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What does the word equilibrium mean to you?
Warm Up
Chapter 18.2Reversible 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
2SO2(g) + O2(g) 2SO3(g)
Forward Reaction
Reverse Reaction
2SO2(g) + O2(g) 2SO3(g)
2SO2(g) + O2(g) 2SO3(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(80%)
B(20%)
Equilibrium does not mean 50:50!
Equilibrium Position:Kf and Kr rate constants
A B(20%) (80%)
Kf
kr
Kf is the rate constant for the forward reactionKr 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
Solving for Keq
aA + bB cC + dD
Moles of B
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) Cu(s) + 2AgCl(aq)
Keq = [AgCl]2
[CuCl2]
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 equilibriumIf Keq < 1; More reactants at equilibrium
Cool Down
H2(g) + I2(g) 2 HI(g)
What is the equilibrium expression for this reaction?
What is the equilibrium expression for this reaction?
SnO2(s) + 2CO(g) Sn(s) + 2CO2(g)
Keq =[HI]2
[H2] x [I2]
Keq =[CO2]2
[CO]2