matriculation chemistry ( reaction kinetics ) part 5
TRANSCRIPT
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CONCENTRATIONS OF REACTANTS:Reaction rates generally increase as theconcentrations of the reactants are increased.
TEMPERATURE:
Reaction rates generally increase rapidly as
the temperature is increased.
PARTICLE SIZE:
The rate increases as the smaller the size of
reacting particles .
CATALYSTS:
Catalysts speed up reactions.
Factors affecting rate of reaction
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A) CONCENTRATIONS OF REACTANTS
The frequency of collision increases increases with the
concentration
Reaction rate
collision
time
4 particle system
(2 and 2) 4 collision
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A) CONCENTRATIONS OF REACTANTS
A concentration of reactants increases, the frequency
of collision increases.
This would also result in the increase in the quantity of
effective collision. Thus the reaction rate increases.
5 particle system
(3 and 2) 6 collision
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A) CONCENTRATIONS OF REACTANTS
This observation correlates with the RATE LAW that has
been previously discussed
Reaction rate = k[ A ]x
[ B ]y
Based on this equation,
(A & B = reactants)(x & y = rate order)
Reaction rate concentration of reactants
REMINDER!Only in zero order reactions, the rate of reaction is not
dependant upon the concentration of the reactants.
(depending on its rate order)
Reaction rate = k[ A ]0 = k (constant)
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B) TEMPERATURE
As temperature increases, kinetic energy, ofmolecules increases
So, more collisions occur in a given time
Furthermore, the higher the kinetic energy, the
higher the energy of the effective collisions.
So more molecules will have energy greaterthan activation energy, E
a
Thus, the rate of reaction increases
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Distribution of Kinetic Energies of Molecules
Represent total number of molecules
with kinetic energy greater than Ea
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ARRHENIUS EQUATION
In 1889, Svante Arrhenius proposed the followingmathematical expression for the effect of temperature on
the rate constant, k:
B) TEMPERATURE
k = A e-E aRT
Wherek= rate constant
A = constant known as the collisionfrequency factore = natural log exponent
Ea = activation energy for the reaction
R = universal gas constant (8.314 J mol-1 K-1)
T = absolute temperature
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ARRHENIUS EQUATION
The relation ship between the rate constant, kandtemperaturecan be seen in the k vs Tgraph:
B) TEMPERATURE
k= A e
-E aRT
1/T (K-1
)
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ARRHENIUS EQUATION - DERIVATION The relationship between kand Tis clearer when
we further derive the Arrhenius Equation
B) TEMPERATURE
RT
Ea
Aek
=
).ln(ln RTEa
eAk
=
)ln(lnln RTE
a
eAk
+=
AeRT
Ek
a
lnlnln+
=
ATR
Ek
a ln)(ln +
=1
Natural log both ends
(But ln e = 1)
Thus
See the linear relationship?
y=m x
+ C
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Graph Representation Of The Arrhenius Equation
Plotting a lnkvs 1/T graph would show a clearerrelationship between k(Rate constant) and temperature
B) TEMPERATURE
ATR
E
ka
ln)(ln +
=
1
Where,Ea = Activation Energy
R = 8.314 Jmol-1K-1
T = Absolute Temp
A = Collision freq. factor
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If the value ofA (collision frequency factor) is not known and the same reaction
conducted at two different temperatures.The Arrhenius equation at each temperature
can be written and combined to formed the equation shown in the box.
B) TEMPERATURE
ATR
E
k
a
ln)(ln +
=1
1
1
Since Ais a constant
A
TR
Ek
a ln)(ln +
=
2
2
1
ATR
Ek
a ln)(ln =+2
2
1A
TR
Ek
a ln)(ln =+1
1
1
and
Rearranging the equations would give
)(ln)(ln2
2
1
1
11
TR
Ek
TR
Ek
aa+=+
)()(lnln
12
21
11
TR
E
TR
Ekk
aa=
)(ln122
111
TTR
E
k
ka
=
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DATA:
SOLUTION:
Exercise: the Activation energy
The decomposition of hydrogen iodide,
has rate constants of 9.51 x 10-9 L mol-1 s-1 at 500 K and
1.10x10-5 L mol-1 s-1 at 600 K. Find Ea.
Ea = 1.76 x 105 J/mol = 176 kJ/mol
2 HI (g) H2(g) + I2(g)
k1 = 9.51 x 10-9 L mol-1 s-1 T1 = 500K
k2 = 1.10 x 10-5 L mol-1 s-1 T1 = 600K
)(ln 122
111
TTR
E
k
ka
=
1
122
111
= ))(ln(TTk
kRE
a
1
9
5
500
1
600
1
10519
10101
3148
= ))(.
.ln().(aE
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A catalystis a substance that increases the rate of achemical reaction without itself being consumed.
Addition of a catalyst increases the reaction rate by
increasing the frequency of effective collision. That is by
Decreasing the Ea, and Correct orientation
C) CATALYST
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Addition of a catalyst changes the value ofk(rateconstant) .
C) CATALYST
Reaction rate = k[ A ]x
[ B ]y
(A & B = reactants)(x & y = rate order)
The catalyst reacts by reducing the Ea and increasing A,
thus increasing the k.
ATR
Ek
a ln)(ln +
=1
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uncatalyzed catalyzed
rateuncatalyzed < ratecatalyzed
When Ea decreases, kincreases,
REACTION RATEincreases
C) CATALYST
Ea > Ea
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Reaction pathway
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D) PARTICLE SIZE
The smaller the size of reacting particles, the greater is
the total surface area exposed for reaction and
consequently the faster the reaction. In the case of
heterogeneous systems, in which the reactants are indifferent phases, the area of contact between the reacting
substances will influence the reaction rate considerably.