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