Chemical Kinetics - Lecture

7/17/2019 Chemical Kinetics - Lecture

http://slidepdf.com/reader/full/chemical-kinetics-lecture-569035655d324 1/37



Contents International University of Sarajevo

1! "actors that a##ect the chemical kinetics2! $eaction rates an% rate la&s'! Metho%s o# (etermining $eaction )r%ers*! +al#,Lives an% $a%ioactive (eca -inetics5! .he Collision Mo%el o# Chemical -inetics/! Catalsis



1! "actors .hat ##ect$eaction $ates

Chemical kinetics – Study of reaction rates, or the changes in the concentrations of

reactants and products with time

– By studying kinetics, insights are gained into how to control

reaction conditions to achieve a desired outcome

Chemical kinetics of a reaction depend on

various factors 1 !eactant concentrations

" #emperature

$ %hysical states and surface areas of reactants

& Solvent and catalyst properties



Concentration ##ects

#wo su'stances cannot react with each other unless their

constituent particles come into contact( if there is no

contact, the rate of reaction will 'e )ero

#he more reactant particles that collide per unit time, the

more often a reaction 'etween them can occur

#he rate of reaction usually increases as the

concentration of the reactants increases



ample



.emperature ##ects

Increasing the temperature of asystem increases the averagekinetic energy of its constituentparticles

*s the average kinetic energyincreases, the particles movefaster, so they collide morefre+uently per unit time andpossess greater energy whenthey collide, causing increases inthe rate of the reaction

!ate of all reactions increaseswith increasing temperature anddecreases with decreasing

temperature



+omogenous 3S+eterogeneous solutions

Homogeneous Mixtures

are composed of two or more components that are e+ually

uniformly- distri'uted throughout the system, e.amples/ Salt

dissolved in water, Soapy water, 0ydrochloric acid water, *lcohol

water, Sugar dissolved in water,

0eterogeneous 2i.tures

are composed of two or more components that are une+ually not

uniformly- distri'uted though out the system, e.amples/ Sand water li+uid solid-, 3il water immisci'le li+uids-, #a'le salt

crystals sugar crystals solids solids-,

$M4($ 66



7hase an% Sur#ace rea ##ects

If reactants are uniformly dispersed in a single homogeneoussolution, the num'er of collisions per unit time depends onconcentration and temperature

If the reaction is heterogeneous, thereactants are in two different phases,and collisions 'etween the reactants canoccur only at interfaces 'etween phases(therefore, the num'er of collisions 'etweenthe reactants per unit time is reduced, asis the reaction rate



Solvent ##ects

#he nature of the solvent can affect the reaction rates of

solute particles

Solvent viscosity is also important in determining

reaction rates

1 In highly viscous solvents, dissolved particles diffuse much more

slowly than in less viscous solvents and collide less fre+uently per

unit time

" !ates of most reactions decrease rapidly with increasing solvent

viscosity



3iscosit 8!$M4($666

#he viscosit of a fluid is a measure of its resistance ongradual deformation 'y shear stress or tensile stress



Catalst ##ects

Catalyst is a su'stance that participates in a chemical

reaction and increases the rate of the reaction without

undergoing a net chemical change itself

Catalysts are highly selective and often determine the

product of a reaction 'y accelerating only one of several

possi'le reactions that could occur



2! $eaction $ates

#he rate of a reaction is the speed at which a

reaction happens

If a reaction has a low rate, that meansthe molecules com'ine at a slower speed than a reaction

with a high rate

Some reactions take hundreds, may'e even thousands, of

years while others can happen in less than one second



$eaction $ates cont!8

– 4.pressed as the concentration of reactant consumed or the

concentration of product formed per unit time

2 ; ' C

– Units are moles per liter per unit time M 5s, M 5min or M 5h-

– #o measure reaction rates

1 initiate the reaction(

" measure the concentration of the reactant or product atdifferent times as the reaction progresses(

$ plot the concentration as a function of time on a graph(

& calculate the change in the concentration per unit time




#here is another 'ig idea for rates of reaction called collisiontheor

#he collision theory says that as more collisions in a system occur,

there will 'e more com<inations of molecules 'ouncing into each

other

If there are a higher num'er of collisions in a system, more

com'inations of molecules can occur #he reaction will go faster and

the rate of that reaction will 'e higher



Concentration

2ore su'stance in a system, greater chance that

molecules will collide and speed up the rate of the reaction

less of something, there will 'e fewer collisions and the

reaction will pro'a'ly happen at a slower speed

Sometimes when you are in a chemistry la', you will add

one solution to another 6hen you want the rate of reactionto 'e slower, you will add only a few drops at a time instead

of the entire 'eaker777



.emperature

!aising the temperature of a system, the molecules'ounce around a lot more 'ecause they have

more energ

Bouncing results in collision and collision in

com'inations 6hen you lower the temperature, themolecules are slower and collide less

#hat temperature drop lowers the rate of the reaction

Back to the chemistry la'7 Sometimes you will mi.

solutions in ice so that the temperature of the system

stays cold and the rate of reaction is slower



7ressure:

%ressure affects the rate of reaction, especially when youlook at gases

6hen you increase the pressure, the molecules have

less space in which they can move #hatgreater %ensit of molecules increases the num'er of

collisions

6hen you decrease the pressure, molecules don8t hiteach other as often #he lower pressure decreases the

rate of reaction



4nteraction o# all #actors




– #he change in the concentration of either the reactant or the

product over a period of time

– 9or a simple reaction *→ B-,

rate : ∆;B< : – ∆;*< ∆t ∆t

– S+uare 'rackets indicate concentration( and ∆ means =change in>

– Concentration of * decreases with time( and the concentration of B

increases with time



$ate La&s cont!8!

? Reaction orders

– 9or a reaction with the general e+uation

a * @ bB → c C @ d A,

the eperimentall %etermine% rate la& has the #orm rate : k ;*<m ;B<n.

– #he proportionality constant, k , is called the rate constant

1 alue is characteristic of the reaction and reaction conditions" * given reaction has a particular value of the rate constant

under a given set of conditions, such as temperature, pressure, and

solvent



Orders of Reaction –summary

In a =ero or%er reaction the rate:k since anything to the

power of is 1 #herefore the rate of reaction does not

change over time and the ;*< for e.ample- changes

linearly

In a #irst or%er reaction, the rate and concentration are

proportional #his means that if the concentration is

dou'led, the rate will dou'le

*nd finally, in a secon% or%er reaction, if the concentration

is dou'led, the rate will increase 'y a factor of & ""- #he

speed at which the ;*< changes



* =ero,or%er reaction is a reaction that proceeds at a rate that isindependent of reactant concentration

#ypically with increasing or decreasing reactants concentrations will not

affecting the o'served reaction #his means that the rate of the reactionis e+ual to the rate constant, k, of that reaction

3ne can write their rate in a form such that the e.ponent of the reactantin the rate law is

rate : – ∆;*< : k ;reactant< : k 1- : k

∆t

>ero,)r%er $eactions



!eaction rate is directly proportional to the concentration of one of thereactantsD depends linearly only on one reactant concentration

– 0ave the general form *→ products

– Aifferential rate for a firstDorder reaction is

rate : – ∆;*< : k ;*<

∆t

– If the concentration of * is dou'led, the rate of the reaction

dou'les( if the concentration of * is increased 'y a factor of 1,

the rate increases 'y a factor of 1

– Units of a firstDorder rate constant are inverse seconds, s –1

– 9irstDorder reactions are very common

"irst,)r%er $eactions



#wo kinds of secondDorder reaction

1 #he simplest kind of secondDorder reaction is one whose rate isproportional to the s+uare of the concentration of the reactant andhas the form "* → products

– Aifferential rate law is rate : – ∆;*< : k ;*<"

"∆t

– Aou'ling the concentration of * +uadruples the rate of the

reaction

– Units of rate constant is 2 –1⋅s –1 or E5mol⋅s

Secon%,)r%er $eactions



" #he second kind has a rate that is proportional to the product

of the concentrations of two reactants and has the form * @ B →

products

– !eaction is first order in * and first order in B

– Aifferential rate law for the reaction is

rate : – ∆;*< : – ∆;B< : k;*< ;B<

∆t ∆t

– !eaction is first order 'oth in * and in B and has an overall

reaction order of "

Secon%,)r%er $eactions



+al#,Lives

*nother approach to descri'e reaction rates is 'ased onthe time re+uired for the concentration of a reactant todecrease to oneDhalf its initial value

#he period of time is called the hal#,li#e of the reaction,

written as t F

#he halfDlife of a reaction is the time re+uired for thereactant concentration to decrease from ;*< to ;*< 5"

If two reactions have the same order, the faster reactionwill have a shorter halfDlife and the slower reaction willhave a longer halfDlife



$a%ioactive (eca $ates

!adioactivity, or radioactive decay, is the emission of aparticle or a photon, that results from the spontaneousdecomposition of the unsta'le nucleus of an atom

#he rate of radioactive decay is an intrinsic property ofeach radioactive isotope, independent of the chemicaland physical form of the radioactive isotope



$a%ioactive (eca $ates

• Because radioactive decay is a firstDorderprocess, the time re+uired for half of the nucleiin any sample of a radioactive isotope to decayis a constant, called the half-life of the isotope

? Isotopes with a short halfDlife decay morerapidly, undergoing a greater num'er ofradioactive decays per unit time than do

isotopes with a long halfDlife



$a%ioisotope (ating.echni?ues

• Using the halfDlives of isotopes, one can estimate theages of geological and archaeological artifacts

? #echni+ues that have 'een developed for this applicationare known as radioisotope dating techniques.

$a%iometric or radioactive dating is a techni+ue used to

date materials such as rocks or car'on, usually 'ased ona comparison 'etween the o'served a'undance of anaturally occurring radioactive isotope and its decayproducts, using known decay rates



ctivation nerg

* minimum energy activation energ@ E a- is re+uired

for a collision 'etween molecules to result in a chemical

reaction

!eacting molecules must have enough energy to

overcome electrostatic repulsion and a minimum

amount of energy to 'reak chemical 'onds so that new

ones may 'e formed



Catalsis

– Su'stances that increase the rate of a chemical

reaction without 'eing consumed in the process

– * catalyst does not appear in the overallstoichiometry of the reaction it cataly)es, 'ut it must

appear in at least one of the elementary steps in the

mechanism for the cataly)ed reaction

– Cataly)ed pathway has a lower E a activation

energy -

– Because of its lower E a, the rate of a cataly)ed



Catalsis

* catalyst decreases the height of the energy 'arrier, and its

presence increases the rates of both the forward and the reverse

reactions 'y the same amount

#here are three major classes of catalysts

1 0eterogeneous catalysts

" 0omogeneous catalysts $ 4n)ymes



n=mes

? n=mes are catalysts that occur naturally in livingorganisms and are almost all protein molecules

? * reactant in an en)ymeDcataly)ed reaction is called asu<strate!

A 4n)ymes can increase reaction rates 'y enormous

factors and tend to 'e very specific, typically producingonly a single product in +uantitative yield



n=mes

? 4n)ymes are e.pensive, and often cease functioning attemperatures higher than $GHC, and have limited sta'ilityin solution

? n=me inhi<itors cause a decrease in the rate of anen)ymeDcataly)ed reaction 'y 'inding to a specificportion of an en)yme and thus slowing or preventing a

reaction from occurring



$ea%ings 8

9irst ,follow the slides and get more infofrom the Book

0omework & topic su'mit 'y "th 2ay-/

0ow we determine the age of dinosaurs J

6rite one page essay

Chemical Kinetics - Lecture

Documents

Transcript of Chemical Kinetics - Lecture