Rates ofreactionandequilibrium slides

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Reaction Rates &Reaction Rates & Equilibrium

HS Ch i t R id L i S i

Rapid Learning Centerwww.RapidLearningCenter.com/© Rapid Learning Inc. All rights reserved.

HS Chemistry Rapid Learning Series

Wayne Huang, PhDKelly Deters, PhDRussell Dahl, PhD

Elizabeth James, PhD



Learning Objectives

What Kinetics studies.

C lli i Th

By completing this tutorial you will learn…

Collision Theory.

How factors affect rate.

Dynamic equilibrium.

Equilibrium constants.

Reaction Quotients.

L Ch t li ’ P i i l

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Le Chatelier’s Principle.

Concept Map

Chemistry

Studies

Previous content

New content Kineticsstudies

Matter

ReactionsChemicalReactions

Undergo

CollisionCollisionTheory

Can be studied with

ReactionReactionRate Equilibrium

Equilibrium

Equation with Ratio of products : reactants

When it’s disturbed

Forward = reverse

uses

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Theory EquilibriumConstant

Expression Le Chatelier’sLe Chatelier’sPrinciple

disturbed, follow



Kinetics and Reaction Coordinate Diagrams

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Definition - Kinetics

Kinetics – Study of the rates of reactions.Reactants Products

Reaction Rate Rate at which reactantsReaction Rate – Rate at which reactants produce products, i.e. how fast a reaction takes place.

Rate = Δ[Product]/Δt = -Δ[Reactant]/Δt

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Definition - Collision Theory

Collision Theory – Defines 3 circumstances to be met for a collision to occur.

Reactants must collide.

Collision must be at the correct orientation.

1

2

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Collision must have minimum energy for reaction to occur.

3

Only a small fraction of collisions meet the requirements and results in a successful reaction.

Collisions Must OccurIn order for two molecules to react, they must come into contact with one another.

F

FN

O

O

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There’s no way they’ll ever react if they don’t run into one another!



Collision with Correct OrientationFor a collision to result in a chemical reaction, it must occur with the correct orientation.

F

FN

O

O

F

FN

O

O

F F NO

O

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Side-to-Side: This is not the correct orientation. The reaction will not happen.

Head-to-Head:This is the correct orientation. The reaction will happen.

Collision with Enough EnergyFor a collision to result in a chemical reaction, it must occur with the minimum energy for reaction.

NO

O

F

FN

O

O

F F NO

O

F

F

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Slow Speed: The collision does not have enough energy to produce a reaction.

High Speed: This collision had more energy (faster moving molecules). A reaction will occur.



Definition - Activation Energy

A ti ti EActivation Energy –Energy that must be overcome for a reaction to occur.

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Reaction Coordinate Diagram

Activated complexA ti ti E

Reaction coordinate diagrams show the energy changes throughout the reaction.

Products

Activated complex(Also called the transition state)

Activation Energy

Ener

gy

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ReactantsEnergy change for reaction

Reaction proceeds



Activated ComplexWhat is an “activated complex”?

gy

Reactants

Activated Complex

F2 + NO2

F NO

F + FNO2

F

Reaction proceeds

Ener

gN

OF F NO

Activated Complex

Products

F2…NO2

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FN

O

F F NO

Definition - Endo & Exothermic

Endothermic Reaction –The reaction takes inenergy the productsenergy…the products have more energy than the reactants.

Exothermic Reaction –The reaction gives off

inheat

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genergy…the products have less energy than the reactants.

Endothermic vs Exothermic Mnemonic: Endo- is to take in heat and Exo- is to give off heat = “Endo is to Enter (in) and Exo is to Exit (off)!”

offheat



Endo- and Exothermic DiagramsReaction coordinate diagrams show whether a reaction is endothermic or exothermic.

EndothermicProducts are higher energy than reactants.

Reaction proceeds

Ener

gy

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ExothermicProducts are lower energy than reactants.

Reaction proceeds

Ener

gy

Factors Affecting Reaction Rates

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Surface Area of ReactantsHow does the surface area of the reactants affect the reaction rate?

Reactants must collide in order

to react.

Larger surface area means more

particles can come in contact with each other

at the same time.

More reactants can collide at the same time and a fraction of those

will result in reaction.

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As surface area increases, reaction rate increases.

Concentrations of ReactantsHow does the concentration of reactants affect the reaction rate?

Only a small fraction of the collisions meet

the requirements and result in a

reaction.

More reactants mean more

collisions will occur.

If more collisions occur, more will

meet the requirements and result in a

reaction.

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As the reactant concentration increases, the reaction rate increases.



TemperatureHow does temperature affect the reaction rate?

If molecules are With higher Reactants must collide with at least energy equal to the activation

energy.

If molecules are at a higher

temperature, they have a

higher average kinetic energy.

genergy

molecules, collisions will have higher

energy and more often result in

reaction.

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For most reactions, as temperature increases, reaction rate increases.

Definition - Catalysts

Catalysts – Substance that increases the rate of reaction without being used upof reaction without being used up.

A + B + C D + C

“C” is the catalyst…it is present in the beginning and in the end.

Without Catalyst

20/54Ex: Enzymes are catalysts in our body.

Reaction proceeds

Ener

gy

With Catalyst (lowering activation energy)



CatalystsHow do catalysts help speed up the reaction without being used?

They increase the chances that a collision will successfully produce a reaction.

For example, catalysts hold one or more of the reactants in place to allow collisions to occur with the correct orientation.

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Once the reaction has occurred, the catalyst releases the molecule(s) and finds another one to help.

Equilibrium

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Definition - Equilibrium

Reversible Reaction – A chemical reaction that can proceed in both directions (represented b a “ ”)(represented by a “ ”).

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Equilibrium – When the rate of the forward reaction equals the rate of the reverse reaction, i.e. Δ[Product]/Δt = -Δ[Reactant]/Δt

Establishing EquilibriumIt takes time to establish equilibrium.

At first, there are only reactants present. Only the forward reaction is possible.

Reactants Products

p

But once there are products as well, they can begin to reform

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p , y greactants.The reverse reaction becomes possible. Forward rate slows and reverse rate increases until they are the same.Once the rate of the forward and reverse process are equal, it is at equilibrium.When equilibrium is established, the numbers of products and reactants doesn’t change…but the reaction keeps going.



Definition - Dynamic Equilibrium

Dynamic Equilibrium – The reaction continues toreaction continues to proceed in both directions, but at the same rate.

Th b f d t d t t l

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The number of products and reactants no longer change, it may look as though the reaction has stopped…

But the reaction continues!

Equilibrium Constants

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Definition - Equilibrium Constant

Equilibrium Constant Expression –Equation showing the ratio of the concentrations of prod cts toconcentrations of products to reactants at equilibrium.

Equilibrium Constant (K) – The

Concentration is symbolized with brackets “[A]”.

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Equilibrium Constant (K) The number calculated from the equilibrium constant expression.

“K” is different for every reaction at every temperature!

Writing Equilibrium Constant Expressions

Write the concentration of products on the top -take each one to a power of the coefficient in the b l d ti

1

To write an equilibrium constant expression:

balanced equation.Write the concentration of reactants on the bottom -also take each to the power of the balanced equation coefficient.

2

Example: Write the equilibrium constant expression for the following:2 H2 (g) + O2 (g) 2 H2O (g)22

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[H2O]

[H2] [O2]K =



Definition: Homo- and Heterogeneous Equilibrium

Homogeneous Equilibrium – All of the species are the same state of mattermatter.

Heterogeneous Equilibrium –There are at least 2 states of

2 H2 (g) + O2 (g) 2 H2O (g)

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matter.

2 H2 (g) + O2 (g) 2 H2O (l)

Concentrations of Solids and Liquids Pure solids and pure liquids have constant “concentrations”.

If concentration (Molarity) = molelitliters

And Density = gramsliters

And Molar Mass = gramsmole

Th f lid li id M l it grams / liters

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Then for a pure solid or liquid, Molarity = grams / litersgrams / mole

Or, Molarity = Density .Molar Mass

Both Density and Molar Mass are constants—they don’t change. Therefore, “concentration” of a pure solid or liquid is a constant.



“K” Expressions with Solids or Liquids

How does this affect the writing of Equilibrium Constant Expressions?If the “concentration” of a pure solid or liquid is constant, th it ill t h d i ilib i d it i t ittthen it will not change during equilibrium and it is not written in the “K” expression.

2 H2 (g) + O2 (g) 2 H2O (g)][][

][

22

2

22

OHOHK =

1

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2 H2 (g) + O2 (g) 2 H2O (l) ][][1

22

2 OHK =

H2O is not included in this “K” expression because it’s a liquid.

Only gases and solutions are included in “K” expressions!

Calculating “K” Example Example: Solve for equilibrium constant for

Fe2O3 (s) + 3 H2 (g) 2 Fe (s) + 3 H2O (g) if the following are concentrations at equilibrium: [H2] = 0.45 M and [H2O] = 0.18 M

32

32

][][

HOHK =

[H2]eq = 0.45 M[H2O]eq = 0.18 MK = ?

Note that Fe2O3 and Fe were not included in the Kexpression as they are solids!

3

3

]450[]18.0[

=K K = 0.064

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3]45.0[

Most instructors and textbooks do not require units for “K” as each one would be different.



Meaning of Equilibrium Constant What general meaning can you get from the magnitude of the equilibrium constant?

If K is very large… [Products][Reactants]

There is a much larger ratio of products to reactants at equilibrium.

The reaction is said to “lie to the right” (products are on the right).

If K is very small [Products]

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If K is very small… [Products]

[Reactants]

There is a much smaller ratio of products to reactants at equilibrium.

The reaction is said to “lie to the left”.

Using “K” to Find Equilibrium Concentration

Example: Find the equilibrium concentration for NO if the equilibrium constant for N2 (g) + O2 (g) 2 NO (g) is 1.24×10-4, and the other equilibrium concentrations are [N2] = 0.166 M and [O2] = 0.145 M

]][[][

22

2

ONNOK =

[N2]eq = 0.166 M[O2]eq = 0.145 MK = 1.24×10-4

[NO]eq = ? M

)145.0)(166.0(][1024.1

24

MMNO

=× −

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[NO]eq = 0.00173 M

( )( )( ) ][145.0166.01024.1 4 NOMM =× −



Reaction Quotient

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What is the Reaction Quotient?Reaction Quotient is “Q”.

K Q

Equilibrium Constant Reaction Quotient

Expression is ratio of products to reactants with balanced equation coefficients as powers.

Only includes gases and

Expression is ratio of products to reactants with balanced equation coefficients as powers.

Only includes gases and

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Only includes gases and solutions.

To solve for K, plug in concentrations at equilibrium.

Only includes gases and solutions.

To solve for Q, plug in concentrations at any time.



The Difference between K and QWhat exactly is the difference?

2 H2 (g) + O2 (g) 2 H2O (g)][][

][2

22

OHOHK =

][][ 22 OH

2 H2 (g) + O2 (g) 2 H2O (g)][][

][

22

2

22

OHOHQ =

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The expressions for K and Q are the same.

To solve for “K”, plug in concentrations at equilibrium only.

To solve for “Q”, plug in concentrations at any time.

Using Reaction QuotientReaction Quotient is used to determine if a system is at equilibrium…and if it’s not, which way does it need to go to get there.

[products now]

[reactants now]= Q

[products at equilibrium]

[reactants at equilibrium]=K

Q = K [now] = [equilibrium] System is at equilibrium.

[Products now] too S t ill k

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Q > Klarge.[Reactants now] too small.

System will make more reactants to reach equilibrium.

Q < K

[Products now] too small.[Reactants now] too large.

System will make more products to reach equilibrium.



Reaction Quotient ExampleExample: For N2 (g) + O2 (g) 2 NO (g), if [N2] = 0.81 M, [O2] = 0.75 M

and [NO] = 0.030 M, is the reaction at equilibrium if K = 0.0025? If not, which way will it go to reach equilibrium?

]][[][

22

2

ONNOQ =

[N2] = 0.81 M[O2] = 0.75 M[NO] = 0.030 MK = 0.0025At equilibrium = ? Q = 0.0015)75.0)(81.0(

)030.0( 2

MMMQ =

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Q < K

Reaction is not at equilibrium.

More products will need to be made (and also thereby reducing reactants) to have Q = K.

Reaction will go to the right (products) to reach equilibrium.

Le Chatelier’s Principle

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Definition - Le Chatelier’s Principle

Le Chatelier’s Principle – If a system at equilibrium is disturbed, it will shift to re-establish the equilibriumestablish the equilibrium.

A system will try to undo whatever you’ve done (de-stress or count-change).

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Le Chatelier’s Principle Mnemonic: Le Chatelier is to lesser the change (stress) added = “Le Chatelier is Lessen (the) Change.”

How does adding a reactant or product affect a system at equilibrium? Reactants Products

Increasing Concentrations

R ti hift

Adding a reactant.

Q becomes too small.

Reaction shifts to right.

(To get rid of extra reactants and make more

products.)

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Adding a product.

Q becomes too large.

Reaction shifts to left.

(To get rid of extra products and make more

reactants.)



How does removing a reactant or product affect a system at equilibrium? Reactants Products

Decreasing Concentrations

Removing a reactant.

Q becomes too large.


(make more reactants.)

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Removing a product.

Q becomes too small.


(make more products.)

How does changing the pressure affect a system at equilibrium? i.e. 2 Reactants 3 Products

Changes in Pressure

Reaction shiftsDecrease volume.

(i.e. Backward reaction ).

Pressure increases.

(i.e. Favors backward reaction).

Reaction shifts to the side with less moles (or

volume, i.e. left) of gas to decrease pressure.

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Increase volume.

(i.e. Forward reaction ).

Pressure decreases.

(i.e. Favors forward

reaction).

Reactions shifts to the side with the more moles (or volume, i.e. right ) of gas to

increase pressure.



Definition - Endo & Exothermic Reactions

Endothermic Reaction – The reaction takes in heat energy…the products have more energy than the reactants.

Energy is a reactant in the reaction.Reactants + heat Products

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Exothermic Reaction – The reaction gives off heat energy…the products have less energy than the reactants.

Energy is a product in the reaction.Reactants Products + heat

For endothermic, think of temperature (or energy) as a “reactant”: Reactants + heat Products

Temperature and Endothermic

R ti hiftIncrease

temperature of endothermic

reaction.

Increasing a “reactant”.


(get rid of extra reactants and

make more products.)

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Decrease temperature of endothermic

reaction.

Remove a “reactant”.


(make more reactants.)



For exothermic, think of temperature (or energy) as a “product”: Reactants Products + heat

Temperature and Exothermic

Increase temperature of

exothermic reaction.

Increasing a “product”.


(get rid of extra products and make more reactants.)

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Decrease temperature of

exothermic reaction.

Remove a “product”.


(make more products.)

Some changes have no effect because they do not affect the value of “Q”.

Changes that Have No Effect

Adding a pure solid or liquid reactant or g p qproduct.

Increasing pressure by adding an inert gas.

Changing the volume of a reaction with an equal number of moles of gas on each side of the reaction

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each side of the reaction.

Adding a catalyst.A catalyst will speed up how fast equilibrium is established—but not the number of reactants and products once it’s at equilibrium.



Le Chatelier’s ExamplesExample: Which way will the reaction shift for each of the

following changes:NH4Cl (s) NH3 (g) + HCl (g)

Removing NH4Cl No change (it’s a solid)Removing NH4Cl

Adding HCl

Adding Ne (g)

Decreasing volume

Example: Which way will the reaction shift for each of the following changes:

No change (it’s an inert gas)

No change (it s a solid)

(Adding a product)

(Goes to side with least gas moles)

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2 SO2 (g) + O2 (g) 2 SO3 (g) + Heat … an exothermic reaction

Increasing volume

Raising temperature

Adding O2

Removing SO2

(Goes to side with most gas moles)

(Energy is a product)

(Adding a reactant)

(Removing a reactant)

When companies need to make large amounts of product, a reaction with a very small K is a problem.

Le Chatelier in Industry

Small KSmall ratio of products to reactants.

Lots of reactants left over (wasting money) and few products made

(not making money).

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They can push the reaction towards the products.

e.g. Remove the products as they’re made, adjust pressure or temperature as needed to push it to the right.



The reaction to produce ammonia, NH3, is very important to manufacturing.

Haber Process

N + 3 H 2 NH (an exothermic reaction: ΔH = 92 kJ/mol)

In order for the reaction to occur at a reasonable rate, the temperature must be very high.

N2 + 3 H2 2 NH3 (an exothermic reaction: ΔH = -92 kJ/mol)Rate = Δ[NH3]/Δt = -(1/2)Δ[N2]/Δt = -(3/2)Δ[H2]/Δt

But when the temperature is high, the equilibrium constant is very low.

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A compromise is made and a catalyst is added to increase the rate at the lower temperature.

The reaction yields 20%...the leftover reactants are recycled and put back into the reaction again.

Le Chatelier’s Principle h ti t

Le Chatelier’s Principle h ti t

Kinetics is the study of the rates of reaction, which are affected by

Kinetics is the study of the rates of reaction, which are affected by

Learning Summary

Th ilib i t tTh ilib i t t

governs how a reaction at equilibrium will change

when disturbed.

governs how a reaction at equilibrium will change

when disturbed.

which are affected by surface area,

concentration, temperature and

catalysts.

which are affected by surface area,

concentration, temperature and

catalysts.

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The equilibrium constant give the ratio of product :

reactants with the stoichiometric ratios as

the powers.

The equilibrium constant give the ratio of product :

reactants with the stoichiometric ratios as

the powers.

Dynamic equilibrium is established when the rates of the forward and reverse

reactions are equal.

Dynamic equilibrium is established when the rates of the forward and reverse

reactions are equal.



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Reaction Rates & EquilibriumEquilibrium

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