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Ch 15 Rates of Chemical Ch 15 Rates of Chemical ReactionsReactions
Chemical Kinetics is a study of the rates of chemical reactions.
Part 1 macroscopic levelwhat does reaction rate mean?how are reaction rates determined experimentally?how do factors like temp and conc influence rates?
Part 2 microscopic levelreaction mechanism detailed pathway taken by atoms and
molecules in the reaction in order to control the reaction
Ch 15 Rates of Chemical Ch 15 Rates of Chemical ReactionsReactions
The rate of a chemical reaction is the change The rate of a chemical reaction is the change of concentration of a substance (reactant) per of concentration of a substance (reactant) per unit time.unit time.
Generally Generally conc reactant conc reactant
timetime
Slope of the line changes Slope of the line changes
When Conc is large, Time is small, When Conc is large, Time is small,
When Conc is small, Time is long.When Conc is small, Time is long.
Ch 15 Rates of Chemical Ch 15 Rates of Chemical ReactionsReactions
Reaction Rate TermsReaction Rate Terms
Concentration in mol/L Concentration in mol/L [ square [ square brackets ]brackets ]
Changes Changes X = XX = Xfinal - final - XX initial initial
Units for reaction rates mol / L Units for reaction rates mol / L .. time time Rate Expression for this reactionRate Expression for this reaction
2 N2 N22OO55 -> 4 NO -> 4 NO2 2 + O + O22
1 1 [N[N22OO55]]1 1 [NO[NO22]][O[O22]]
4 4 t t 2 2 t t tt
Ch 15 Rates of Chemical Ch 15 Rates of Chemical ReactionsReactions
For a chemical reaction to occur, reactants For a chemical reaction to occur, reactants must must physically collide, physically collide, with sufficient energy, with sufficient energy, With appropriate geometry With appropriate geometry For the reaction to occur. Rates are dependent For the reaction to occur. Rates are dependent
on these variables. on these variables.
Ch 15 Rates of Chemical Ch 15 Rates of Chemical ReactionsReactions
Factors that affect the speed of a reactionFactors that affect the speed of a reaction
Concentration - greater concentration of Concentration - greater concentration of reactions results in the faster rate.reactions results in the faster rate.
Temperature - for endothermic reactions, Temperature - for endothermic reactions, faster moving molecules react at a faster rate.faster moving molecules react at a faster rate.
Catalyst - substances that accelerate a Catalyst - substances that accelerate a chemical reaction, but but are not transformed chemical reaction, but but are not transformed by the reaction.by the reaction.
Effect of Concentration on Reaction Effect of Concentration on Reaction RateRate
The rate of a reaction is proportional to the The rate of a reaction is proportional to the concentration of reactants concentration of reactants
aA + bB -> xXaA + bB -> xX Rate = k [A]Rate = k [A]mm[B][B]nn
k is the proportionality constant called k is the proportionality constant called the rate constantthe rate constant [A] and [B] are the concentrations of [A] and [B] are the concentrations of A and BA and B
m and n are determined experimentallym and n are determined experimentally
Effect of Concentration on Reaction Effect of Concentration on Reaction RateRate
Rate = k [A]Rate = k [A]mm[B][B]nn
The order of a reactant is the exponent m or nThe order of a reactant is the exponent m or n The order of a reaction is the sum of the The order of a reaction is the sum of the
exponentsexponents If the exponent is 1, doubling the concentration, If the exponent is 1, doubling the concentration,
doubles the ratedoubles the rate If the exponent is 2, doubling the concentration, If the exponent is 2, doubling the concentration,
quadruples the rate.quadruples the rate. If the exponent is 0, doubling the concentration If the exponent is 0, doubling the concentration
has no effect on the rate.has no effect on the rate.
Effect of Concentration on Reaction Effect of Concentration on Reaction RateRate
Rate = k [A]Rate = k [A]mm[B][B]nn
The rate constant is kThe rate constant is k k is a proportionality constant that relates rate k is a proportionality constant that relates rate
and concentration at a given temperature.and concentration at a given temperature.
Effect of Concentration on Reaction Effect of Concentration on Reaction RateRate
Determining the Rate Equation for a Determining the Rate Equation for a Reaction.Reaction.
Rate = k [CO]Rate = k [CO]mm[NO[NO22]]nn
Determining the Rate Equation for a Determining the Rate Equation for a Reaction.Reaction.
Rate = k [NO]Rate = k [NO]mm[O[O22]]nn
Determining the Rate Equation
for a Reaction.
Concentration-Time Relationships:Concentration-Time Relationships:Integrated Rate LawIntegrated Rate Law
ln ln [R][R]tt = - kt = - kt
[R][R]oo
Concentration-Time Relationships:Concentration-Time Relationships:Integrated Rate LawIntegrated Rate Law
Second-Order Reactions suppose R -> is Second-Order Reactions suppose R -> is second order then second order then
- - [R][R] = k[R] = k[R]22 tt
1 1 _ _ 1 1 = kt = kt
[R][R]tt [R] [R]oo
Concentration-Time Relationships:Concentration-Time Relationships:Integrated Rate LawIntegrated Rate Law
1 1 _ _ 1 1 = kt = kt
[R][R]tt [R] [R]oo
Concentration-Time Relationships:Concentration-Time Relationships:Integrated Rate LawIntegrated Rate Law
1 1 _ _ 1 1 = kt = kt
[R][R]tt [R] [R]oo
Concentration-Time Relationships:Concentration-Time Relationships:Integrated Rate LawIntegrated Rate Law
Zero-Order Reactions suppose R -> is zero Zero-Order Reactions suppose R -> is zero order then order then
- - [R][R] = k[R] = k[R]00 tt
[R][R]oo - [R] - [R]tt = kt = kt
Concentration-Time Relationships:Concentration-Time Relationships:Graphical Analysis MethodGraphical Analysis Method
[R][R]oo - [R] - [R]tt = kt = kt
Concentration-Time Relationships:Concentration-Time Relationships:Graphical Analysis MethodGraphical Analysis Method
ln ln [R][R]tt = - kt = - kt
[R][R]oo
Concentration-Time Relationships:Concentration-Time Relationships:Graphical Analysis MethodGraphical Analysis Method
1 1 _ _ 1 1 = kt = kt
[R][R]tt [R] [R]oo
Concentration-Time Relationships:Concentration-Time Relationships:Graphical Analysis MethodGraphical Analysis Method
Concentration-Time Relationships:Concentration-Time Relationships:Integrated Rate LawIntegrated Rate Law
Concentration-Time Relationships:Concentration-Time Relationships:Integrated Rate LawIntegrated Rate Law
Concentration-Time Relationships:Concentration-Time Relationships:Graphical AnalysisGraphical Analysis
Half-Life and First-Order ReactionsHalf-Life and First-Order Reactions
Concentration-Time Relationships:Concentration-Time Relationships:Integrated Rate LawIntegrated Rate Law
Half-Life and First-Order ReactionsHalf-Life and First-Order Reactions
Concentration-Time Relationships:Concentration-Time Relationships:Integrated Rate LawIntegrated Rate Law
Half-Life and First-Order ReactionsHalf-Life and First-Order Reactions
ln [R]t = - kt
[R]o
Concentration-Time Relationships:Concentration-Time Relationships:
ln ln [R][R]tt = - kt = - kt
[R][R]oo
Particulate View of Reaction RatesParticulate View of Reaction RatesCollision TheoryCollision Theory
The reacting molecules must collide with The reacting molecules must collide with one anotherone another
The reacting molecules must collide with The reacting molecules must collide with one another with sufficient energyone another with sufficient energy
The reacting molecules must collide in an The reacting molecules must collide in an orientation that can lead to rearrangement orientation that can lead to rearrangement of the atoms.of the atoms.
Particulate View of Reaction RatesParticulate View of Reaction RatesCollision TheoryCollision Theory
The reacting molecules must collide with The reacting molecules must collide with one anotherone another
The rate of reactions is primarily related to the The rate of reactions is primarily related to the number of collisions which is related to the number of collisions which is related to the concentration of molecules.concentration of molecules.
The rate of a reaction is related to the The rate of a reaction is related to the concentration of each reactant.concentration of each reactant.
Collision Theory: Concentration and Collision Theory: Concentration and Reaction Rate.Reaction Rate.
The rate of molecular reactions is related to the number of collisionsWhich is related to the concentration. Collisions directly related to Concentrations
Collision Theory: Temperature, Reaction Collision Theory: Temperature, Reaction Rate and Activation EnergyRate and Activation Energy
Recall the Boltzman distribution of molecular Recall the Boltzman distribution of molecular energiesenergies
The reacting molecules must collide with one another The reacting molecules must collide with one another with sufficient energy with sufficient energy
Activation Energy, EActivation Energy, Eaa, minimum energy required for , minimum energy required for molecules to react.molecules to react.
Collision Theory: Temperature, Reaction Collision Theory: Temperature, Reaction Rate and Activation EnergyRate and Activation Energy
Increasing the Temperature Increases the Increasing the Temperature Increases the number of number of molecules with sufficient energy to react.molecules with sufficient energy to react.
Temperature, Reaction Rate and Activation Temperature, Reaction Rate and Activation EnergyEnergy
The reacting molecules must collide in an orientation The reacting molecules must collide in an orientation that can lead to rearrangement of the atoms. A that can lead to rearrangement of the atoms. A graph of this is described as the reaction pathway.graph of this is described as the reaction pathway.
Reactant molecules approach each other with Kinetic Reactant molecules approach each other with Kinetic energyenergy Kinetic energy decreases, potential increasesKinetic energy decreases, potential increases
Reactant Molecules collide and bonds rearrangeReactant Molecules collide and bonds rearrange Highest potential energyHighest potential energy
Transition stateTransition state Activated complexActivated complex
Product molecules convert potential energy to Kinetic Product molecules convert potential energy to Kinetic energy as they move apart from each other.energy as they move apart from each other.
Temperature, Reaction Rate and Activation Temperature, Reaction Rate and Activation EnergyEnergy
Temperature, Reaction Rate and Activation Temperature, Reaction Rate and Activation EnergyEnergy
Reaction Pathway diagramsReaction Pathway diagrams
Reaction Pathway diagramsReaction Pathway diagrams
Example:Example:
The following are the data from an experiment to assess the The following are the data from an experiment to assess the disinfection of wastewater with a given dose of chlorine. disinfection of wastewater with a given dose of chlorine. Assuming first-order kinetics, determine the rate constant.Assuming first-order kinetics, determine the rate constant.
Time, min % E. coli remaining Time, min % E. coli remaining
00 100100
1010 7070
2020 2121
3030 6.36.3
6060 0.60.6
Time, min % E. coli remaining Time, min % E. coli remaining
00 100100
1010 7070
2020 2121
3030 6.36.3
6060 0.60.6
Effect of temperature on biological reaction rate
The effect of temperature on reaction rate is given by the Arrhenius equation:
EA= activated energy, J/mol
R = Universal gas constant 8.31J/mol-K
T = Temperature in Kelvin = (oC + 273)
A = Constant (not significantly affected by small temp. change
Arrhenius EquationArrhenius Equation
Arrhenius Equation Describes all variablesArrhenius Equation Describes all variables Describes the dependence of reaction rates on Describes the dependence of reaction rates on
energy, frequency of collisions, temperature energy, frequency of collisions, temperature and collision geometryand collision geometry
rate constant = k = Arate constant = k = Aee-Ea/RT-Ea/RT
Where A represents the frequency factor andWhere A represents the frequency factor andThe rest of the equation represents the fraction of the The rest of the equation represents the fraction of the molecules with minimum energy for collisionmolecules with minimum energy for collision
R = 8.31 x 10R = 8.31 x 10-3-3 kJ/mol kJ/mol..KK
Reaction Rates: Arrhenius EquationReaction Rates: Arrhenius Equation
rate constant = k = Arate constant = k = Aee-Ea/RT-Ea/RT
R = 8.31 x 10R = 8.31 x 10-3-3 kJ/mol kJ/mol..KK
Arrhenius Equation and Activation EnergyArrhenius Equation and Activation Energy
Graph the data above and calculateThe slope. The slope = - Ea / R
Arrhenius Equation and Activation EnergyArrhenius Equation and Activation Energy
Effect of Catalysts on Effect of Catalysts on Reaction RateReaction Rate
Substances that speed up the rate of a Substances that speed up the rate of a chemical reaction by lowering the reaction chemical reaction by lowering the reaction barrier (changing the mechanism)barrier (changing the mechanism)
Heterogeneous catalysts (solid in solution)Heterogeneous catalysts (solid in solution) Homogeneous catalysts -same phaseHomogeneous catalysts -same phase
Temperature, Reaction Rate and Activation Temperature, Reaction Rate and Activation EnergyEnergy
A catalyst effects the rate of reaction by A catalyst effects the rate of reaction by impacting collision geometryimpacting collision geometry
http://www.800mainstreet.com/7/0007-http://www.800mainstreet.com/7/0007-005-rea-t-cat.html005-rea-t-cat.html
Reaction MechanismReaction Mechanism
Most reactions are bimolecularMost reactions are bimolecular An intermediate molecule is produced and An intermediate molecule is produced and
then used in the subsequent reaction(s) then used in the subsequent reaction(s) Each elementary step has its own EEach elementary step has its own Ea a and and k k
which combine to give the overall reactionwhich combine to give the overall reaction
Reaction MechanismReaction Mechanism
Mechanisms are postulated from Mechanisms are postulated from experimental data.experimental data.
Molecularity unimolecular, bimolecular Molecularity unimolecular, bimolecular and termolecularand termolecular
Reaction Mechanism, Reaction Mechanism, Molecularity, and Rxn RateMolecularity, and Rxn Rate
Reaction Mechanism, Reaction Mechanism, Molecularity, and Rxn RateMolecularity, and Rxn Rate
The molecularity of an elementary step The molecularity of an elementary step and its order are the sameand its order are the same
The rate is determined by the slowest step The rate is determined by the slowest step The rate law included all reactions The rate law included all reactions up to up to
the rate determining stepthe rate determining step
Reaction Mechanism, Reaction Mechanism, Molecularity, and Rxn RateMolecularity, and Rxn Rate
The rate is determined by the slowest step The rate is determined by the slowest step Step 1 A + B -> X + M Step 1 A + B -> X + M kk11 slow rate E slow rate Eaa is large is large
Step 2 M + A -> Y Step 2 M + A -> Y kk2 2 fast rate E fast rate Eaa is small is small
Overall Reaction 2A + B -> X + YOverall Reaction 2A + B -> X + Y
Rate = kRate = k11 [A] [B] [A] [B]
Reaction Mechanism, Reaction Mechanism, Molecularity, and Rxn RateMolecularity, and Rxn Rate
Reaction Mechanism, Reaction Mechanism, Molecularity, and Rxn RateMolecularity, and Rxn Rate
The first bimolecular step is the rate-determining step, theRate equation is Rate = k [NO2] [F2]F is an intermediate, produced in # 1 and used in #2
Reaction Mechanism, Reaction Mechanism, Molecularity, and Rxn RateMolecularity, and Rxn Rate
If this were bimolecular, this would beThe rate equation
Reaction Mechanism, Reaction Mechanism, Molecularity, and Rxn RateMolecularity, and Rxn Rate
Reaction Mechanism, Reaction Mechanism, Molecularity, and Rxn RateMolecularity, and Rxn Rate
Reaction Mechanism and Reaction Reaction Mechanism and Reaction Rate EquationsRate Equations
Experiments determine effect of conc on Experiments determine effect of conc on raterate
Proposed reaction mechanism is used to Proposed reaction mechanism is used to derive a rate equation (no intermediates)derive a rate equation (no intermediates)
A rate equation can only be based on A rate equation can only be based on elementary steps (not over all rate elementary steps (not over all rate equations)equations)
More than one mechanism can be More than one mechanism can be proposed.proposed.