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Rates of Chemical

Reactions

13.1 Rates of Chemical Reactions

13.2 Expressions of Reaction Rates in Terms of

Rates of Changes in Concentrations of

Reactants or Products

13.3 Methods of Measuring Reaction Rates13.4 Factors ffecting Reaction Rates

13

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Chemical Kinetics

A study of

(1)reaction rates

(2) the factors affecting reaction rates

(3) reaction mechanisms

(the detailed steps involved in reactions)

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Explosive reactions2H2(g) + O2(g) 2H2O(l)

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otassium reacts !ith!ater vigorously

Vigorous reactions2K(s) + 2H2O(l) 2KOH(a") + H2(g)

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Very rapid reactions

Ag+(a") + Cl#(a")AgCl(s)

$ormation of insolu%le salts

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$e3+(a") + 3OH#(a")$e(OH)3(s)


$ormation of insolu%le %ases

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H+(a") + OH#(a")H2O(l)Acid&al'ali neutraliation reactions

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*1

Ag+(a") + Cl#(a")AgCl(s)

$e3+(a") + 3OH#(a")$e(OH)3(s)

H+(a") + OH#(a")H2O(l)

All involve oppositely charged ions

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Rapid or moderate reactions

n(s) + 2Ag+(a") n2+(a") + 2Ag(s)

,isplacement reactions of metals - &

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Rapid or moderate reactions

n(s) + 2Ag+(a") n2+(a") + 2Ag(s)

Cl2(a") + 2.r(a") 2Cl(a") + .r2(a")

,isplacement reactions of metals - &

,isplacement reactions of halogens - &

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Slow reactions$ermentation of glucose

C/H12O/(a") 2C2H0OH(a") + 2CO2(g)

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Slow reactions

2nO(a") + 0C2O2(a") + 1/H+(a")

2n2+(a") + 1CO2(g) + 4H2O(l)

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Very slow reactions5usting of iron

$e(s) + 3O2(g) + 2nH2O(l) 2$e2O36 nH2O(s)

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Extremely slow reactions

CaCO3(s) + 2H+(a") Ca2+(a") + CO2(g) + H2O(l)

.efore corrosion After corrosion

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7!o 8ays to 9:press 5eaction 5ates

1* Average rate2* ;nstantaneous rate

(rate at a given instant)

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Amount is usually e:pressed in

Concentration mol dm#3

Mass g

Volume cm3

or dm3

Pressure atm

occurtochangetheforta'entime7otal

reactantaorproductaofamountinchange7otal

reactionofrateAverage

=

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Q. *3/ gof magnesium reacted !ith 0* cm3of 1* hydrochloric acid to give 3/ cm3of

hydrogen under room conditions*7he reaction !as completely in

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133

scm*

s

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g(s) + 2HCl(a")gCl2(a") + H2(g)2*(c)

mol*10molg2*3

g*3/gofmolesof=o*

1

==

mol*0dm*0dmmol1*HClofmolesof=o* 33 ==

g is the limiting reactant

mol*3mol*102reactedHClofmolesof=o* ==

,ecrease in concentration of HCl(a") in s

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g(s) + 2HCl(a") gCl2(a")+ H2(g)2*(d)

;ncrease in concentration of gCl2

(a") in

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2* ;nstantaneous rate

7he rate at a particular instant of thereaction is called the instantaneous rate*

$or the chemical reaction

a! " #$ cC " d%

)d1(

dtd@,)

c1(

dtd@C)

%1(

dtd@.)

a1(

dtd@A

rateous;nstantane

==

=

=

@B ? molarity of B

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2* ;nstantaneous rate

7he rate at a particular instant of thereaction is called the instantaneous rate*

$or the chemical reaction

a! " #$ cC " d%

)d1(

dtd@,)

c1(

dtd@C)

%1(

dtd@.)

a1(

dtd@A

rateous;nstantane

==

=

=

&nits- mol dm3s1 mol dm3min1 mol dm3h1Detc*

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'rap(ical Representation o) ReactionRates * Rate curves

! rate curveis a graph plotting the amountofa reactant or product against time*

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Consider the reaction

A . + C (reactant) (product)

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At any time t the instantaneous rateof thereaction e"uals t(e slope o) t(e tangent to t(ecurveat that point*

7he greater the slope the higher the rate of thereaction*

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&veslope of curve of reactant A

@A !ith time

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+veslope of curve of product .

@. !ith time

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7he rate at tis usually the )astestand is calledthe initial rate*7he curve is the steepest!ith the greatest

slopeat time t*

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7he rate of the reaction gradually as thereaction proceeds*

$lat curvereaction completed

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Q.3 B + E 2

Time of reaction !min"

Conc

entrationofproduct

#

!moldm

3"

A

B

C

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31Time of reaction !min"

Conc

entrationofproduct

#

!moldm

3"

A

B

C

133

mindmmol*3

dmmol0*

2

1rateAverage

==

B + E 2

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Conc

entrationofproduct

#

!moldm

3"

A

B

C

Aatrateous;nstantane

min*)&(1*/

dmmol*)&(/*

2

1 3=

13 mindmmol1*<

=

1*/

B + E 2

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Conc

entrationofproduct

#

!moldm

3"

A

B

C

.atrateous;nstantane

min1*)&(3*

dmmol2*>)&(0*1

2

1 3=

13 mindmmol*/

=

B + E 2

2*>

0*1

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Conc

entrationofproduct

#

!moldm

3"

A

B

C

Catrateous;nstantane =

B + E 2

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ethods of easuring 5eaction 5ates

A*hysical measurements

1* Continuous measurements

2 ;nitial rate measurements(Cloc' reactions)

.* Chemical measurements (7itration)

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1* Continuous measurements

9:periment is done in +,Eta'e*

7he reaction rates are determined %ymeasuring continuously a convenient property

!hich is directly proportional to t(econcentrationof any one reactant or productof the reaction mi:ture*

roperties to %e measured - FGas volume Gas pressure ass Color intensity 9lectrical

conductivity

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1*1 easurement of large volume changes

9:amples-

(1) CaCO3(s) + 2HCl(a") CaCl2(a") + H2O(l) +

CO2(g)

(2) n(s) + H2

IO

(a") nIO(a") + H2(g)

(3) 2H2O2(a") 2H2O(l) + O2(g)

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A typical la%oratory set&up for measuring thevolume of gas formed in a reaction

1*1 easurement of large volume changes

7emperature is'ept constant

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$olum

eofgasformed!cm

3"

n(s) + H2IO(a") nIO(a") + H2(g)

ratedtdJslope =

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*

(2) n(s) + H2IO(a") nIO(a") + H2(g)

Jolumeof CO2

Sigmoid curve

H2(g) is sparingly solu%le in !ater !hileCO2is "uite solu%le in !ater*

Rate

Rate

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1*2 easurement of small volume changes& ,ilatometry

i"uid phase reaction

mi:ture

Capillary tu%e

CH3COOH(l) + CH3CH2OH(l) CH3COOCH2CH3(l) + H2O(l)

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1*3 easurement of mass changes

CaCO3(s) + 2HCl(a") CaCl2(a") + H2O(l) + CO2(g)

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stop!atch

cotton !ool plug

limestone piecesof 'no!n mass

measured volume ofstandard

hydrochloric acidelectronic%alance

7he cotton !ool plug is to allo! the escape of CO2(g)%utto prevent loss of acid spraydue to spurting*

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CaCO3(s) + 2HCl(a") CaCl2(a") + H2O(l) + CO2(g)

n(s) + H2IO(a") nIO(a") + H2(g)

8hich reaction is more suita%le to %e follo!ed%y mass measurement L

Hydrogen is a very light gas*

7he change in mass of the reaction mi:turemay %e very small*

7he electronic %alance used in the schoolla%oratory may not %e sensitive enough todetect the small change*

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time

Kossof

mass

(m) mfinal? total mass loss

ratedt

dm

slope =

dtd@Hslope

+

time

mfinal&m

t

mfinal? mfinalF m

? rate 2

( m ? )

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1* Colorimetry

colour intensity @coloured species

dt

intensity)d(colourrate

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H2O2(a") + 2H+(a") + 2;(a") -a/0+ 2H2O(l)

CH3COCH3(a") + -a/0CH3COCH2;(a") + H+(a") + ;(a")

$ra/0+ HCOOH(a")2H+(a") + 2.r(a") + CO2(g)

Mn+a/0+ 1/H+(a") + 0C2O

2(a")

2n2+(a") + 1CO2(g) + 4H2O(l)

colour intensity as reaction proceeds

colour intensity as reaction proceeds

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A colorimeter

cuvettes

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.lue solutionEello!filter

Complementary colours

Eello!light

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airs of opposite colours are complementary

colours

5ed Cyan

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colours

5ed CyanGreenagenta

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colours

5ed Cyan

CEK

Green agenta

.lue Eello!

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8hen mi:ed in the proper proportion

complementary colours produce a neutral color(grey !hite or %lac')*

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--2

;3? intensity %efore a%sorption;? intensity after a%sorption

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--2

1M

;

;ncetransmittaM

=

=

;

;logA%sor%ance 1

;f ; ? ;

4? 1M

!? log11 ?

ero a%sorption

;f ; ?

4? M

!log1

complete a%sorption

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A ? %C

.eerNs la!

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C

A,eviation at higherconcentrations

A cali%ration curve is firstconstructed for AC conversion

@; 0

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time

@;2

time

A

ratedt

d@;slope 2 ==

ratedt

dAslope =

*0

1 0 f l i l

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1*0 easurement of electricalconductivity

,a"+5a/0 "C53C++5a/0 C53C++

,a"a/0" 5+l0

conducting mo#ility 6 +5 7 C53C++

conductivity

as t(e rx proceeds

1 0 f l i l

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1*0 easurement of electricalconductivity

Mn+a/0 " 185"a/0 " 9C+

a/0

Mn"a/0" 12C+g0 " :5+l0

total num#er o) ions

electrical conductivity

as t(e rx proceeds

1 / t f h

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1*/ easurement of pressure changes

dt)d(rate 7

7? total pressure of the reactionmi:ture

/

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(i) 2=O(g) + 2H2(g) =2(g) + 2H2O(g)

(ii) 3H2(g) + =2(g) 2=H3(g)

*/

At fi:ed J and 7 7n

;n %oth reactionsn as the reactions proceed

7as the reactions proceed

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g(s) + 2HCl(a") gCl2(a") + H2(g)

to data;loggerinter)ace and computer

pressure sensor

magnesium ri##on

suction

)lasy=

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t

1

9:pt* @I2O32

(a")()

@H+

(a")()

7ime ta'en

(t)to mas' the

mar' s

s1

1 *1 1 1

2 *4 1 13

3 * 1 20

*2 1 0

Q @

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Q.@

@I2O32(a")

t

1

:2

32

QQ

(a")O@I'

=t1

inear : ? 1

+t( E l ) Cl < R ti

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+t(er Examples o) Cloc< Reactions 6 ;

0;(a") + ;O3(a") + /H+(a") 3;2(a") + 3H2O(l)

Imall and fi:ed amounts of I2O32(a") and starch are

added to the reaction mi:tures in all runs*

7ime ta'en for the reaction mi:ture to turn deep %lueis measured*

;2(a") + starch deep %lue comple:

(e:cess) (fi:ed)

;2(a") + 2I2O32(a") 2;(a") + IO/2(a")(fi:ed) (fi:ed)

+t( E l ) Cl < R ti

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0;(a") + ;O3(a") + /H+(a") 3;2(a") + 3H2O(l)

;2(a") + 2I2O32(a") 2;(a") + IO/2(a")

(fi:ed) (fi:ed)

;2(a") + starch deep %lue comple:

(e:cess) (fi:ed)

.y changing the concentration of any one of thereactants deep %lue colour !ill appear in differenttime lapses a chemical cloc' R

Hallo!een cloc'

+t( E l ) Cl < R ti

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0.r(a") + .rO3(a") + /H+(a") 3.r2(a") +3H2O(l)

%&

+

%&

'r'r

'r

3.r2

(fi:ed) (fi:ed)

.r2 + methyl red colourless(e:cess) (fi:ed)

Ad f h i l

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Advantages of physical measurements

1* Iuita%le for fastreactions*

2* Imall sample sie

3* ore accurate than chemical method

(titration)

* =o interruption continuous measurements

0* Can %e automated*

,i d t f h i l t

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,isadvantages of physical measurements

1* ore sophisticated

2* ore e:pensive

3* ore specific F only suit a limited num%er of

reactions*

$ C( i l M t 4it ti M t( d 0

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$. C(emical Measurements 4itration Met(ods0

1* Itart a reaction !ith all reaction conditions%ut one fi:ed*

2* 8ithdra! and "uenchfi:ed amounts of the

reaction mi:ture at different times*

Quenching methods:

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,ilutingthe reaction mi:ture !ith a

sufficient amount of cold !ater or anappropriate solvent*

Coolingthe reaction mi:ture rapidly in ice*

Quenching methods:

7emperature

Concentration

5emovingone of the reactants or the

catalyst (if any) %y adding anotherreagent*

$ C( i l M t 4it ti M t( d 0

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$. C(emical Measurements 4itration Met(ods0

1* Itart a reaction !ith all reaction conditions%ut one fi:ed*

2* 8ithdra! and "uenchfi:ed amounts of the

reaction mi:ture at different times*

3* 7itrate the "uenched samples to determinethe concentration of one of the reactants or

products*

H+ as catalyst

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H as catalystC53C+C53 " - C53C+C5- " 5-

*4

7he reaction is "uenched %y adding to it =aHCO3(a")that removes the catalyst*

HCO3(a") + H+(a") H2O(l) + CO2(g)

H+as catalyst

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H as cata ystC53C+C53 " - C53C+C5- " 5-

*olume of ladded at the end

point (cm3)

* 1

1 &

-ime after mi/ing

(min)

>olume of l

added at the endpoint (cm3)

* &

1 1

4ption 6

4ption =

Example 13-3A

13.3 Methods of Measuring Reaction Rates !(' p.11"

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Example 13 3A

!c" (odium h

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Example 13 3A

(d) 8ame a suitable indicator for thetitration.

Ans!er!d" Meth

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Example 13 3B

6 student recorded the follo"inge/perimental results for the reaction of ?incand dilute h!drochloric acid.

@n(s) 2l(a0)

@nl2(a0)

2(g)

-ime(min)

.

1.

2.

3.

+.

*. A.

.

&.

%.

>olumeof 2(g)

produced (cm3)

1* 2A 33 3& + +1 +2 +2 +2

Example 13-3B


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Example 13 3B

(a) Blot a graph of volume of h!drogen gasproduced against time.Ans!er

!a"

Example 13-3B


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Example 13 3B

(b) Cescribe the change in the rate of thereaction using !our graph in (a).Ans!er

!+" s sho>n in the graph in !a"? the olume of heen the 1st and the 2nd minute" is

greater than that near the end of the reaction !e.g. in

the time interal +et>een the /th and the 6th minute".

Therefore? the rate of the reaction decreases >ith time.

Example 13-3B


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Example 13 3B

(c) 5/plain ho" !ou can measure the initialrate of the reaction graphicall!.Ans!er

!c" The initial rate can +e found +< determining the slope of

the tangent to the cure at time Aero.

Example 13-3B


$ac

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p

(d) Cetermine graphicall! the rate of thereaction at the *thminute. tate the unit.

Ans!er

!d" From the graph in !a"?

rate of reaction

, slope of the tangent to the cure at the ) minute

,

, 2 cm3min1

min"20!

cm34"!4/ 3

Chec' oint 13&313.3 Methods of Measuring Reaction Rates !(' p.1)"

$ac

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uggest an e/perimental method fordetermining the rate of each of the follo"ingreactions

(a) 24&2#(a0) 2$#(a0)

24+2#(a0) $2( a0)

(b) 3443(a0) $2(a0)

3442$(a0) $(a0)

(c) 2Dn4+#(a0) *24+

2#(a0) 1A(a0)

2Dn2(a0) 142(g) &24(l)

(a0)Ans!er!a" Colorimetric measurement @ titration

!+" Colorimetric measurement

!c" Colorimetric mesurement @ titration

13.4 Factors ffecting Reaction Rates !(' p.16"

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9:plain !hy sa!dust %urns e:plosively in pureo:ygen %ut slo!ly in air*

BetDs 4(in< 1

$ac

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2 Ch13 (EE) (1).pptx