2 ch13 (ee) (1).pptx
TRANSCRIPT
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1
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)
Very rapid reactions
$ormation of insolu%le %ases
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Very rapid reactions
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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32Time of reaction !min"
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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33Time of reaction !min"
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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34Time of reaction !min"
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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39Time of reaction !min"
$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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airs of opposite colours are complementary
colours
5ed CyanGreenagenta
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airs of opposite colours are complementary
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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+t(er Examples o) Cloc< Reactions 6 ;
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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+t(er Examples o) Cloc< Reactions 6 ;
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
13.3 Methods of Measuring Reaction Rates !(' p.13"
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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
13.3 Methods of Measuring Reaction Rates !(' p.13"
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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
13.3 Methods of Measuring Reaction Rates !(' p.13"
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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
13.3 Methods of Measuring Reaction Rates !(' p.13"
$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