Transcript Ch 18.2 Reversible Reactions and Equilibrium

Ch 18:
Chemical Equilibrium
Section 18.2
Shifting Equilibrium
Standards and Objectives
Standard 9
a. Students know how to use LeChatelier’s
principle to predict the effect of changes in
concentration, temperature, and pressure.
b. Students know equilibrium is established when
forward and reverse reaction rates are equal.
 Objective
 We will discuss the factors that disturb
equilibrium, discuss reactions that go to
completion, & describe the common ion effect.

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 SO (g)
3
 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.
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.
CONCENTRATION
 If 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.
TEMPERATURE
 If 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.
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 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)
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.
Common Ion Effect
 This is the phenomenon in which the
addition of an ion common to two solute
brings about precipitation or reduced
ionization.
 If you have a NaCl solution and you add HCl
acid, the extra Cl- ions from the acid will mix
with the Na+ ions to form NaCl precipitate.
Homework
 18.2 pg 604 #1-5
Section 18.1
The Nature of
Chemical Equilibrium
Standards and Objectives
Standard 9.c.
 Students know how to write and calculate an
equilibrium constant expression for a reaction.
 Objective
 We will write chemical equilibrium expressions
and carry out calculations involving them.

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
2

HI 
Keq 
H 2 I 2 
2

Hg O2 
Keq 
HgO2
2

SO3 
K eq 
2
SO2  O2 
2

NH 3 
K eq 
N 2 H 2 3
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 x 10-3
mol/L of N2, 1.7 x 10-3 mol/L of O2, and 1.1 x
10-5 mol/L of NO. What is the Keq for this
system?
N2 + O2 ⇄ 2NO
Keq = 1.1 x 10-5
Homework
 18.1 pg 595 #1, 3, 6-9