Chemical Changes and Structure: Controlling The Rate of a Chemical Reaction 3
Chemical Changes and Structure
CONTROLLING THE RATE OF A CHEMICAL
REACTION 3
At the higher temperature of T1 + 10°C, many more particles now have an energy equal to or
greater than the activation energy for the reaction. For example, the particles at point C now
have enough energy for successful collisions. The relatively small increase in temperature
has caused the coloured area under the curve to the right of EA, which represents the
number of molecules with sufficient energy to react, to increase significantly.
Energy distribution diagrams
Energy distribution diagrams show how many particles are moving with each value of
kinetic energy.
On an energy distribution
diagram, the area under
the curve to the right of
EA represents the number
of particles which have
sufficient energy to react.
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Point D on this graph shows
that there are also only a
small number of very fastmoving particles with high
kinetic energy values.
B
Number of particles
DON’T FORGET
Point A on this graph shows
that there are only a small
number of slow-moving
particles with low kinetic
energy values.
A
D
Kinetic energy
EA
For a chemical reaction to occur, the minimum energy required by the colliding particles
for a collision to be successful is known as the activation energy, EA. Only those particles
with an energy greater than or equal to the activation energy, EA, will take part in successful
collisions. For example, those particles represented at point D have an energy greater than
the EA, but those at points A, B and C do not. The total number of particles with an energy
greater than the minimum activation energy required is represented by the dark green
coloured area to the right of the vertical line representing the activation energy, EA.
CHANGING THE TEMPERATURE
The effect on the kinetic energy of the particles of changing the temperature is seen in the
following diagram. The curve labelled T1 is the original curve shown above. The curve labelled
T1 + 10°C shows the energy distribution when the temperature has increased by 10°C.
DON’T FORGET
Increasing the temperature
means that more particles
have an energy greater than
the activation energy.
Number of particles
T1
T1 + 10°C
A
10
CfE_H_Chemistry.indb 10-11
EA
reactants
products
Function progress
The effect that this has on
the number of reacting particles which can have successful collisions is shown below. The
broken red line shows the activation energy for the reaction when no catalyst is present
and only those particles in the area coloured orange have enough energy for reaction.
The broken purple line
represents the lowered
activation energy when a
catalyst is used. Now the
particles within the area
coloured pink, as well
as those within the area
coloured orange, have an
energy greater than the
activation energy for the
catalysed reaction. Now
there are more successful
collisions and therefore a
faster rate of reaction.
EA (with more
efficient catalyst)
EA (with catalyst)
Kinetic energy
Explain the different ways in which a catalyst increases the number of reacting particles
with the minimum energy required (the activation energy) compared with how increasing
the temperature increases the number of particles with the minimum energy required.
contd
DON’T FORGET
Catalysts provide an
alternative reaction
pathway with a lower
activation energy.
EA (no catalyst)
THINGS TO DO AND THINK ABOUT
D
Kinetic energy
EA
uncatalysed
EA
catalysed
The broken blue line represents the lowered activation energy when an even more
efficient catalyst is used. Now the particles within the area coloured blue, as well as those
within the areas coloured orange and pink, have an energy greater than the activation
energy for the reaction.
B
C
A catalyst speeds up a
chemical reaction. The
catalyst takes part in the
reaction, but is regenerated
at the end of the reaction. In
other words, the catalyst is the
same at the end as it was at
the beginning of the reaction.
All catalysts provide an
alternative reaction pathway
with a lower activation
energy. Often a difficult
single-step reaction is
replaced by a series of
much easier reactions. Less
energy is needed and so the
activation energy for the
reaction is lowered.
C
Point B shows that the
greatest number of particles
are moving with medium
kinetic energy values.
USING A CATALYST
Potential energy
In solids, liquids or solutions, the particles present are in continual motion. Some will be
moving very slowly and others will be moving more quickly. The more quickly a particle is
moving, the greater its kinetic energy.
Number of particles
KINETIC ENERGY DISTRIBUTIONS
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ONLINE TEST
Take the ‘Controlling
the rate of a chemical
reaction’ test at www.
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HigherChem
11
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Chemistry in Society: Chemical Analysis 1
Chemistry in Society
CHEMICAL ANALYSIS 1
CHROMATOGRAPHY
What is chromatography?
Chromatography is a method used to analyse mixtures. It can be used to identify the
substances present in a mixture and, in some cases, it can tell us how much of each
substance is in the mixture.
There are different types of chromatography, but they all involve separating the
components in a mixture by passing the mixture through a suitable medium in which the
different components move at different speeds.
DON’T FORGET
There are different types
of chromatography.
Each type involves an
interaction between:
•the mixture to be
separated
•the mobile phase
•the stationary phase.
The medium is an adsorbent material and is often referred to as the stationary phase.
The different components can be held to the stationary phase by van der Waal’s
attractions. The stronger the van der Waal’s attractions formed between the component
and the medium, the more the component is held up and therefore it travels more slowly
through the medium. The weaker the bonds formed between the component and the
medium, the less the component is held up and so it passes through the medium more
quickly. As the different components move at different speeds through the medium, they
are separated from each other.
Usually a liquid or gas is used to carry the mixture through the adsorbent stationary
phase – because this moves through the medium, it is known as the mobile phase.
Chromatography works because it exploits the fact that different molecules experience
different types and strengths of intermolecular forces as the mobile phase carries them
through the stationary phase. The strengths and types of these intermolecular forces
depend on the differences in the polarity and size of the molecules being separated.
DON’T FORGET
Molecular size and the
polarity of molecules affect
the speed at which molecules
travel in chromatography;
the differences in these
properties are why
components in a mixture
separate out.
DON’T FORGET
In all types of
chromatography, the
components in a mixture
are carried by the mobile
phase through the stationary
phase. The mobile phase and/
or the stationary phase are
different in the different
types of chromatography.
Molecules that form stronger intermolecular forces with the mobile phase than with
the stationary phase will move more quickly than other molecules that form stronger
intermolecular forces with the stationary phase.
Using our knowledge of the different types of intermolecular forces, we can make
predictions about which substances travel through the medium at a faster rate. Remember
that ‘like dissolves like’. If the mobile phase is polar and the stationary phase is non-polar,
then we would expect any polar molecules to move further and faster than non-polar
molecules as they are carried through the stationary phase by the mobile phase.
There are other types of chromatography in addition to paper chromatography. You are
not expected to remember any details about them, but some information is given in the
table below:
Type of chromatograpy
Mobile phase
Stationary phase
Paper chromatography
Liquid solvent
Paper
Thin-layer chromatography Liquid solvent
Plastic film with fine coating of silica
Gas chromatography
Non-volatile liquid sticking to an unreactive solid
Unreactive gas
The finished result in both paper and thin-layer chromatography is known as a
chromatogram and the different substances show up as spots. If the spots are colourless,
then a compound known as a locating reagent can be sprayed on to the chromatogram to
make the different compounds show up as coloured spots.
contd
Gas–liquid
chromatography is much
more complicated and is
carried out using special
apparatus, as shown in
the diagram on the right.
The injection port is
where the gas mixture or
liquid mixture enters. The
stationary phase is coiled
inside an oven to vaporise
any liquids that are to be
separated and identified.
After separation, a detector
produces a signal whenever
a compound leaves the
column. The results are then
recorded on a graph.
injection
port
flow
controller
Watch a video introducing
Paper Chromatography at
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CfE_H_Chemistry.indb 74-75
VIDEO LINK
detector
column
carrier
gas
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chromatography at www.
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HigherChem
column
oven
ONLINE TEST
The y-axis of the graph gives an indication of the amount of the component present and
the x-axis gives the retention time of that component. The retention time is the time
taken for a particular component to travel through the apparatus.
Take the ‘Chemical
analysis’ test at www.
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HigherChem
You will see examples of this below and on the next page.
THINGS TO DO AND THINK ABOUT
Chromatography is a useful analytical and forensic technique.
1 Chromatography can be used to find out if a substance is pure. For example, carrying
out a thin-layer chromatography experiment using a pure substance results in only
one spot on the developed chromatogram. If the substance had an impurity present,
the impurity would show up as another spot. Consider the following example.
An organic chemist is attempting to synthesise a fragrance
compound by the following chemical reaction:
A
B
compound X + compound Y → fragrance compound
After one hour, a sample is removed and compared with
pure samples of compounds X and Y using thin-layer
chromatography.
X
Y
sample
C
X
Y
sample
X
Y
sample
D
Which of the following chromatograms shows that the
reaction has produced a pure sample of the fragrance
compound?
2 Under a definite set of experimental conditions for
chromatographic analysis, a given substance will always
travel a fixed distance relative to the distance travelled
by the solvent front. This ratio of distances is called the
Rf value:
Rf =
distance travelled by substance
distance travelled by solvent front
Let’s illustrate how we would calculate the Rf value of a
substance given its chromatogram:
As the Rf value is characteristic for any given compound
(provided that the same stationary and mobile phases
are used), it can provide evidence about the identity of
the compound.
74
VIDEO LINK
recorder
X
Y
sample
solvent front
Rf =
a
b
b
a
position of
original spot
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