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Chapter 17

Reaction Rates and Equilibrium

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Collision Theory

When one substance is mixed with another, the two substances do not react on a macroscopic basis, but react as their individual particles (atoms, molecules, or ions) come together.

Collision theoryvideo

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Lab 1Lab 1

Factors Affecting Rates of Reaction Factors Affecting Rates of Reaction

Create Data TablesCreate Data Tables Do Part C,B,D,ADo Part C,B,D,A Graph Parts C, DGraph Parts C, D

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The factors that affect how these particles collide are the factors that influence the rate of a reaction.

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Factors influencing the likelyhood of a reaction occurring:

1) Force of collision is sufficient

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2) Orientation of colliding particles during collision is exact

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3) Must meet the minimum activation energy requirement for

the specific reaction

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In order for the reaction to occur, the particles involved must collide with each other, the more often the particles collide, the faster the reaction occurs. Not every collision results in a reaction.

activated complex

The activated complex is the temporary arrangement of atoms as they change from reactants into products.(transition state)

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Successful collision

Unsuccessfulcollision

Orientation

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The colliding molecules must have enough energy to react and form an activated complex. This minimum amount of energy is called the activation energy.

activation energy

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Rate of a Reaction = speed of reactants turning into products

Can be measured in :

grams of reactant consumed/secondor

grams of product produced/second

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Effect of the Nature of the Reactant

The nature of the reactants involved will determine the kind of reaction that occurs.

Reactions with bond rearrangements or electron transfer take longer than reactions

without these changes. Ionic reactions (such as double displacement or

neutralization) occur almost instantly.

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Effect of Temperature on Reaction Rate

Increasing the temperature increases

the rate of a reaction. At higher temperatures, the particles have more energy, move faster and collide more

frequently.

Increasing the temperature 10 degrees celcius generally doubles the reaction

rate.

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Effect of Concentration on Reaction Rate

Increasing the concentration of the reactants increases the rate of a reaction. When there are more

particles per unit volume, they will collide more frequently, causing the

rate to increase.

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Effect of Particle Size/Surface Area

Decreasing particle size/increasing surface area increases the rate of a reaction. When the surface area is increases, there is more contact

between the reactants, the number of collisions increase, and therefore the

rate of reaction also increases.

2020

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Not Cattle List!!

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Catalyst - any substance that is added to the reaction to

INCREASE the rate. The catalyst DOES NOT react with any substance to produce product.

Inhibitor - Any substance added to slow down the rate of

reaction. Works against the catalyst.

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C2H2

catalyst

H2 C2H4

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A catalyst increases the rate of a reaction by lowering the activation energy.

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Equation relating the rate of a reaction to the concentrations

of the reactants and the specific rate constant.

Rate Law Expression

2NO + 2H2 N2 + 2H20

Rate = k[NO]2[H2]2

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Rate = k[NO]2[H2]2

Rate = speed of reactants turning into products[ ] = the concentration ofk = specific rate constant for a reaction - if the rate is fast k will be high - if the rate is slow k will be low

2 (exponent) is used to predict the order of the reaction. The order reflects the affect of doubling the concentration of a reactant on the overall speed or rate of reactants becoming products.

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Order of reaction affect on the rate of rxn when [ ] of reactant is “doubled”

0

1st

2nd

3rd

4th

No affect

Rate doubles (2)1 = 2x faster

Rate quadruples (2)2 = 4x faster

Rate octuples (2)3 = 8x faster

Rate goes crazy (2)4 = 16x faster

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Rate = k[NO]2[H2]2

What is the affect of doubling the concentrationOf each of the reactants in the above expression?

N2 + 3H2 2NH3

Write the rate law expression for the above reaction and predict the affect of doubling the concentration of each of the reactants. Also predict the overall reaction rate

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In-Depth Rate lawIn-Depth Rate law

Rate LawsRate Laws

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Reaction Mechanism

Most reactions occur in a series of steps. Each step normally involves the collision of only two particles.

There is little chance of three or more particles colliding with the proper position and sufficient energy to cause a reaction.

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If a reaction consists of several steps such as the following:

A B B C C final product

One of the steps will be slower than all the others. This step is called the rate determining step. The other faster steps will not affect the rate of the reaction. The series of steps that must occur for a reaction to go to completion is called the reaction mechanism.

Reaction Mechanism

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Old Lady Driveror

The rate of a reaction isdetermined by the slowest

step in the reaction mechanism

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Elementary steps -1st step is the rate determiningstep

intermediate

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Assignment 1Assignment 1

Calculate tangents (rates of reaction) on Part C Calculate tangents (rates of reaction) on Part C graphs of Lab 1graphs of Lab 1

(see figure 17.8 as an example)(see figure 17.8 as an example) Do Q 4,6,21-28 Chp 17Do Q 4,6,21-28 Chp 17 Do 17ADo 17A OBWS 1-4OBWS 1-4

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Entropy

Systems tend to go from a state of order(low entropy)To a state of maximum disorder (high entropy)spontaneously.

A measure of the disorder of system

Law of disorder

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Entropy changes can be predicted

I2 (s) I2 (g)

Solids – low entropyLiquids –avg entropyGases – high entropy

OrYou can predict the entropy by comparingThe # of moles in the equation.

2H20(g) + 2Cl2(g) 4HCl(g) + O2(g)

Fewer moles means lower entropy

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2H2S + 3O2 2H2O + 2SO2

(188.7)(205.O)(205.6J/Kmol) (248.5)=+ +2mol 3 2 2

1026.2 J/K = 874.4J/K

A (-) entropy indicates a decrease in entropy

S = Products - Reactants

S =874.4J/K - 1026.2J/K

S = -151.8J/K

Entropy( S) Calculations

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Spontaneous Reactionsare reactions that are known to produce the written products.

Copy table 17.2 page 408

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A reaction tends to be spontaneous if :

-the reaction is exothermic heat is released

(- Enthalpy)

-the entropy of the products is greater than the entropy of the reactants (+ Entropy)

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When a reaction occurs some energy known as free energy of the system becomes available to do work.

Free Energy

Free energy may be available but is not always used efficiently. - Auto engine uses only 30% of free energy of burning gasoline. 70% is lost as friction and heat.

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Spontaneous reactions release free energy and are said to be exergonic.

(- Gibbs free energy)

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Nonspontaneous reactions absorb free energy and are said to be endergonic.

(+ Gibbs free energy)

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Nonspontaneous reactions can be made spontaneous by:

-increasing the temperature

-by linking it to a spontaneous reaction that produces a large amount of free energy

Which makes the nonspontaneous reaction go!!

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C + O2 CO2

(0.0kj/mol) + (0.0kj/mol) = (-394.4kj/mol)

(0.0kj/mol) + (0.0kj/mol) = (-394.4kj/mol)1mol 1 1

A (-) free energy means the reaction is spontaneous

G = Products - Reactants

G = -394.4kj - 0.0kj

G = -394.4kj

Gibb’s Free Energy Calculation

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Free Energy Calculations

G = H - T S

H = Enthalpy (heat energy) (kJ/mol)Table 8.1 Pg 190

G = Gibb’s Free energy (kJ/mol)Table 17.4 Pg 414

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T = temperature (Kelvin)

C + 273= K

S = entropy (disorder) (j/K-mol)

Table 17.1 pg 407

G = H - T S

*All are calculated using Products – Reactants Method

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CaCO3 CaO + CO2 at 25 C

G = -178.5kJ/mol – [(298K)x(164.7J/Kmol)]

G = -227.6 kJ/mol

G = H - T S

Negative G means the reaction is spontaneous

= -178.5 kJ/mol – 49080.6 J/mol

= -178.5 kJ/mol – 49.1 kJ/mol

Is this reaction spontaneous? Try getting the Gibb’s Free energy.

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Assignment 2Assignment 2

17B Entropy Problems17B Entropy Problems 17C Gibb’s Free Energy Problems17C Gibb’s Free Energy Problems

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Reversible Reactions

The conversion of reactants into productsand the conversion of products intoreactants occuring at the same time.

A + B C

Note double arrows

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When the forward and reverse reactionsare occurring at the same rate(speed) the reaction is said to be in a state of chemical equilibrium

The equilibrium position can be shownby the double arrows and a 3rd arrow

A B1% 99%

A B99% 1%

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Catalysts will have no effect on the equilibrium because they speed upboth the forward and reverse reactionsequally.

A Bcatalyst

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Assignment 3Assignment 3

Lab E1 : Analogy for an Equilibrium Lab E1 : Analogy for an Equilibrium ReactionReaction

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Equilibrium constants (Keq)-Indicate if the reverse reaction will

favor reactants or products at equilibrium

aA + bB cC + dD

Keq = [C]c [D]d

[A]a [B]b

Keq > 1 more products than reactants at eqKeq < 1 more reactants than products at eq

Keq expression

productsreactants

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Assignment 4Assignment 4

IIinitial initial CChange hange EEquilibriumquilibrium

Section 16.3 Equilibrium Calculations for Section 16.3 Equilibrium Calculations for Homogeneous Systems pg 656-661,665Homogeneous Systems pg 656-661,665

(see Section 16.3 Summary pg 665)(see Section 16.3 Summary pg 665)

17D Equilibrium Calculations17D Equilibrium Calculations

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Henry Louis Le Chatelier

If a chemical system at equilibrium experiences a change in concentration, temperature, or total pressure; the equilibrium will shift in order to

minimize that change.

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Le Châtelier’s Principle

If a stress is applied to a system at equilibrium, the equilibrium will shift to relieve the stress.

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Equilibrium = A state in which no net change takes

place in a system

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In a In a reaction equilibriumreaction equilibrium, both the forward , both the forward and reverse reactions are taking place and reverse reactions are taking place simultaneously, at the same rate, so that no simultaneously, at the same rate, so that no net change occurs.net change occurs.

N2 (g)   +   3H 2(g)                           2NH3 (g)

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Dynamic Equilibrium =

An equilibrium in which two or

more changes are taking place

simultaneously, but at the same rate.

Also called…

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How can you shift a system thatis already in equilibrium?

By adding a stress on the system!!

Why would anyone want to shift a system in equilibrium?

To make more product!!

LIKE…

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Concentration Change

Reactants Products

Increase [reactant] = shifts to use up the added reactants and produce more products Shifts right

Increase [product] = shifts to use up added product and produce more reactants Shifts left

Reactants Products

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Decrease [reactant] = shifts to produce more reactants Shifts left

Decrease [product] = shifts to produce more product Shifts right

Reactants Products

Reactants Products

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6565Adding heat to increase stress

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Effect of Temperature on a System Effect of Temperature on a System In EquilibriumIn Equilibrium

VideoVideo

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A + B C + heat

A + B C + heat

Increase = favors the endothermic direction.

The reaction shifts in the direction that uses up the added energy.

Temperature Change

Decreasing = favours the exothermic direction.

The reaction shifts in the direction that produces energy.

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NO (g) NO2 (g)

colorless brown

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Pressure/Volume Change

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Increasing Pressure or Decreasing Volume favors fewer gas molecules.

The reaction shifts to relieve the pressure.

2A + B C

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Decreasing Pressure or Increasing Volume favors more gas molecules.

The reaction shifts to restore the pressure.

2A + B C

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Assignment 5Assignment 5

Online Lab : Le Chatelier’s PrincipleOnline Lab : Le Chatelier’s Principle Complete OBWSComplete OBWS Review Sheet 12Review Sheet 12 TestTest