KineticsKineticsChapter 12Chapter 12

Reaction RatesReaction Rates

Kinetics is concerned with studying the reaction Kinetics is concerned with studying the reaction mechanism of a reaction.mechanism of a reaction.

An average reaction rate describes how fast (not An average reaction rate describes how fast (not how spontaneous) a given reaction is overall. how spontaneous) a given reaction is overall.

Only positive rates are considered. In the event Only positive rates are considered. In the event of a decrease in concentration, we insert a of a decrease in concentration, we insert a negative sign into the equation.negative sign into the equation.

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Reaction Rates on a GraphReaction Rates on a Graph

Graph Graph concentration and concentration and time of reaction.time of reaction.

Slope of a line Slope of a line tangent to a point tangent to a point gives gives instantaneous rate.instantaneous rate.

2NO2 2NO + O2

Reaction Rate SummaryReaction Rate Summary

Average rate is not always equal to Average rate is not always equal to instantaneous rate.instantaneous rate.

Concentration of the products and reactants Concentration of the products and reactants change over time, producing logarithmic curves.change over time, producing logarithmic curves.

Because concentration changes, rate is Because concentration changes, rate is changing.changing.

Coefficients in the balanced reaction indicate the Coefficients in the balanced reaction indicate the relationship between the rates of reactions AND relationship between the rates of reactions AND the concentrations.the concentrations.

Rate Laws IntroRate Laws Intro

All chemical reactions are reversible.All chemical reactions are reversible. When the rate of the forward and reverse When the rate of the forward and reverse

reactions are equal, it is called equilibrium. reactions are equal, it is called equilibrium. Having the reverse reaction occur Having the reverse reaction occur

simultaneously with the forward messes simultaneously with the forward messes with concentration.with concentration.

We manipulate conditions to inhibit the We manipulate conditions to inhibit the progress of the reverse reaction when progress of the reverse reaction when measuring rates.measuring rates.

Rate Law EquationRate Law Equation

There are two types of rate laws.There are two types of rate laws. IntegratedIntegrated RegularRegular

Integrated Rate Law

Compares concentration and time

Rate = [A]/t

Regular Rate Law

Compares concentration with rate of reaction

Rate = k[A]n

Things to watch for:How is the rate defined (in reference to which reactant).

Only reactants are included in rate law.

Determining the Order of Each Determining the Order of Each Reactant in the Rate LawReactant in the Rate Law

The order is the superscript on the rate law.The order is the superscript on the rate law. These values are determined from data that These values are determined from data that

is derived from several experiments that is derived from several experiments that measure initial concentration and initial rate measure initial concentration and initial rate of reaction.of reaction.

You must scrutinize the relationship You must scrutinize the relationship between concentration and rate to between concentration and rate to determine the order of a given reactant. determine the order of a given reactant.

Each reactant can have a different order.Each reactant can have a different order.

Types of OrdersTypes of Orders

Zero OrderZero Order Change in concentration elicits no change in rate. Change in concentration elicits no change in rate. This will eliminate the reactant from the rate law.This will eliminate the reactant from the rate law.

First OrderFirst Order Direct, equally proportional relationship between Direct, equally proportional relationship between

concentration and rate.concentration and rate. ½ the concentration, ½ the rate½ the concentration, ½ the rate 3x the concentration, 3x the rate, etc.3x the concentration, 3x the rate, etc.

Second OrderSecond Order Exponential relationshipExponential relationship

½ concentration, ¼ rate.½ concentration, ¼ rate. 3x concentration, 9x rate3x concentration, 9x rate

Calculating ordersCalculating orders

Only compare one concentration variable at a time…look for reactions in which one variable

is held constant.

Reaction MechanismsReaction Mechanisms

A reaction mechanism is a series of A reaction mechanism is a series of smaller reactions by which the overall smaller reactions by which the overall reaction occurs. reaction occurs.

The smaller reactions must add together to give The smaller reactions must add together to give the complete balanced overall reaction.the complete balanced overall reaction.

The experimentally derived rate law must agree The experimentally derived rate law must agree with the rate law given in the slowest step of the with the rate law given in the slowest step of the reaction mechanism.reaction mechanism.

Types of elementary stepsTypes of elementary steps

Rate Determining StepRate Determining Step

A reaction is only as fast as its slowest step.A reaction is only as fast as its slowest step.

NO2 + NO2 NO3 + NO

NO3 + CO NO2 + CO2

We have to hypothesize the slowest step based upon an experimentally known rate.

NO2 + CO NO + CO2 has a rate law:

Rate = k[NO2]2Which is the rate

determiningstep?

Try MeTry Me The balanced equation for the reaction of The balanced equation for the reaction of

gases nitrogen dioxide and fluorine is gases nitrogen dioxide and fluorine is 2NO2NO2(g)2(g) + F + F2(g)2(g) 2NO 2NO22FF(g)(g)

The experimentally determined rate law is The experimentally determined rate law is

Rate = k[NORate = k[NO22][F][F22]]

A mechanism for this reaction is:A mechanism for this reaction is:

NONO22 + F + F22 NO NO22F + F F + F

F + NOF + NO22 NO NO22FF

Which part of the mechanism do you suppose is Which part of the mechanism do you suppose is the most likely Rate Determining Step?the most likely Rate Determining Step?

More PracticeMore Practice A proposed mechanism for a reaction is A proposed mechanism for a reaction is

slow Cslow C44HH99Br Br C C44HH9 9 ++ + Br + Br --

fast Cfast C44HH9 9 ++ + H + H22O O C C44HH99OHOH2 2

++

fast Cfast C44HH99OHOH2 2 ++ + H + H22O O C C44HH99OH + HOH + H33OO++

Write the rate law for this mechanism. What is Write the rate law for this mechanism. What is the overall balanced equation for the reaction? the overall balanced equation for the reaction? What are the intermediates in the proposed What are the intermediates in the proposed mechanism?mechanism?

Collision Theory and KineticsCollision Theory and Kinetics

Molecules must collide to react.Molecules must collide to react.

Only some Only some collisions collisions yield a reaction.yield a reaction.

Activation energy Activation energy must be overcome must be overcome to break and form to break and form necessary bonds. necessary bonds.

Increase Productivity!Increase Productivity!

TemperatureTemperature Increase temperature, increase KEIncrease temperature, increase KE # effective collisions = (total collisions)*e# effective collisions = (total collisions)*e-E-Eaa/RT/RT

Molecular OrientationMolecular Orientation What touches in a collision will determine if What touches in a collision will determine if

something happens or not.something happens or not.

Calculating Activation EnergyCalculating Activation Energy

Arrhenius Equation:Arrhenius Equation:

Natural log of Arrhenius Equation:Natural log of Arrhenius Equation:

k = [A]e-Ea/RT

ln(k) = -(Ea/R)(1/T)+ ln[A]

Try Me!!Try Me!!

The Reaction: 2 NThe Reaction: 2 N22OO55 4NO 4NO22 + O + O22

yielded the following data. Calculate Eyielded the following data. Calculate Eaa..

k (s-1) T (oC)2.0 x 10-5 207.3 x 10-5 302.7 x 10-4 409.1 x 10-4 502.9 x 10-3 60

Derive This Equation:Derive This Equation:

ln(k2/k1) = (Ea/R)(1/T1 – 1//T2)

From These Equations:From These Equations:

ln(k1) = -(Ea/R)(1/T1)+ ln(A)

ln(k2) = -(Ea/R)(1/T2)+ ln(A)

CatalysisCatalysis

Catalyst: a substance that speeds up a Catalyst: a substance that speeds up a reaction without being consumed during reaction without being consumed during the process.the process.

How? How? The presence of a catalyst causes the reaction to The presence of a catalyst causes the reaction to

take a different reaction mechanism. take a different reaction mechanism. The new reaction mechanism has a different slow The new reaction mechanism has a different slow

step.step. The new slow step has a lower activation energy.The new slow step has a lower activation energy.

What do you already know about how a catalyst works?

Heterogeneous CatalystsHeterogeneous Catalysts

Reactants are ADSORBED Reactants are ADSORBED on the surface of the catalyst.on the surface of the catalyst.

Reactants migrate on the Reactants migrate on the surface of the catalyst. surface of the catalyst.

Reactants form bonds and Reactants form bonds and are desorbed from the are desorbed from the surface of the catalyst.surface of the catalyst.

• Homogeneous catalysts exist in the same phase as the reactants…ie freon gas• Heterogeneous catalysts are in a phase…ie Pt metal