CH 18:

CHEMICAL E

QUILIBRIU

M

SECTION 1

8.2

SHIFTI

NG

EQUIL

IBRIU

M

REVERSIBLE REACTIONS

Reversible Reactions: one in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously.

The double arrow tells you that the reaction is reversible.

2 SO2(g) + O2(g) ⇄ 2 SO3(g)

Chemical Equilibrium: when the rates of the forward and reverse reactions are equal, the reaction has reached a state of balance.

At chemical equilibrium, no net change occurs in the actual amounts of the components of the system.

The relative concentrations of the reactants and products at equilibrium constitute the equilibrium position of a reaction.

Think of a 2-story mall with an escalator. You have 100 people on the top floor and 50 people on the bottom floor. If 10 people want to move to the top floor, how can this happen while still keeping 100 people on the top and 50 people on the bottom?

You will have to have 10 people go down the escalator at the same time as 10 people go up the escalator.

People are moving at the same RATE!

FACTORS AFFECTING EQUILIBRIUM: LE CHÂTELIER’S PRINCIPLE

Le Châtelier’s Principle: If a stress is applied to a system in dynamic equilibrium, the system changes in a way that relieves the stress.

There are three factors that we will talk about that can affect equilibrium.

CONCENTRATIONIf you add more of a reactant, the

reaction goes toward the products.If you take away some of a reactant, the

reaction goes toward the reactants. If you add more of a product, the

reaction goes toward the reactants.If you take away some of a product, the

reaction goes toward the products.

TEMPERATUREIf heat is a product and you add heat, the

reaction shifts toward the reactants.If heat is a reactant and you add heat,

the reaction shifts toward the products. If heat is a product and you take away

heat, the reaction shifts toward the products.

If heat is a reactant and you take away heat, the reaction shifts toward the reactants.

PRESSURE – ONLY GASES!!If you increase pressure, the

reaction shifts towards the side with less moles.

If you decrease pressure, the reaction shifts towards the side with more moles.

VOLUME – ONLY GASES!!If you increase volume, it is the same as decreasing pressure, so the reaction shifts towards the side with more moles. If you decrease volume, it is the same as increasing pressure, so the reaction shifts towards the side with less moles. Because it is the same as changing pressure, we do not consider this to be a 4th factor.

CATALYSTSAdding a catalyst has no effect on the equilibrium, it will only help to reach equilibrium faster.

REACTIONS TO COMPLETION

A reaction is considered to “go to completion”, when almost all of the ions are removed from the solution.

This depends on the solubility of the product formed, and if it is soluble, then on its degree of ionization.

FORMATION OF A GAS

Gases are not very soluble, so when a gas is formed and the reaction container is open to the air, the gas will escape and the reaction will go almost to completion.

FORMATION OF A SLIGHTLY IONIZED PRODUCT

This occurs with the neutralization reactions of acids and bases.

HCl(aq) + NaOH(aq)

H3O+(aq) + Cl-

(aq) + Na+(aq) + OH-

(aq) Na+

(aq) + Cl-

(aq) + 2H2O(l)

H3O+(aq) + OH-

(aq) 2H2O(l)

Water only slightly ionizes, so it exists as mainly H2O molecules.

FORMATION OF A PRECIPITATE

If a product is insoluble (a precipitate), then when the product forms, it cannot dissolve to allow the reaction to go in the reverse direction.

NaCl(aq) + AgNO3(aq) NaNO3(aq) + AgCl(s)

SECTION 1

8.1

THE

NATURE

OF

CHEMIC

AL

EQUIL

IBRIU

M

EQUILIBRIUM EXPRESSIONS

Equilibrium Constant: Keq is the ratio of product concentrations to reactant concentrations at equilibrium.

nA + mB ⇄ xC + yD

Keq = [C]x [D]y

[A]n [B]m

WRITE EQUILIBRIUM EXPRESSIONS:

1. H2 + I2 ↔ 2HI

2. 2HgO ↔ 2Hg + O2

3. 2SO2 + O2 ↔ 2SO3

4. N2 + 3H2 ↔ 2NH3

22

2

IH

HIKeq

22

2

23

OSO

SOKeq

322

23

HN

NHKeq

2

22

HgO

OHgKeq

EQUILIBRIUM CONSTANTS

To find an Equilibrium Constant, plug in the concentrations of the reactants and products into the equilibrium expression and solve!

Keq > 1, products favored at equilibrium

Keq < 1, reactants favored at equilibrium

Keq does not have any units.

CALCULATING Keq

A liter of a gas mixture at equilibrium at 10°C contains 0.0045 mol of N2O4 and 0.030 mol of NO2. Write the expression for the equilibrium constant and calculate Keq.

N2O4(g) ⇄ 2NO2(g)

Keq = [NO2]2 = (0.030 mol/L)2

[N2O4] = 0.0045 mol/L

Keq = 0.20

CALCULATING Keq

An equilibrium mixture of N2, O2, and NO gases is determined to consist of 6.4 mol/L of N2, 1.7 mol/L of O2, and 1.1 mol/L of NO. What is the Keq

for this system?

N2 + O2 ⇄ 2NO

Keq = [NO]2

[N2] x [O2]

Keq = (1.1)2

(6.4) x (1.7) Keq = 0.11