Chemical Kinetics Chapter 22 p721, 732743

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Unit 8 Kinetics and Equilibrium

• Chemical Kinetics Chapter 22 p721, 732-743
• The branch of Chemistry concerned with the
• Reaction rate
• Measured in
• Mechanism

rates of reactions and the mechanisms by which
reactions occur.
the speed at which a rxn occurs.
moles of product made ( or reactant used) per
unit time.
a series of steps by which reactant molecules
become product molecules.
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A. Collision Theory a simple mechanism

• Rxns occur when reactant molecules collide with
  enough energy (activation energy) and at the
  correct angle (orientation)
• This is called an effective collision.
• ex. H2(g) I2(g) ? 2HI(g)

Enough energy
Correct orientation
Activated complex
Not enough energy
H2 I2 ? H2I2 ? 2HI
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Collision, Not Enough Energy
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Collision, Incorrect Orientation
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Effective Collision correct angle and
activation energy
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B. Factors Effecting Reaction Rate

• Anything that can increase the number of
  collisions and/or make the collisions that do
  occur more effective, will speed up a rxn.
• Temperature (Heat) inc temp ? molecules move
  faster ? more collisions and more effective
  collisions ? inc rxn rate
• Concentration of reactants inc concentration ?
  more molecules ? more collisions ? inc rate
• Pressure on Gases inc pressure ? dec volume ?
  more collisions ? inc rate

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Collisions Changing Temperature
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Collisions Changing Concentration
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Collisions Changing Pressure
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• 4. Surface Area inc surface area between phases
  in a heterogeneous system ? more collisions ? inc
  rate rate
• 5. Catalyst a substance that inc the rate of a
  rxn without being used up in the reaction ?
  causes collisions to be more effective ? inc rate
• 6. Nature of reactants ? more reactive substance
  ? more effective collisions ? inc rate
• ex. Al HCl ? AlCl3 H2 moderate
  rate
• Cu HCl ? CuCl2 H2 slow rate
• Ca HCl ? CaCl2 H2 fast rate

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Collisions Surface Area
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C. Energy Changes During Chemical Rxns

• Activation Energy (EA) Energy required to start
  a chemical rxn.
• Potential Energy of Reactants (Hreact) the
  amount of energy contained in the bonds of the
  reactant molecules.
• Potential Energy of Products (Hprod) products
• Heat of Reaction ( ?Hrxn) the heat change
  during a chemical reaction
• ?Hrxn Hprod H react

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D. Endothermic and Exothermic Reactions

• Exothermic Products have less energy than
  reactants, Hprodlt Hreact then ?H neg. and is
  written as a product when written in an equation.
• ex. 2H2 O2 ? 2H2O 483.6kJ ?H
  483.6kJ
• Endothermic Products have more energy than
  reactants, Hprod gt Hreact then ?H pos. and is
  written as a reactant when written in an
  equation.
• ex. N2 2O2 66.4kJ ? 2NO2 ?H 66.4kJ
• Values for Heats of Reaction are given in Table
  I.
• What is the ?H value for the combustion of CH4?

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E. Potential Energy Diagrams

• A Potential Energy Diagram is a graph of the P.E.
  change over time as a reaction occurs.
• ex. H2 I2 ? H2I2 ? 2HI

POTENTIAL
H2I2
ENERGY
EA
2HI
HAct Comp
?HRxn
HProd
H2 I2
HReact
TIME (REACTION COORDINATE)
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Potential Energy Diagram an Unsuccessful
Collision
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Potential Energy Diagram a Successful Collision
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F. Endo/Exo and the Potential Energy Diagram

• Endothermic Rxns absorb energy, HprodgtHreact
• and, on the P.E. Diagram Hprod is higher than
  Hreact.

• Exothermic Rxns emit energy, HprodltHreact and,
• on the P.E. Diagram Hprod is lower than Hreact.

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G. Catalyst and the P.E. Diagram

• A Catalyst Changes the reaction mechanism by
  making a different activated complex that is more
  stable, lower energy.

HAct Comp
EA
Cat
Catalyzed reaction pathway
EA and HAct Comp become lower Cat changes Act!
HAct Comp

• This gives the reaction a lower energy pathway to
  make the products, with a lower activation
  energy. This makes collisions more effective!

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H. Reverse Rxn. A rxn in which product
molecules can collide to reform reactant
molecules. ex. rechargeable battery
A B ? AB
A B ? AB
EA For
EA Rev
A B
?H
AB
FWD
RVS
The only value that changes is EA because the
reverse reaction starts with the products. ?H is
the same value, but has opposite sign.
Notice ?H EA For EA Rev
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II. Equilibrium Chap. 16 p535 to 540 and p548
to 555

• Equilibrium can only exist for a reversible
  process in a closed container. ( called a closed
  system)
• At equilibrium, the rate of a forward process
  equals the rate of a reverse process!
• At equilibrium, a change appears to stop.
• At equilibrium, amounts of reactant and products
  no longer change but amounts are not equal.

Ratefor Raterev
Amt. of reactant ? Amt. of product
These values dont change Amounts, Pressure,
Color,
Temperature, Volume
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Example of an Equilibrium
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Chemical Equilibrium
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A. Types of Equilibrium

• Phase Equilibrium Two phases of a pure
  substance in a closed container (thermos bottle)
  will reach equilibrium. ex. H2O(l) ? H2O(g)

• Solution Equilibrium Saturation equilibrium
  between dissolved and undissolved solute.

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ex. C12H22O11(s) ? C12H22O11(aq)
Saturated sugar solution
Dissolved sugar molecules
Undissolved sugar cube

• Chemical Equilibrium A reversible chemical rxn
  in a closed container will reach equilibrium.
• ex. H2(g) I2(g) ? 2HI(g)

Start of Rxn, only reactant molecules
5 H2s 4 I2s
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During Rxn
4 H2s 3 I2s 2 HIs
At Equilibrium
2 H2s 1 I2 6 HIs
Notice when reactants change to products, other
products change to reactants
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H2
I2
HI
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For.
Rev.
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B. LeChateliers Principle

• When a stress is placed on a system at
  equilibrium, the equilibrium point will shift to
  relieve the stress.
• What is a stress? the addition of more of a
  reactant or product, changing temperature or
  pressure. All of these changes put a stress on a
  closed system.
• What is a shift? shift right means that the
  forward rxn speeds up more than the reverse,
  setting up a new equilibrium in which more
  product is made. Shift left means the reverse
  speeds up more making more reactants.

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1. Changing Concentration of a Reactant

• Adding more of a reactant (inc. concentration)
  causes more collisions between reactant
  molecules, making more product

Add more I2 to our previous equilibrium 2H2s,
4I2s, 6HIs
Notice adding more of a reactant makes more
product and less of the other reactant a shift
right! 1H2, 3I2s, 8HIs
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• Eventually, as more product is made, the reverse
  reaction speeds up too until its rate equals the
  rate of the forward reaction, and a new
  equilibrium is reached!
• This new equilibrium will contain more of the
  products and less of all reactants except the one
  added.
• Add react. ? Eq. shifts right ? more prod. made
• Add prod. ? Eq. shifts left ? more react. made
• Remove react.? Eq. shifts left? more react.
• Remove prod.? Eq. shifts right? more prod.
• Add/Away Take/ Toward

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• ex. 2CoCl2 2HCl heat ? 2CoCl3 H2O
• If more HCl is added to the above
  equilibrium, in which direction will the
  equilibrium shift?
• How will the amount of CoCl2 change?
• Amount of CoCl3?
• H2O? amount of or
  concentration of
• If more H2O is added?
• CoCl2?

HCl?
CoCl3?
If H2O is removed?
CoCl2?
HCl?
CoCl3?
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• ex. 2K2CrO4 2HCl ? K2Cr2O7 2KCl H2O
• If HCl (an acid) is removed by neutralizing
  it with NaOH (a base), which direction will the
  equilibrium shift and how will amounts of other
  substances change?
• Remove (take) HCl, equilibrium shifts to the
  left
• amount of
  K2CrO4 increases
• amount of
  K2Cr2O7 decreases
• amount of KCl
  decreases
• amount of H2O
  decreases

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2. Changing Temperature

• When you increase the temperature on a system at
  equilibrium, you increase the rate of both the
  forward and reverse reactions, but not equally!
• The endothermic direction will speed up more than
  the other direction causing an equilibrium shift
  in that direction.
• Since changing temp. changes the amount of heat
  energy to the reaction, and since heat energy can
  be written into reactions, we can still use the
  add/away, take/toward rule!

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• ex. 2CoCl2 2HCl heat ? 2CoCl3 H2O
• If the temperature is increased at constant
  pressure, in which direction will the equilibrium
  shift? How will the amounts of all chemicals
  change?
• Inc Temp? Add Heat ? Rxn shifts to the Right
• CoCl2 and HCl will decrease
• CoCl3 and H2O will increase
• ex. N2(g) 3H2(g) ? 2NH3(g) 91.8kJ
• Should the temperature be increased or
  decreased in order to make more product, NH3?

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3. Changing Pressure

• Inc Pressure on an equilibrium causes the Volume
  of Gases only to decrease.
• The reaction will shift to relieve that stress
  Shift toward the side with fewer gas molecules.
  ex. N2(g) 3H2(g) ? 2NH3(g)

Original Equilib.
Inc. Pressure
Molecules too close together. Fewer
molecules lowers pressure
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• ex. N2(g) 3H2(g) ? 2NH3(g)
• If the pressure is increased on the above
  equilibrium, in which direction will the
  equilibrium shift and how would the concentration
  of all substances change?
• Inc P ? Dec Vol? shift to side with fewer gas
  mol. Shift Right N2 and H2 dec
• NH3 inc
• If the pressure on the reaction is decreased,
  in which direction will equilibrium shift?
• Dec P? Inc Vol ? shift to side with more gas
  mol.
• Shift Left

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4. Adding a Catalyst

• Adding a Catalyst speeds up a rxn in both the
  forward and reverse directions.

EA
EA
for
rev

• It does this by lowering the Activation Energy
  for both the forward and reverse rxn equally,
  both rxns speed up equally. Therefore, a
  catalyst does not shift an equilibrium.

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III. Thermodynamics and Spontaneous Rxns

• Thermodynamics is the study of energy changes.
• For a reversible rxn, one direction will be
  observed to occur spontaneously in nature, the
  other direction will only occur by the addition
  or removal of energy. (One direction is said to
  be favored over the other.)
• There are two rules in Thermodynamics that decide
  which direction is favored, that is, the
  spontaneous direction.

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A. Rules for Determining Favored Direction

• First Law of Thermodynamics (Rule 1) In an open
  system, changes tend to occur spontan-eously if
  they lead to lower energy (enthalpy).
• Enthalpy potential energy or heat content (H)
• ?H also called change in Enthalpy
• If ?H is negative (exothermic), forward rxn is
  more likely to be favored.
• If ?H is positive (endothermic), reverse rxn is
  more likely to be favored.

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2. Second Law of Thermodynamics (Rule 2)

• In an open system, changes tend to occur
  spontane-ously if they lead towards more disorder
  (entropy).
• Entropy (S) the measure of the disorder in a
  system ?S the entropy change
• Low Entropy ?S Pos
  High Entropy

Solid Liquid
Gas
Molecule Smaller Molecules Individual
Atoms
Substance Solution
Heterogeneous Mixture
?S Positive forward rxn is more likely
spontaneous ?S Negative reverse rxn is more
likely spontaneous
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• ex. Tell the changes lead to an increase (?S
  pos) or a decrease (?S neg) in Entropy.
• 1. CaCO3(s) ? CaO(s) CO2(g)
• 2. S8(s) 8O2(g) ? 8SO2(g)
• 3. NaOH(s) Na(aq) OH (aq)

H2O
B. Combining the Two Rules
?H ?S Is Forward Rxn Spontaneous?
( ) ( )
Yes
( ) ( )
No, Reverse is.
( ) ( )
Maybe, at higher temp.
( ) ( )
Maybe, at lower temp.