TOPIC 6
CHEMICAL KINETICS
6.1
Collision Theory and
Rates of Reaction
ESSENTIAL IDEA
The greater the probability that molecules
will collide with sufficient energy and proper
orientation, the higher the rate of reaction.
NATURE OF SCIENCE (2.7)
The principle of Occam’s razor is used as a guide to
developing a theory – although we cannot directly see
reactions taking place at the molecular level, we can
theorize based on current atomic models. Collision
theory is a good example of this principle.
INTERNATIONAL-MINDEDNESS
Depletion of stratospheric ozone has been
caused largely by the catalytic action of CFCs
and is a particular concern in the polar regions.
These chemicals are released from a variety of
regions and sources, so international action and
cooperation have been needed to ameliorate the
ozone depletion problem.
THEORY OF KNOWLEDGE
The Kelvin scale of temperature gives a
natural measure of the kinetic energy of gas
whereas the artificial Celsius scale is based
on the properties of water. Are physical
properties such as temperature invented or
discovered?
UNDERSTANDING/KEY IDEA
6.1.A
Species react as a result of
collisions of sufficient energy and
proper orientation.
COLLISION THEORY
In order to react, particles must collide
with kinetic energy greater than the
activation energy and have the correct
collision geometry.
 Restating: To have a successful
reaction you need 2 factors:

Sufficient energy (greater than Ea)
 Correct geometry

Endothermic
Reactions
PE
POTENTIAL ENERGY DIAGRAM
Exothermic Reactions
PE
POTENTIAL ENERGY DIAGRAM
UNDERSTANDING/KEY IDEA
6.1.B
The rate of reaction is expressed as
the change in concentration of a
particular reactant/product per unit
time.
 Define
the term rate of reaction.
 The rate of a chemical reaction is
the increase in concentration of
products or the decrease in
concentration of reactants per unit
time.
 Units for rate = mol dm-3s-1
2NO2(g)  2NO(g) + O2(g)
Reaction Rates:
1. Can measure
disappearance of
reactants
2. Can measure
appearance of
products
3. Are proportional
stoichiometrically
2NO2(g)  2NO(g) + O2(g)
Reaction Rates:
4. Are equal to the
slope tangent to
that point
[NO2]
t
5. Change as the
reaction proceeds,
if the rate is
dependent upon
concentration
[ NO2 ]
 constant
t
APPLICATION/SKILLS
Be able to analyze graphs and
numerical data from rate
experiments.
GUIDANCE
Be able to calculate reaction rates
from tangents of graphs of
concentration, volume or mass vs
time.
GUIDANCE
Be able to interpret graphs of
changes in concentration, volume
or mass against time.
ANALYSE DATA
You may be given a set of data such
as concentration or changes in
volume or mass vs time.
 You should be able to graph the data
and find the rate at any point using the
tangent.
 Be careful with units.

To find the tangent at a certain point,
take the change in concentration
(change in “y”) divided by the change
in time (change in “x”).
 Rates of reaction are always
expressed as positive numbers even if
the slope is negative.

APPLICATION/SKILLS
Investigate rates of reactions
experimentally and evaluate the
results.
APPLICATION/SKILLS
Be able to explain the effects of
temperature,
pressure/concentration and particle
size on rate of reaction.
 Predict
and explain, using the
collision theory, the qualitative
effects of particle size,
temperature, concentration and
pressure on the rate of a reaction.
FACTORS AFFECTING
THE RATE OF REACTION

There are 5 factors which will increase
the number of effective collisions.
TEMPERATURE
Increasing the temperature increases
the kinetic energy of the particles.
 By increasing the temperature, you
will increase the number of collisions.
 You will also have a higher number of
particles with enough KE to overcome
the activation energy barrier.
 Increasing temperature increases the
rate of reaction.

CONCENTRATION
Increasing the concentration of
reactants increases the frequency of
collisions.
 By increasing the concentration, you
will increase the number of successful
collisions and therefore increase the
rate of reaction.

PARTICLE SIZE

A sugar cube dissolves much more slowly
than the same amount of sugar in granulated
form.


This is because the sugar cube only has its 6
sides in contact with the water; whereas, the
granulated sugar has significantly more
surface area in contact with the water so
dissolving is faster.
Decreasing particle size increases the rate of
reaction by providing greater surface area
which allows for more contact and collisions.
PRESSURE OF GASES
Increasing the pressure increases the
rate of reaction.
 By increasing the pressure of a gas,
you are compressing it into a smaller
area which increases its concentration
which increases the number of
collisions.

CATALYST
A catalyst increases the rate of both the
forward and reverse reactions without
undergoing chemical change.
 A catalyst provides an alternate route for the
reaction with a lower activation energy.
 It is incorrect to say that the catalyst lowers
the activation energy.
 It essentially ensures that without raising the
temperature, more particles have sufficient
KE to overcome the Ea barrier.

UNDERSTANDING/KEY IDEA
6.1.C
Concentration changes in a
reaction can be followed indirectly
by monitoring changes in mass,
volume and color.
In all of the following techniques, the
goal is to measure the change in
concentration vs time.
 Reaction rate is dependent upon
temperature so it is crucial to control the
temperature.
 This is best done by a thermostatically
controlled water bath.

Techniques for measuring
rate of reaction
Concentrations are not usually measured
directly. There is usually some sort of signal
which relates to the change in concentration
of either the reactants or the products.
 The raw data collected from these “signals”
usually have units other than the units for
concentration which is measured in mol/dm3.
 There are six common techniques which will
be discussed.

1. CHANGE IN VOLUME
OF GAS PRODUCED
If one of your products is a gas, then you can
collect the gas and measure the change in
volume at regular time intervals.
 The graph is change in volume vs time.
 Collection can be done with a gas syringe or
by water displacement in a burette.

Water displacement is limited if the gas is
soluble in water.
 Warm water should be used since most gases
are less soluble in warm water.

2. CHANGE IN MASS
If a reaction involves a change in mass, you can
measure this directly.
 The reaction vessel can be placed directly upon
a balance and the mass can be recorded
against time by being hooked to a data
collection set up.
 The graph would be mass against time.

3. CHANGE IN TRANSMISSION OF
LIGHT – COLORIMETRY OR
SPECTROPHOTOMETRY
If one of the reactants or products is
colored, you can measure the change
in the color on a colorimeter.
 Continuous readings can be made of
the light transmission.
 The graph is absorbance vs time.

4. CHANGE IN CONCENTRATION
MEASURED USING TITRATION



This is not a continuous method because as
you are titrating one sample, the rest is still
reacting.
Samples must be withdrawn at regular time
intervals and then analyzed by titration.
In order to stop the reaction at each time
interval, a substance is introduced to stop
the reaction at that moment. This is called
quenching.
5. CHANGE IN
CONCENTRATION MEASURED
USING CONDUCTIVITY


Conductivity is a measure of the number of
ions in a solution.
If a reaction is using up or producing ions,
you can measure this by a conductivity
meter.
6. NON-CONTINUOUS METHODS
OF DETECTING CHANGE DURING
A REACTION – CLOCK REACTIONS
There are times when you cannot record
the continuous change in the rate of a
reaction.
 It may be more convenient to measure the
time is takes for a reaction to reach a
certain fixed point.
 This means something you have chosen to
use as an indicator to “stop the clock”.
 The limitation is this method gives only an
average rate over the time interval.

UNDERSTANDING/KEY IDEA
6.1.D
Activation energy (Ea) is the
minimum energy that colliding
molecules need in order to have
successful collisions leading to a
reaction.
UNDERSTANDING/KEY IDEA
6.1.E
By decreasing Ea, a catalyst
increases the rate of a chemical
reaction, without itself being
permanently chemically changed.
APPLICATION/SKILLS
Sketch and explain energy profiles
with and without catalysts.
EFFECT OF CATALYST
A catalyst increases the rate of the
reaction without itself undergoing
chemical change.
 It provides an alternate route for the
reaction which has a lower activation
energy.
 Be careful not to say “it lowers the
activation energy of the reaction”.

APPLICATION/SKILLS
Be able to describe the kinetic
theory in terms of the movement of
particles whose average kinetic
energy is proportional to
temperature in Kelvin.
KINETIC THEORY OF
MATTER
All matter consists of particles which are
in constant motion.
 The KE for gases is greater than that of
liquids which is greater than that of solids.
 Temperature (measured in Kelvin) is
proportional to the average KE of the
particles in a substance.
 The higher the temperature, the higher the
kinetic energy.

More about kinetics
Kinetics refers to movement.
 Movement in chemistry refers to the
progress of a reaction.
 Therefore, kinetics is the study of how
fast the reaction goes.
 The reaction mechanism is a sequence
of bond breaking and bond making which
suggests “how” the reaction happens.

Even though reaction concentrations
decrease, rate is always expressed as
a positive number.
 The graph is a curve so we can only
measure the tangent to the curve at a
moment in time which will give us the
rate at that moment in time.

APPLICATION/SKILLS
Be able to construct Maxwell –
Boltzmann energy distribution
curves to account for the
probability of successful collisions
and factors affecting these,
including the effect of a catalyst.
 Sketch
and explain qualitatively the
Maxwell-Boltzman energy
distribution curve for a fixed amount
of gas at different temperatures and
its consequences for changes in
reaction rate.
MAXWELL-BOLTZMANN
DISTRIBUTION CURVE
The Maxwell-Boltzmann curve shows
that particles in a gas at a particular
temperature show a range of values
of kinetic energy.
 The area under the curve represents
the total particles in a sample.

Citations
International Baccalaureate Organization. Chemistry Guide, First
assessment 2016. Updated 2015.
Brown, Catrin, and Mike Ford. Higher Level Chemistry. 2nd ed.
N.p.: Pearson Baccalaureate, 2014. Print.
Most of the information found in this power point comes directly
from this textbook.
The power point has been made to directly complement the Higher
Level Chemistry textbook by Catrin and Brown and is used for direct
instructional purposes only.