ChemicalEquilibrium

A Balancing Act

Equilibrium

two opposing processes occurring at the same rate

a system at equilibrium is in balance

During a game, players enter and leave.

Always the same number of players on field.

H2O(l) H2O(g)

Photochromicsunglasses

AgCl + light Ag + Cl(transparent) (dark)

Reversible reaction

a chemical reaction that can occur in both the forward and the reverse directions

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

Chemical equilibrium

a state in which the forward and reverse reactions balance each other because they take place at equal rates

dynamic state; no net change

Law of chemical equilibrium

At a given temperature, a chemical system might reach a state in which a particular ratio of reactant and product concentrations has a constant value.

Equilibrium constant expression

ratio of molar concentrations of products to reactants; each raised to a power equal to coefficient in balanced equation

aA + bB cC + dD

Equilibrium constant (Keq)

numerical value of the ratio of product to reactant concentrations

constant only at a specific temperature

Value of equilibrium constant (Keq) shows the extent to which reactants are converted into products.

Keq > 1: Products are favored at equilibrium

Keq < 1: Reactants are favored at equilibrium

Homogeneous equilibrium

all reactants and products in same phase

Write an equilibrium constant expression for:

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

Keq =[NH3]2 ___________

[N2] [H2]3

Heterogeneous equilibrium

reactants and products present in more than one physical state

H2O(l) H2O(g)

Heterogeneous equilibrium

Since concentrations of pure liquids and solids remain constant, these substances are omitted from the equilibrium constant expression.

Write an equilibrium constant expression for:

H2O(l) H2O(g)

Keq = [H2O(g)]

Equilibrium constant expression Keq = . . .

Products over reactants raised to power of coefficient; leave out pure solids and liquids.

Equilibrium concentrations can vary from trial to trial.

Equilibrium position

H2(g) +I2(g) 2HI(g)

Each set of equilibrium concentrations represent an equilibrium position.

A system has only one value for Keq at a specific temperature, however there are unlimited number of equilibrium positions.

A system at equilibrium must:

•take place in closed system•temperature remain constant•all reactants and products are present (both reactions can occur)

Write the equilibrium constant expression for

N2(g) + O2(g) 2NO(g)

Calculate the value of Keq

if [N2] = 0.20 mol/L,

[O2] = 0.15 mol/L, and

[NO] = 0.0035 mol/L.Keq = 4.1 x 10-4

What does the value of Keq tell you about the equilibrium?

What happens when a system is at equilibrium and you upset the balance?

Le Chatelier’s Principle

If a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.

Le Chatelier’s Principle

Used to predict how a equilibrium system will react to changes in concentration, pressure (volume) and temperature.

Changes in Concentration

Add reactant

1. Forward rxn speeds up.

2. Over time, forward rxn slows down and reverse rxn speeds up.

3. At equilibrium, forward and reverse rxns occur at same rate, new equilibrium position established.

4. Equilibrium has shifted right, value

of Keq unchanged.

Changes in Concentration

Remove reactant

1. Reverse rxn speeds up.

2. Over time, reverse rxn slows down and forward rxn speeds up.

3. At equilibrium, forward and reverse rxns occur at same rate, new equilibrium position established.

4. Equilibrium has shifted left; value of

Keq unchanged.

Changes in Pressure (Volume)

1. For a gas, decreasing volume of container increases pressure; particles have less space, collide more frequently.

2. System will respond by trying to relieve stress (decrease pressure.)

3. Shifts to the side with fewer moles of gas.

4. Forward rxn speeds up.

5. Over time, forward rxn slows down and reverse rxn speeds up.

6. At equilibrium, forward and reverse rxns occur at same rate, new equilibrium position established.

7. Equilibrium has shifted right, value

of Keq unchanged.

Changes in Pressure (Volume)

Note: If volume is decreased concentrations of all gaseous substances increases. Side with fewer moles of gas increases more.

Changes in TemperatureReview: If H is negativerxn is exothermicheat written as product

If H is positiverxn is endothermicheat written as reactant

N2O4(g) 2NO2(g)

Changes in Temperature

H = 55.3 kJ

55.3 kJ + N2O4(g) 2NO2(g)

light dark

55.3 kJ + N2O4(g) 2NO2(g)

Le Chatelier’s Principle

Used to predict how a equilibrium system will react to changes in concentration, pressure (volume) and temperature.

Chemists can change conditions to make rxn more productive.