rate laws and order of reaction read 6.3 p 372-378 q 1-6 p 377
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Rate Laws and Order of Reaction Read 6.3 p 372-378 Q 1-6 p 377. Concentration and Rate. Each reaction has its own equation that gives its rate as a function of reactant concentrations. this is called its Rate Law - PowerPoint PPT PresentationTRANSCRIPT
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ChemicalKinetics
Rate Laws and Order of Reaction
Read 6.3 p 372-378
Q 1-6 p 377
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ChemicalKinetics
Concentration and Rate Each reaction has its own equation that
gives its rate as a function of reactant concentrations.
this is called its Rate Law
• A rate law is an equation expressing the rate of a reaction in terms of the [molar] of the species involved in the reaction.
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ChemicalKinetics
aA + bB cC + dD
• The rate is expressible as rate = k[A]m[B]n
• The powers m and n are kinetic orders,• A and B are the chemical substances• k is the rate constant, different values for
each rxn
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ChemicalKinetics
Order of a Reaction• The “order” of a chemical reaction is the number
of chemical [] terms upon which the rate depends
• Rate = k[A] - first order rate; the rate only depends on one [] term
• Rate = k[A]2 - second order rate
• Rate = k[A][B] - second order rate
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ChemicalKinetics
Concentration and Rate
Compare Experiments 1 and 2:when [NH4
+] doubles, the initial rate doubles.
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ChemicalKinetics
Concentration and Rate
Likewise, compare Experiments 5 and 6: when [NO2
-] doubles, the initial rate doubles.
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ChemicalKinetics
Concentration and Rate
This equation is called the rate law, and k is the rate constant.
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ChemicalKinetics
Rate Laws• Exponents tell the order of the reaction with
respect to each reactant.• This reaction is
First-order to [NH4+]
First-order to [NO2−]
• The overall reaction order can be found by adding the exponents on the reactants in the rate law.
• This reaction is second-order overall.
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ChemicalKinetics
Recall………………………..• A rate law shows the relationship between the reaction
rate and the concentrations of reactants. For gas-phase reactants use Partial Pressure instead of [A].
• k is a constant that has a specific value for each reaction.• The value of k is determined experimentally.
“Constant” is relative here- k is unique for each rxnk changes with T
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ChemicalKinetics
Temperature and Rate
• Generally, as temperature increases, so does the reaction rate.
• This is because k is temperature dependent.
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ChemicalKinetics
Determining Rxn Order from Experimental Data – Ex.1
• Four experiments were conducted to discover how the initial rate of consumption of BrO3
- ions in the rxn varies as the concentrations of the reactants are changed.
• Use the experimental data in the table below to determine the order of the reaction with respect to each reactant and the overall order.
• Write the rate law for the rxn and determing the value of k.
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ChemicalKinetics
Data – Ex. 1
Initial [ ]mol L-1
Initial Rate
Experiment BrO3- Br- H+ mol L-1 s-1
1 0.10 0.10 0.10 1.2x10-3
2 0.20 0.10 0.10 2.4x10-3
3 0.10 0.30 0.10 3.5x10-3
4 0.20 0.10 0.15 5.4x10-3
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ChemicalKinetics
Homework
• Pg. 377 # 1-6
• Lab Exercise 6.4.1 The Sulfur Clock “determine the order of the reaction” p.404-405
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ChemicalKinetics
Review of Chemical KineticsFirst order Second order Second order
Rate Laws
Graph of rate vs concn
Linear Exponential Exponential
-dependent on one molecule
-dependent on two molecules
-dependent on two molecules
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ChemicalKinetics
First-Order ProcessesConsider the process in which methyl isonitrile is converted to acetonitrile.
CH3NC CH3CN
How do we know this is a first order rxn?
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ChemicalKinetics
Reaction MechanismsRead 6.4 p 383-390
The sequence of events that describes the actual process by which reactants become products is called the reaction mechanism.
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ChemicalKinetics
Reaction Mechanisms
• Reactions may occur all at once or through several discrete steps.
• Each of these processes is known as an elementary reaction or elementary process.
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ChemicalKinetics
Reaction Mechanisms
• The molecularity of a process tells how many molecules are involved in the process.
• The rate law for an elementary step is written directly from that step.
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ChemicalKinetics
Multistep Mechanisms
• In a multistep process, one of the steps will be slower than all others.
• The overall reaction cannot occur faster than this slowest, rate-determining step.
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ChemicalKinetics
Slow Initial Step
• The rate law for this reaction is found experimentally to be
Rate = k [NO2]2
• CO is necessary for this reaction to occur, but the rate of the reaction does not depend on its concentration.
• This suggests the reaction occurs in two steps.
NO2 (g) + CO (g) NO (g) + CO2 (g)
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ChemicalKinetics
Slow Initial Step• A proposed mechanism for this reaction is
Step 1: NO2 + NO2 NO3 + NO (slow)
Step 2: NO3 + CO NO2 + CO2 (fast)
• The NO3 intermediate is consumed in the second step.
• As CO is not involved in the slow, rate-determining step, it does not appear in the rate law.
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ChemicalKinetics
Fast Initial Step
• The rate law for this reaction is found (experimentally) to be
• Because termolecular (= trimolecular) processes are rare, this rate law suggests a two-step mechanism.
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ChemicalKinetics
Fast Initial Step• A proposed mechanism is
{Step 1 is an equilibrium- it includes the forward and reverse reactions.}
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ChemicalKinetics
Fast Initial Step
• [NO] [Br2] = [NOBr2]
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ChemicalKinetics
Questions
• Q 2 p 390• Q 1-3 p 391
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ChemicalKinetics
Unit Review
• Chapter 5: Self Quiz Q 1-18 p 355 Q 2,4,6,9,14,16
• Chapter 6: Self Quiz 1-18 p 407, Q 4,7,910,13-16 p 408-409