known as solubility equilibrium.

C6H12O6(s) C6H12O6(aq)

Any reversible physical process, in which the rate of the forward process is equal to the rate of the reverse process, is known as physical equilibrium.

Other examples of physical equilibria include:

· Vapour Pressure· Solubility · Solid-liquid (melting point) · Liquid-vapour (boiling point)

Chemical Equilibrium

Car exhaust:

smog2 NO2 (g) N2O4 (g)

smog

At equilibrium (te):1. Rate of the forward reaction EQUALS the

rate of the reverse reaction – indefinitely.

2. Concentrations of both reactants and products will be CONSTANT – not same, but constant.

- constant macroscopic properties -

aA + bB cC + dDEquilibrium RegionKinetic Region

aA + bB cC + dDEquilibrium RegionKinetic Region

Chemical equilibrium:

State where the concentrations of reactants and products remain constant over time because the rates of the forward reaction equal the reverse reaction.

The Equilibrium Law

Law of mass action (equilibrium law):

A ratio to show the relationship between [reactants] and [products] at equilibrium.

The value of this ratio is called the equilibrium constant.

aA + bB cC + dD

kf [A]a[B]b = kr [C]c[D]d

At equilibrium rates are equal:

rateforward = kf [A]a[B]b ratereverse = kr [C]c[D]d

If the forward and reverse reactions were elementary reactions, the rate laws would be:

aA + bB cC + dD

The ratio of rate constants was condensed to one constant, Kc, called the equilibrium constant.

The law of mass action or equilibrium law then becomes

=

kf

kr [A]a[B]b

[C]c[D]d

=Kc [A]a[B]b

[C]c[D]d =Kc Reactants

Products

Homogeneous equilibria:Reactants and products are all in either gas (g) or aqueous (aq) phase.

N2 (g) + 3 H2 (g) 2 NH3 (g)

Writing the Equilibrium Law

=Kc [N2][H2]3[NH3]2

Heterogeneous equilibria - reactants and products in more than one state.

When writing the mass action expression, omit solids (s) or liquids (l).

Concentration of solids and liquids don't change at constant temperatures.

C(s) + H2O(g) CO(g) + H2(g)

=Kc[CO][H2]

[H2O]

The Equilibrium Constant

As with most constants, the only factor affecting Kc is temperature.

The Kc can indicate whether there are more products or reactants at equilibrium:

=Kc [A]a[B]b

[C]c[D]d =Kp PA

a PBb

PCc PD

d

Rat

e

Con

cen

trat

ion

Case 1: If Kc = 1 [products] = [reactants]

aA + bB cC + dD

Con

cen

trat

ion

Rat

e

Case 2:If Kc > 1 [product] > [reactant]

Products are favoured.

aA + bB cC + dD

Con

cen

trat

ion

Rat

e

Case 3:If Kc < 1 [product] < [reactant]

Reactants are favoured.

aA + bB cC + dD

· Most chemical reactions are reversible.

· Chemical equilibrium can only occur in a closed system.

· Equilibrium is when rates are equal AND the [ ]s remains constant.

· Kc can be used to determine the equilibrium position.