DEPARTMENT OF NATURAL SCIENCE SUBJECT: CHEMISTRY

DUY TAN UNIVERSITY

DEPARTMENT OF NATURAL SCIENCE
SUBJECT: CHEMISTRY


CHEMISTRY

RATE OF A CHEMICAL REACTION

2015


Reaction rate

1. Definition
The rate of a chemical reaction is a measure of how fast the
reaction occurs.

2015


Reaction rate
The rate of a chemical reaction is measured as a change in the
amounts of reactants or products divided by the change in time.

Generic reaction:

aA + bB → cC + dD

1 ∆  A 

1 ∆  B  1 ∆ C  1 ∆  D 
Rate = −
=−
=
=
a ∆t
b ∆t
c ∆t
d ∆t
2015


Reaction rate
Exercise 1
The equation for a reaction is:
2NO2(g) + H2O(g) + 1/2O2(g) → 2HNO3 (g)
Which one of the following is not numerically equal to the others?

1 ∆ [ NO2 ]
A. − .
2
∆t

∆ [ H 2O ]
C. −
∆t

1 ∆ [ O2 ]
B. − .
2 ∆t

1 ∆ [ HNO3 ]
D.
2
∆t
Answer: B


Reaction rate
For example
Consider the following balanced chemical equation

+
H2O2 (aq) + 3I (aq) + 2H (aq) → I3 (aq) + 2H2O(l)

In the first 10,0 second of the reaction, the concentration of I

-

from 1.000M to 0,868M.
Calculate the average of the reaction rate in this time interval.

2013

dropped


2. The rate Law:
The effect of concentration
Consider this reaction:

aA + bB → cC + dD
m
n
Rate = k[A] . [B]
k: rate constant
m: the reaction order with respect to A
n: the reaction order with respect to B
m + n: the overall order
The rate law for any reaction must always be determined by experiment.


2. The rate Law:
The effect of concentration

For example:

2NO2(k) + F2(k) → 2NO2F(k)

Rate = k[NO2][F2]


2. The rate Law:
The effect of concentration
EXAMPLE

Consider the reaction and the obtained rate / concentration data:
A(aq) + B(aq) → C(aq)

[A]

[B]

rate

1

0.10

0.10

3.10

2

0.10

0.20

6.10

3

0.20

0.10

-2
1.2 .10

-3
-3


3. The Effect of Temperature on Reaction Rate

The Arhenius equation
− Ea

k = A.e RT
R: the gas constant (8.314 J/mol.K)
A: constant called the frequency factor

Ea : the activation energy (is an energy barrier or hump that must be
surmounted for the reactants to be transformed into products)


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