LECTURE 4 a - Reaction Kinetics

7/24/2019 LECTURE 4 a - Reaction Kinetics

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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$



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$



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 rate is the change in the concentration

of a reactant or a (roduct &ith time.

/ nit of rate mol '1 s'13

/ rate

dt ) (eriod of time

Time

1∝

d*A

dt'



A +

time

rate ) ' d*A

dt

rate )d*+

dt *A 4

*+ 5



/ The average rate is 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 the initial 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



Reaction6

72!2(aq) → 72!(l) 8 9 !2(g)

Reaction rates are obtained

from the slo(es of the straight

lines

An average rate from the

(ur(le line.

The instantaneous rate att ):00 s from the red line.

The initial rate from the blue

line.

Rate of reaction

blue red

(ur(le



instantaneous rate = rate at a specifc time

+r 2 aq3 8 7C!!7 aq3 2+r ' aq3 8 278 aq3 8 C!2 g 3

average rate ) 'd*Br2

dt) '

*+r 2final ; *+r 2initial

t final ' t initial



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



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 "2 is 1<: the rate of disa((earance of 72 and 1<2

the rate of formation of "7:.



Consider the reaction, 27 → 72 8 2,

determine the rate of disa((earance of 7 &hen the rate of 2 formation 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



E&ERC'SE 1:

7%drogen gas (roduced non(olluting (roduct &hich

is &ater va(our &hen reacted in !2 .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 '1 s'1, at

&hat rate is *72! increasing

(0.46 mol L-1

s-1

)



Consider the reaction,

2"!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 "!2 being formed

0.0>>

a3 At &hat rate is molecular o-%gen reacting

0.0::

Exercise (:



Consider the reaction,

"2g3 8 :72g3 → 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

a3At &hat rate is ammonia being formed0.0:

b3 At &hat rate is molecular nitrogen reacting

0.02B

Exercise :

LECTURE 4 a - Reaction Kinetics

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