© 2013 Pearson Education, Inc. Chapter 9, Section 1 General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake Chapter 9 © 2013 Pearson

© 2013 Pearson Education, Inc. Chapter 9, Section 1

General, Organic, and Biological Chemistry

Fourth EditionKaren Timberlake

Chapter 9

© 2013 Pearson Education, Inc.

Reaction Rates and Chemical Equilibrium

9.1Rates of Reactions

Lectures


Collision Theory of Reactions

A chemical reaction occurs when collisions between reactant molecules have sufficient

energy to break their bonds. molecules collide with the proper orientation. bonds between atoms of the reactants (N2 and O2)

are broken and new bonds (NO) form.

2


Collision Theory of Reactions

A chemical reaction does not take place if the collisions between reactant molecules do not have

sufficient energy to break their bonds, or if molecules are not properly aligned.

3


Activation Energy

Activation energy is the minimum energy needed for a reaction to take place or break the bonds between atoms of reactants.

4


Reaction Rate

The rate of reaction is determined by measuring theamount of reactant used up or product formed in acertain period of time.

The rate or reaction is affected by temperature changes. changes in concentrations. the presence of a catalyst.

5


Reaction Rate and Temperature

At higher temperatures, the increase in kinetic energy causes the reacting

molecules to move faster, more collisions occur, and more colliding molecules have sufficient energy to

react and form products.

For example, we refrigerate perishable foods to slowdown the reactions that cause spoilage.

6


Reaction Rate and Concentration

Increasing theconcentration of reactants increases the number of

collisions and increases the reaction

rate.

7


Reaction Rate and Catalysts

A catalyst speeds up the rate of a reaction. lowers the energy of activation. is not used up during the reaction.

Insert reaction rate and catalysis top pg 5

8


Learning Check

State the effect of each change on the rate of reactionas increases, decreases or no change.

1. increasing the temperature

2. removing some of the reactants

3. adding a catalyst

4. placing the reaction flask in ice

5. increasing the concentration of a reactant

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Learning Check

Indicate the effect of each factor listed on the rate of thefollowing reaction as increases, decreases, or does not change.

A. raising the temperature

B. removing some O2

C. adding a catalyst

D. lowering the temperature

10




9.2Chemical Equilibrium

Chapter 9Reaction Rates and

Chemical Equilibrium

© 2013 Pearson Education, Inc.Lectures


Reversible Reactions

A reversible reaction proceeds in both the forward andreverse directions. As a result there are two reactionrates: the rate of the forward reaction and the rate of thereverse reaction.

When molecules begin to react, the rate of the forward reaction is faster than the rate of the reverse reaction.

As reactants are consumed and products accumulate, the rate of the forward reaction decreases, whereas the rate of the reverse reaction increases.

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© 2013 Pearson Education, Inc. Chapter 9, Section 2 13




Suppose SO2 and O2 are present initially. As they collide,the forward reaction begins.

2SO2(g) + O2(g) 2SO3(g)

As SO3 molecules form, they also collide in the reversereaction that forms reactants. This reversible reaction iswritten with a double arrow.

forward

2SO2(g) + O2(g) 2SO3(g) reverse



At equilibrium, the rate of the forward

reaction becomes equal to the rate of the reverse reaction.

the forward and reverse reactions continue at equal rates.

no further changes occur in the concentration of reactants and products.

Insert bottom page 7 right column, reaction diagram.

15


Equilibrium

Given the reaction, H2(g) + I2(g) 2HI(g) atequilibrium, the forward and reverse reactions occur atthe same time and are shown together using a doublearrow. Forward reaction: H2(g) + I2(g) 2HI(g)

Reverse reaction: 2HI(g) H2(g) + I2(g)

H2(g) + I2(g) 2HI(g)

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Equilibrium H2(g) + I2(g) 2HI(g)

Initially, the reaction flask contains only the reactants H2 and I2. The forward reaction begins to produce HI. As the reaction proceeds, there is less H2 and I2 and more HI, which

increases the rate of the reverse reaction. At equilibrium, the concentrations of reactants and product are constant. The reaction continues, with the rate of the forward reaction equal to the

rate of the reverse reaction.17


Learning Check

Write the forward and reverse reactions for the following.

CH4(g) + 2H2S(g) CS2(g) + 4H2(g)


Learning Check

Complete with equal, not equal, forward, reverse,changes, or does not change.1. Reactants form products in the ________ reaction.

2. At equilibrium, the reactant concentration _______.

3. When products form reactants, it is the _______ reaction.

4. At equilibrium, the rate of the forward reaction is ______ to the rate of the reverse reaction.

5. If the forward reaction is faster than the reverse, the amount of products ________.




9.3Equilibrium Constants





For the reaction,

the equilibrium constant expression, Kc, gives the

concentrations of the reactants and products at equilibrium,

the square brackets indicate the moles/liter of each substance, and

the coefficients b and a are written as superscripts that raise the moles/liter to a specific power.

Equilibrium Constants

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Guide to Writing the Kc Expression

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Writing a Kc Expression

Write the Kc expression for the following reaction.

Step 1 Write the balanced chemical equation.

Step 2 Write the concentrations of the products as the numerator and the reactants as the denominator.

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Writing a Kc Expression

Write the Kc expression for the following reaction.

Step 3 Write any coefficient in the equation as an exponent.

24


Learning Check

Which of the following is the correctly written Kc expression for the reaction shown below?

A. B.

C. D.

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Homogeneous, Heterogeneous Equilibrium In a homogeneous equilibrium, all reactants and

products in the reaction are in the same physical state.

In a heterogeneous equilibrium, the reactants and products are in two or more physical states.

The concentration of solids and liquids are constant,and therefore omitted from the equilibrium constantexpression.

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Learning Check

Write the equilibrium constant expression for thefollowing reaction at equilibrium.

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Guide to Calculating the Kc

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Calculating Equilibrium Constants

What is the Kc for the following reaction at 427 ˚C ifthe equilibrium concentrations are [H2] = 0.20 M,

[I2] = 0.20 M and [HI] = 1.47 M?

Step 1 Write the Kc expression for the equilibrium.

29


Calculating Equilibrium Constants

What is the Kc for the following reaction at 427 ˚C ifthe equilibrium concentrations are [H2] = 0.20 M,

[I2] = 0.20 M and [HI] = 1.47 M?

Step 2 Substitute equilibrium (molar) concentrations and calculate Kc.

30


Learning Check

Calculate the Kc for the reaction,

with the following equilibrium concentrations,

31




9.4Using Equilibrium

Constants





Reaching Chemical Equilibrium

The values of equilibrium constants can be large orsmall.

The size of the Kc depends on whether equilibrium isreached with more products than reactants (Kc is large), or

more reactants than products (Kc is small).


Kc is Large.

When a reaction has a large equilibrium constant, itmeans the forward reaction produced a large amount of

products when equilibrium was reached. the equilibrium mixture contains mostly products.


Kc is Large.


Kc is Small.

When a reaction has a small equilibrium constant, it means

the reverse reaction converted a large amount of products back to reactants.

the equilibrium mixture contains mostly reactants.


Kc is Small.


Summary of Kc Values

A reaction that favors products has a large Kc. with about equal concentrations of products and

reactants has a Kc close to 1. that favors reactants has a small Kc.


Large and Small Kc Values


Learning Check

For each Kc, indicate whether the reaction at equilibrium

contains mostly reactants or products.


Guide to Using the Kc Value


At equilibrium, the reaction, has a Kc of and containsWhat is the equilibrium concentration of PCl5? Problem Facts

Calculating Equilibrium Concentration


At equilibrium, the reaction, has a Kc of 4.2 x 10–2 and containsWhat is the equilibrium concentration of PCl5?

Step 1 Write the Kc expression for the equilibrium equation.

Step 2 Solve the Kc expression for the unknown concentration.




At equilibrium, the reaction, has a Kc of and containsWhat is the equilibrium concentration of PCl5?

Step 3 Substitute the known values into the rearranged Kc expression and calculate.


Learning Check

The Kc is 2.0 for the reaction,

If the equilibrium concentrations are and [ what is the equilibrium concentration of Br2?

A. 0.39 M

B. 0.78 M

C. 1.3 M




9.5Changing Equilibrium

Conditions:Le Châtelier’s Principle





Le Châtelier’s Principle

Chemical equilibrium can be disturbed by a change inconcentration, volume, or temperature. Altering any ofthese conditions puts the system under stress.

Le Châtelier’s principle states that when a system at equilibrium is disturbed, the system

will shift in the direction that will reduce that stress. there will be a change in the rate of the forward or

reverse reaction to return the system to equilibrium.


Changing Concentrations

For the following reaction at equilibrium,

the rate of the forward reaction is increased to relieve the stress when more H2 or I2 is added, or HI is removed.

the rate of the reverse reaction is increased to relieve the stress when H2 or I2 is removed or more HI is added.


Adding Reactant or Product

The equilibrium shifts toward products when H2(g) or I2(g) is added.

reactants when HI(g) is added.

Add H2 or I2

Add HI


Removing Reactant or Product

The system shifts toward

a reverse reaction when H2 or I2 is removed.

a forward reaction when HI(g) is removed.


Concentration Changes and Equilibrium

(a) The addition of H2 places stress on the equilibrium system ofH2 (b) To relieve the stress, the forwardreaction converts some reactants, to product, HI(g).(c) A new equilibrium is established when the rates of the forwardreaction and the reverse reaction become equal.


Effect of a Catalyst

Adding a catalyst lowers the activation energy of the forward reaction. increases the rate of the forward reaction. lowers the activation energy of the reverse reaction. increases the rate of the reverse reaction. decreases the time to reach equilibrium. has no effect on the concentrations at equilibrium.


Learning Check

Predict any shift in the forward or reverse reactions foreach of the following changes on the reaction.

1. H2S(g) is added.

2. NH3(g) is removed.

3. A catalyst is added.


Effect of Volume Change

Changing the volume of a gas mixture at equilibrium willchange the concentrations of gases in the mixture,upsetting the equilibrium. Decreasing the mixture volume will increase the

concentration of gases. Increasing the mixture volume will decrease the

concentration of gases.


Effect of Volume Change

Decreasing the volume of the gas mixture shifts theequilibrium towards the fewer number of moles.

Increasing the volume of the gas mixture shifts theequilibrium toward the larger number of moles.


Volume Decrease and Equilibrium

A decrease in the volume of the container causes the system to shift in the direction of fewer moles of gas.

An increase in the volume of the container causes the system to shift in the direction of more moles of gas.


Temperature Change and Endothermic ReactionsFor an endothermic reaction at equilibrium, heat is areactant. A decrease in temperature (T) removes heat, shifting

the equilibrium toward the reactants. An increase in temperature adds heat, shifting the

equilibrium toward the products.


Temperature Change and Exothermic ReactionsFor an exothermic reaction at equilibrium, an increase in temperature adds heat and the

equilibrium shifts toward the reactants. a decrease in temperature (T) removes heat and the

equilibrium shifts toward the products.


Effects of Changes on Equilibrium


Learning Check

Indicate if each change on a reaction at equilibrium shifts

towards the products or reactants or does not change theequilibrium.

1. adding NO(g)

2. raising the temperature

3. increasing the volume

Documents

© 2013 Pearson Education, Inc. Chapter 9, Section 1 General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake Chapter 9 © 2013 Pearson