Post on 21-Dec-2015
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