Matriculation Chemistry ( Reaction Kinetics ) part 5

8/14/2019 Matriculation Chemistry ( Reaction Kinetics ) part 5

1/18

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


2/18

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


3/18


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


4/18


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)


5/18

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


6/18

Distribution of Kinetic Energies of Molecules

Represent total number of molecules

with kinetic energy greater than Ea


7/18

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


8/18

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

)


9/18

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


10/18

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


11/18

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

=


12/18

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


13/18

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


14/18


15/18

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


16/18

uncatalyzed catalyzed

rateuncatalyzed < ratecatalyzed

When Ea decreases, kincreases,

REACTION RATEincreases

C) CATALYST

Ea > Ea


17/18

Reaction pathway


18/18

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.

Matriculation Chemistry ( Reaction Kinetics ) part 5

Documents

Transcript of Matriculation Chemistry ( Reaction Kinetics ) part 5