matriculation chemistry reaction kinetics part 1.pdf
TRANSCRIPT
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8/12/2019 Matriculation Chemistry Reaction Kinetics part 1.pdf
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Objectives:
1. Define reaction rate, average rate,
instantaneous rate and initial rate.2. Determine the reaction rate based on a
differential equation.
11.0 REACT!" #"ETC$
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Chemical kinetics is the stud% of the rates of
chemical reactions, the factors that affect theserates, and the reaction mechanisms b% &hich
reactions occur.
'Time'!(timum %ield'!(timum conditions
control over reaction,
obtain (roducts economicall%,
using o(timum conditions
m(ortant
industrial (rocess
REACT!" #"ETC$
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rate )d*+
dt d*+ ) change in concentration of +
+ecause *A decreases &ith time, d*A is negative.
rate ) d*A ) change in concentration of A
E-am(le A B
Rate of reaction
/Reaction rateis the change in the concentrationof a reactant or a (roduct &ith time.
/nit of rate mol '1 s'13/rate
dt ) (eriod of time
Time
1
d*A
dt'
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A +
time
rate ) ' d*A
dt
rate )d*+
dt *A 4
*+ 5
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/ The average rateis the rate over a (eriod of time.
/ The rate of reaction at a given time is called an
instantaneous rate of reaction./ The instantaneous rate at the beginning of a
reaction is called theinitial rate of reaction.
/ nstantaneous rate is determined from a gra(h of
concentration vs time b% dra&ing a line tangent tothe curve at that (articular time.
Rate of reaction
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Reaction6
72!2(aq)72!(l)8 9 !2(g)
Reaction rates are obtained
from the slo(es of the straight
lines
An average ratefrom the
(ur(leline.
The instantaneous rateatt ):00 s from the redline.
The initial ratefrom the blue
line.
Rate of reaction
blue red
(ur(le
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instantaneous rate= rate at a specific time
+r2aq38 7C!!7 aq3 2+r'aq38 278aq38 C!2g3
average rate) 'd*Br2
dt) '
*+r2final ; *+r2initial
tfinal ' tinitial
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The differential rate equation
Rate )dt
d[D]d1
dtd[C]
c1
dt]d[B
b1
dtd[A]
a1
===
Consider the reaction, aA 8 b+ cC 8 dD
A differential rate equation enables the relationshi(bet&een the rate of disa((earance of reactants and
the formation of (roducts.
a,b,c and d are the stoichiometric coefficients
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E-am(le6
The formation of "7:,
The differential rate equation
"2g3 8 :72g3 2"7:g3
The differential rate equation is
dt
]d[NH
2
1
dt
]d[H
3
1
dt
]d[N 322==Rate )
The equation means that the rate of disa((earance
of "2is 1
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Consider the reaction, 27 728 2,
determine the rate of disa((earance of 7 &hen
the rate of 2formation is 1.= - 10'> ? s'1.
dt]d[I
dt]d[H
dtd[HI]
21 22 ==
Example 1:
Rate )
Rate )dt
]d[I
dt
d[HI]
2
1 2=
dt
]d[I2 ) 1.= 10'>
Solution:
dt
d[HI]) 2 1.= 10'> ) !" 1#$"% s$1
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E&ERC'SE 1:
7%drogen gas (roduced non(olluting (roduct is
&ater va(our &hen react in !2 due to this reaction
has been used for fuel aboard the s(ace shuttle,
and ma% be used b% Earth'bound engines in the
near future. 272g3 8 !2g3 272!g3
/ E-(ress the rate in terms of changes in *72,
*!2 and *72! &ith time.
/ @hen *!2 is decreasing at 0.2: mol '1s'1, at
&hat rate is *72! increasing
(0.46 mol L-1
s-1
)
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Consider the reaction,
"!g3 8 !2g3 2"!2g3.
$u((ose that at a (articular time during the
reaction nitric o-ide "!3 is reacting at the rate of
0.0>> ? s'1
a3 At &hat rate is "!2being formed
b3 At &hat rate is molecular o-%gen reacting
Exercise (:
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Consider the reaction,
"2g3 8 :7
2g3 2"7
:g3
$u((ose that at a (articular moment during the
reaction molecular h%drogen is reacting at the rate
of 0.0B ? s'1
a3 At &hat rate is ammonia being formed
b3 At &hat rate is molecular nitrogen reacting
Exercise :