This document provides an introduction to chemical kinetics. It defines chemical kinetics as the study of reaction rates and mechanisms. Reaction rates are measured as changes in concentration over time and can be determined by monitoring reactants or products. The rate of a reaction is affected by factors like temperature, concentration, catalysts, and surface area. Rate laws relate the rate of reaction to concentrations and are determined experimentally. Elementary reactions make up reaction mechanisms, and the slowest step is rate-determining. The Arrhenius equation models the temperature dependence of reaction rates. Hess's law allows calculating enthalpy changes indirectly. Surfactants reduce surface tension and stabilize bubbles and emulsions.

1. Chapter 2
An Introduction to chemical kinetics

2. What is Chemical Kinetics
 Kinetics means how fast does a reaction proceeds
 It studies the rate at which a chemical process
takes place.
 Reaction speed is measured as the change in
concentration of reactants or products with time.
 Kinetics also tells about the reaction
mechanism (exactly how the reaction occurs).
Atkins, P.W.; Jones, Loretta Chemistry Molecules, Matter and Change 4th Ed. . July 2000

3. Rates of Reaction
Rates of reactions can be determined by monitoring the
change in concentration of either reactants or products as a
function of time t.
[A] = concentration of reactant A

4. • Consider the reaction A  B
• There are two ways of measuring rate of reaction:
(1) the speed at which the reactants are vanished
(2) the speed at which the products are formed
• A general way of measuring the rate of the reaction is in terms of
change in concentration per unit time
Instantaneous rate =
∆[A]
∆t
limits to
d[A]
dt
Most Common Units… Rate = M/s
Where, Molarity (M) = moles/Liter

5. Reaction rate is the change in the concentration of a
reactant or a product with time (M/s).
A B
rate = -
∆[A]
∆ t
rate =
∆[B]
∆ t
∆[A] = change in concentration of A over
time period ∆ t
∆[B] = change in concentration of B over
time period ∆ t
Because [A] decreases with time, ∆[A] is negative.

6. Reaction Rates and Stoichiometry
• For the general reaction;
aA + bB cC + dD
(-) sign indicates
Concentration of
reactants decreases
Concentration of
products increases
(i.e., rate of change in concentration divided by respective stoichiometric
coefficient)

7. Example;
For the reaction:
2N2O5(g)  4NO2(g) + O2(g)
we write the rate of the reaction as follows:
Reaction Rate = −
𝟏
𝟐
∆[N2O5]
∆t
=
𝟏
𝟒
∆[NO2]
∆t
=
∆[O2]
∆t

8. Factors affecting the Reaction Rate Constant
1.Temperature: At higher temperatures, reactant molecules have more kinetic
energy, move faster, and collide more often and with greater energy
•Collision Theory: When two chemicals react, their molecules have to
collide with each other (in a particular orientation) with sufficient energy for
the reaction to take place.
•Kinetic Theory: Increasing temperature means the molecules move
faster.
2.Concentrations of reactants
•As the concentration of reactants increases,
so does the likelihood that reactant molecules will collide.
3.Catalysts
•Speed up reactions by lowering activation energy
4.Surface area of a solid reactant
•More area for reactants to be in contact
5.Pressure of gaseous reactants or products
•Increased number of collisions
Atkins, P.W.; Jones, Loretta Chemistry Molecules, Matter and Change 4th Ed. . July 2000

9. Concentration and Rate
Each reaction has its own equation which expresses
its rate as a function of the concentrations of the
involved species (e.g., reactants, products, catalysts).
This is called its Rate Law

10. Rate Law
• In general, rate of a reaction increases with the increase in
concentration of reactants since there are more collisions
occurring between reactants.
• The overall dependence of reaction rate on the reactant
concentration is given in the form of rate law.
• For the reaction; mA + nB  pC + qD
Rate = k [A]m [B]n
 [A] and [B] are reactant concentrations
 The exponents m and n are reaction order (w.r.t. specific
reactant)
 The constant k is the rate constant
 The overall reaction order is the sum of the reaction orders
m + n

11. F2 (g) + 2ClO2 (g) 2FClO2 (g)
Rate = k [F2][ClO2]
• Rate law, rate constant and orders are determined
experimentally ONLY.
• The order of a reactant is NOT generally related to its
stoichiometric coefficient in a balanced chemical equation.
• For example;
1
Rate Law

12. Reactions with simple rate laws:
Reactions with complex rate laws*:
* imply multi-step reactions (sequence of elementary steps)
Simple and complex rate laws
however, the overall rate cannot
involve intermediate species

13. Always follow simple rate laws
Elementary reactions
Reactant order reflects molecularity of molecules involved in reaction

14. Determination of the order of an equation
For example ;
Exp.No. Concentration of [A]
mol/l or (M)
Concentration of
[B] mol/l or (M)
Rate M/s
1 0.1 0.1 0.002
2 0.2 0.1 0.004
3 0.2 0.2 0.016
0.002 = k[0.1]n[0.1]m and 0.004 = k[0.2]n[0.1]m
Dividing, 0.004/0.002 = k[0.2]n[0.1]m / k[0.1]n[0.1]m
Therefore n = 1
Similarly, 0.016 = k [0.2]n[0.2]m and 0.004 = k [0.2]n[0.1]m
Dividing and simplifying , 4 = 2m ; 22 = 2m ; m = 2
The order of the equation is m + n = 2+1 = 3

15. Order of a Reaction
• zero order -
• 1st order -
• 2nd order -
• 3rd order -
if the change in concentration of the
reactant produces no effect.
if doubling the concentration causes
the rate to double.
A reaction is if doubling the
concentration causes a quadruple
increase in rate.
doubling concentration leads to 23 (or
8 times) the rate. (extremely rare)
Atkins, P.W.; Jones, Loretta Chemistry Molecules, Matter and Change 4th Ed. . July 2000

16. Reaction Mechanism
• The sequence of events which describes the actual
process by which reactants are converted into
products is called the reaction mechanism.
• Reaction may occur at once or through several
separate steps.
• Each of these separate steps are known as
elementary reactions. Most chemical reactions
occur by a series of elementary steps.
• An intermediate is formed in one step and used up
in the subsequent step and hence is never seen as a
product in the overall balanced reaction.

17. • At the molecular level, the overall progress of a chemical
reaction can be represented by a series of simple
elementary steps or elementary reactions.
• The sequence of elementary steps that leads to product
formation is the reaction mechanism.
2NO (g) + O2 (g) 2NO2 (g)
N2O2 is detected during the reaction!
Elementary step: NO + NO N2O2
Elementary step: N2O2 + O2 2NO2
Overall reaction: 2NO + O2 2NO2
+

18. Rate Laws and Rate Determining Steps
 The sum of the elementary reactions must give the
overall balanced equation for the reaction.
 The mechanism must agree with the experimentally
determined rate law.
 The rate-determining step should calculate the same
rate law which is determined experimentally.
 The rate-determining step is the slowest step
in the sequence of elementary reactions
leading to product formation.

19. • Elementary step: any process that occurs in a single
step
• Molecularity: number of molecules present in an
elementary step.
– Unimolecular: reaction involving one molecule in
the elementary step
– Bimolecular: reaction with two molecules in the
elementary step
– Termolecular: reaction with three molecules in the
elementary step.
 (It is very uncommon to see termolecular processes as it is statistically
improbable for an effective collision to occur.)
Elementary Steps and Molecularity

20. Rate Laws of Elementary Steps
• Since it occurs in single step, the stoichiometry can be used to
determine the rate law
• Law of Mass Action: The rate of a one step reaction is directly
proportional to the concentration of the reacting species.
• Notice that the coefficients become the exponents.

21. Activation Energy
• A minimum amount of energy is required for a reaction to
take place: activation energy, Ea.
• A ball cannot get over a hill if it does not roll up the hill with
sufficient energy. In the similar manner, a reaction cannot
occur until and unless the reactant molecules possess
sufficient energy to get over the activation energy barrier.
General chemistry- Principles, patterns and Applications. Bruce Averill, Strategic Energy
Security Solutions. Patricia Eldredge, R.H. Hand, LLC ISBN 13: 9781453322307. Saylor
Foundation. Pages: 186-222

22. Activation Energy
• Molecules must possess a minimum amount of
energy to react because-
 In order to form products, bonds must be broken in
the reactants. Bond breakage requires energy.
 Molecules moving too slowly, with too little kinetic
energy, don’t react when they collide.
• Activation energy, Ea, is the minimum energy
required to start a chemical reaction.

23. A + B C + D
Exothermic Reaction Endothermic Reaction

24. Arrhenius Equation
 Svante Arrhenius developed a mathematical equation for the
relationship between k and Ea:
A = frequency factor
Ea = activation energy
R = gas constant (8.3145 J/K·mol)
T = temperature (in K)
Gallagher, RoseMarie; Ingram, Paul Chemistry Higher Tier Oxford University Press
July 2001

25.  A is the frequency factor, a number that
represents the likelihood that collisions between
the reactant molecules would occur with the
proper orientation for reaction.
 Molecules must collide in order to react.
 Key Factors:
 Activation energy, Ea
 Temperature
 Molecular orientations

26. Arrhenius Equation and Temperature Dependence
of the Rate Constant
Taking the natural logarithm of
both sides,
(y = mx + b)
Ea can be calculated from the slope of a plot of
ln(k) vs. 1/T.

27. Hess’s Law
 Hess’s law - the energy change in a chemical reaction is independent of the path
taken.
 Hess’s law of heat summation states that if we add two or more thermochemical
equations to obtain a final equation, then we can also add the heats of reaction to give
the final heat of reaction.
 Hess’s law allows to determine the heat of reaction indirectly by from the known
heats of reaction of two or more thermochemical equations.
DH = ∑ (DHf of products) - ∑ (DHf of reactants)
Atkins, P.W.; Jones, Loretta Chemistry Molecules, Matter and Change 4th Ed. . July 2000

28. Properties of surfactants
 They are used in making firefighting foam as they reduce the surface tension of the
foam solutions.
 Soap bubbles have very large surface areas with very little mass.
 Bubbles in pure water are unstable.
 The addition of surfactants can have a stabilizing effect on the bubbles.
 Surfactants reduce the surface tension of water by a factor of three or more.
 Emulsions are a type of solution in which surface tension plays an important role.
Tiny fragments of oil suspended in pure water will spontaneously assemble themselves
into much larger masses. But the presence of a surfactant provides a decrease in surface
tension, which permits stability of minute droplets of oil in the bulk of water
Further reading:
https://www.britannica.com/science/surfactant
https://www.researchgate.net/publication/255744917_Surfactants_and_their_Applications
/figures?lo=1

29. References
1. General chemistry- Principles, patterns and Applications. Bruce
Averill, Strategic Energy Security Solutions. Patricia Eldredge, R.H.
Hand, LLC ISBN 13: 9781453322307. Saylor Foundation. Pages: 186-
222
2. Drysdale, D. D. (2011). An introduction to fire dynamics. 2nd ed.,
Wiley, Chichester
3. James Quintiere, Thomson, 1st Ed. 1997, Principles of Fire
Behavior.
4. James G. Quintiere, Delmar 1998, Principles of Fire Behavior.
5. Physics and Chemistry for Firefighters, Fire Service Manual -
Volume 1 1998
6. Friedman, R. (1998).Principles of fire protection chemistry and
physics. 3rd ed., NFPA, Quincy MA
7. East, T.D and McConkey, A (1996), Applied Thermodynamics for
Engineering Technologists.5thEd,Longman.
8. Atkins, P.W.; Jones, Loretta Chemistry Molecules, Matter and
Change 4th Ed. . July 2000
Useful link: http://readinglists.central-lancashire.ac.uk/index