Chemical Kinetics or Reaction Rates

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Chemical Kinetics or Reaction Rates
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You have 6 ball bearings with identical magnetic
strength but different diameters where A gt B gt C
gt D gt E gt K. Which two will hold together the
most the least.
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Reaction Rate

• The speed at which reactants become products in a
  chemical reaction
• M/s

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Kinetic Theory

• All matter is made up of tiny particles
• The particles are in constant motion
• All collisions are elastic ?

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Elastic Collisions

• Collisions in which there is no energy change

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Particles Reacting

• For particles to react, they must collide
• Reaction rate is dependent on collision rate

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Particles Reacting

• When a reaction takes place, an energy change
  occurs
• Collisions (in which reactions occur) are not
  elastic

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Reaction Rate

• Rate - dReactant/dt
• Rate dProduct/dt
• M/s

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Factors Affecting Rxn Rate

• Concentration, Temperature, Agitation, Process,
  Orientation, Catalyst, SA of Solid, Pressure of
  Gas, Reaction Mechanism

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List describe all of the factors affecting rxn
rates.
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Drill

• List at least 5 factors that will affect reaction
  rates

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Rate Expression

• An equation showing how rate depends on the
  amount of reactants in a rate determining
  reaction

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Rate Expression

• Rxn aA(aq) bB(aq)
• P
• Expr Rate kAaBb
• A molarity of A

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Rate Expression

• Rxn aA(g) bB(g)
• Product
• Exp Rate kPAaPBb

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Write Rate Expressions for
H2(g) N2(g) NH3(g) HCl(aq)
Ba(OH)2(aq) BaCl2(aq) H2O (l)
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Write Rate Expressions for
Cl2(g) HOOH (aq) P(aq)
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Reaction Order

• The total number of reactants
• Equals total of all of the coeffecients of R in
  the rxn
• Equals total exponents in rate expression

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Determining Rxn Rate

• Find rate expression ratios
• (Ratio of concentrations)x rate ratio
• x exponent

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Experimental Results
Exp A B C Rate 1 1.0 1.0
1.0 2 2 2.0 1.0 1.0
4 3 1.0 2.0 1.0 8 4 1.0
1.0 2.0 4
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Experimental Results
Exp A B C Rate 1 0.1
0.1 0.1 3 2 0.3 0.1 0.1
9 3 0.1 0.3 0.1 27 4
0.1 0.1 0.3 3
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Experimental Results
Exp R S T Rate 1) 0.25 0.20
0.10 2 2) 0.25 0.60 0.10 18 3)
0.50 0.20 0.10 4 4) 0.25 0.20
0.40 32
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Half-Life

• The time required for one half of a reactant to
  be decomposed in a reaction

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Useful Relationships
Rxn Order Rate Exp 0 Rk 1
RkA 2 RkA2
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Useful Relationships
Rxn Order Half Life 0 Ao/2k
1 0.693/k 2 1/kA
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1st Order Age Dating Formula

• t ln(Ai/Af)t1/2
• 0.693

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Useful Relationships
Rxn Order Lin Plot 0 A vs k
1 lnA vs k 2 1/A vs k
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Useful Relationships

• Rxn Order Calc R E
• 0 -dA/dt k
• 1 -dA/dt kA
• 2 -dA/dt kA2

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Clausius-Claperon Eq
(T2)(T1) k2 (T2 T1) k1
Ea R ln
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Calculate the activation energy of a reaction
whose rate constant is 2.0 x 103 at 27oC and is
2.0 x 106 at 77oC
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Experimental Results
Exp T A B Rate 1) 27oC
0.10 0.10 2 2) 27oC 0.30 0.10 18
3) 27oC 0.10 0.20 4 4) 77oC
0.10 0.10 20
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Reaction Mechanism

• The sequence of steps that make up the reaction
  process

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Reaction Mechanism
Step 1 A lt--gt B fast Step 2 B ---gt C slow Step
3 C lt--gt D fast Total A ---gt D
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Reaction Mechanism

• The rate determining step in a reaction mechanism
  is the slowest step

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Reaction Mechanism

• To solve the rate expression, you must use the
  slowest step

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Solve Rate Expression
A B C D fast 2C A G
fast 2D B K fast 2G 2K Prod.
slow
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Solve Rate Expression
X Y M N fast M N 2G
slow 2N G K fast 2G 2K Prod.
fast
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Solve Rate Expression
X Y M N fast 3M N 2G
fast 2N K fast 4G 2K
Prod. slow
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Solve Rate Expression
A B C D fast 4C A
2G fast 4D B 2K fast G K Q
W slow Q W Prod. fast
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Review
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Experimental Results
Exp T A B Rate 1) 27oC
0.10 0.10 2 2) 27oC 0.30 0.10 18
3) 27oC 0.10 0.20 8 4) 77oC
0.10 0.10 20
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Solve Rate Expression
A B C D fast 4 C A
2G fast 2 K 4D B fast G K
Q W fast 2Q 2W Prod. slow