Transcript Chapter 18.2 Equilibrium

Warm Up
What does the word equilibrium
mean to you?
Chapter 18.2
Reversible Reactions and
Equilibrium
Darren Dressen
Chemistry CP
Los Altos High School
Key Concepts/Tasks
 What does chemical equilibrium really mean
and how does an equilibrium change with
time?
 Be capable of writing an equilibrium
expression by just looking at a reaction.
 What does the value of Keq indicate about
the equilibrium position of a reaction?
Vocabulary
 Reversible Reaction
 Chemical Equilibrium
 Equilibrium Position
 Equilibrium Constant, Keq
 Le Chatelier’s Principle (later)
Khan’s Academy
Chapter 18.1 - Introduction to Kinetics/Reaction Rates
Chapter 18.2 - Reversible Reactions and Equilibrium
Reversible Reaction
F orw ard R eaction
2S O 2 (g) + O 2 (g)
2S O 3 (g)
R everse R eaction
2S O 2 (g) + O 2 (g)
2S O 3 (g)
2S O 2 (g) + O 2 (g)
2S O 3 (g)
Chemical Equilibrium
At chemical equilibrium, no net change
occurs in the actual amounts of the
components (reactants and products) of
the system (chemical reaction).
A common misconception: Once a reaction is
at equilibrium everything stops
Equilibrium is a dynamic process
… the reaction does not stop
Equilibrium Position
A
B
(20%)
(80%)
A
B
(80%)
(20%)
Equilibrium does not mean 50:50!
Equilibrium Position:
Kf and Kr rate constants
Kf
A
(20%)
B
kr
(80%)
Kf is the rate constant for the forward reaction
Kr is the rate constant for the reverse reaction
Equilibrium Constant, Keq
The equilibrium constant (Keq) is the ratio of
product concentrations to reactant concentrations
at equilibrium.
Keq is derived from Kf and Kr …..
Keq = Kf/Kr
Moles of B
Solving for Keq
aA + bB
cC + dD
Moles
of D
Concentration of B (M)
Exception
If a molecule in the reaction is a pure solid
or pure liquid, then it is removed from the
equilibrium expression.
2Ag(s) + CuCl2(aq)
Keq = [AgCl]2
[CuCl2]
Cu(s) + 2AgCl(aq)
Why don’t solids and liquids count?
Since concentrations of pure liquids and
solids remain constant, these substances
can be ignored in equilibrium constant
expression.
Known as a heterogeneous equilibrium
Concept question??? What is the
difference between a homogeneous and
heterogeneous equilibrium?
What Keq Tells Us
aA + bB
cC + dD
If Keq > 1; More products at equilibrium
If Keq < 1; More reactants at equilibrium
Cool Down
What is the equilibrium expression for this reaction?
H 2 (g) + I 2 (g)
2 H I(g)
[H I] 2
K eq
=
[H 2 ] x [I 2 ]
What is the equilibrium expression for this reaction?
SnO 2 (s) + 2C O (g)
Sn (s) + 2C O 2 (g)
[C O 2 ] 2
K eq =
[C O ] 2