Chemical Changes and Structures – Reaction Rates – Factors Affecting the Rate of a Reaction
reaction rates – factors
affecting the rate of a reaction
Rate is a term used to describe what something is doing over time. In chemistry,
it is important to understand why chemical reactions occur at certain rates. In the
chemical industry speeding up slow reactions will make them more economical, as
saving time saves money.
Online test
Collision Theory
Chemists use collision theory to try and
explain why chemical reactions occur at
different rates. In simple terms, this means
that the particles involved in a reaction
must collide into each other. However, only
collisions that have enough kinetic energy
will be successful and lead to a reaction. The
minimum energy required for a reaction to
occur is known as the activation energy.
no reaction
reaction
Nails rusting – a slow reaction
Concentration
Don’t Forget
The concentration of a solution
is measured in moles per litre,
mol/l. A 2 mo/l solution is twice
as concentrated as a 1 mo/l
solution.
VIDEO LINK
http://www.youtube.com/
watch?v=kjKyEdrVXJA
http://www.youtube.com/
watch?v=AfT2Uktx4c
Particle size
In general, increasing the surface area of a reactant will increase the rate of a reaction.
Magnesium powder will react much faster with hydrochloric acid than a piece of
magnesium ribbon of the same mass. This is because the powder has a larger surface and
so more collisions can take place.
1 cm
3 cm
Don’t Forget
Crushing a lump of chalk will
decrease the particle size of
the lump but increase the
surface area.
3 cm
surface area = 30 cm2
14
High concentration – many collisions
cut the
shape into
smaller pieces
1 cm
1 cm
1 cm
surface area = 54 cm2
Sample pages
VIDEO LINK
Low concentration – few collisions
Sample pages
An explosion – a very fast
reaction
In general, increasing the concentration of the reactants will lead to an increase in
reaction rate. More reacting particles in a given volume will mean that there is a greater
chance of a successful collision – the reaction will be faster at higher concentrations.
This also explains why reactions start fast, slow down and stop. There is always a higher
concentration of reactant particles at the start of the reaction. As the reaction proceeds
the reactant particles get used up, there are fewer collisions per second and so the
reaction slows down.
Sample pages
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tests
Sample pages
Chemical Changes and StructurES
VIDEO LINK
Temperature
low temperature
high temperature
In general, increasing the temperature of a
reaction will increase the reaction rate.
Temperature is a measure of the average
kinetic energy particles possess. The higher
the temperature the more energy the particles
will have. Any increase in energy makes
the particles collide more often, but most
importantly, with more force. This means more
particles will be in possession of the activation energy. This makes the reaction much faster.
A temperature increase of around ten degrees Celsius often doubles the rate of a reaction.
Catalysts
Don’t Forget
Usually chemical reactions
are supplied with heat
energy to increase their
rate. However, other forms
of energy, e.g. light, can
also be used to supply
the necessary energy of
activation. Photosynthesis is
an example of this.
Don’t Forget
transition metals
A catalyst is a substance
which speeds up a reaction,
but is chemically unchanged
at the end of the reaction.
When the reaction is
complete, the mass of the
catalyst will be the same as it
was at the beginning. Many
catalysts are either transition metals or compounds which contain transition metals.
Examples include nickel used in the manufacture of margarine from vegetable oils and
platinum in car exhausts.
Consider the reaction
between zinc metal and
sulphuric acid which
produces hydrogen. The gas
can be seen as bubbles in
the reaction mixture.
dilute sulphuric acid
slower reaction
Zinc + Sulphuric acid →
Zinc sulphate + Hydrogen
Copper metal acts as a
catalyst for this reaction.
http://www.youtube.com/
watch?v=NKgEjNfzZ3U
faster reaction
zinc
More particles will possess
the activation energy if the
temperature of a reaction
is increased. This leads to a
faster reaction.
VIDEO LINK
http://www.youtube.com/
watch?v=tXKAAgwR7cg
Don’t Forget
When asked to define what
a catalyst is, many students
will incorrectly indicate that
the catalyst does not take
part in the reaction. This is
a common misconception.
To increase the rate of the
reaction a catalyst must
take part.
copper penny
partly coated
with zinc
THINGS TO DO AND THINK ABOUT
1. Use the collision theory and activation energy to explain the following facts.
(a) Milk stays fresher for longer if it is kept in the fridge.
(b)There is a real danger of explosion in sawmills, coalmines and flour mills
(c) Charcoal on a barbecue glows more brightly when air is blown onto it.
(d)The reaction between magnesium and hydrochloric acid takes place at room
temperature but the reaction of magnesium with oxygen requires a flame to get it
started.
2. Look again at the diagram on particle size. Can you prove the surface area values given?
3. Outline the steps necessary to prove that the copper
catalyst is not used up in the reaction of zinc with
Experiment
A
B
C
sulphuric acid.
Concentration of
0⋅5
1⋅0
1⋅5
4. The table shows the conditions used for six experiments acid/mol/l
on an investigation into the rate of reaction between
Temperature/°C
20
20
20
calcium carbonate and an acid.
Form of calcium
lump
powder powder
carbonate
(a) In which experiment will the reaction be quickest?
(b)Which two experiments could investigate the effect
of temperature on the speed of the reaction?
Don’t Forget
When carrying out
experiments to identify
which factor is altering
the rate of a reaction, it is
very important that only
one factor is changed to
ensure a fair comparison is
made. For example, when
investigating temperature,
only the temperature should
change. Others factors like
concentration and particle
size should remain the same.
D
E
F
0⋅5
1⋅0
1⋅5
30
30
30
powder
lump
powder
15
Chemical Changes and Structures – Reaction Rates – Monitoring the Rate of a Reaction
Reaction Rates – monitoring the
rate of a reaction
A formula for reaction rate
During a chemical reaction the reactants change into products. This means that as the
reaction proceeds the reactants are being used up and the products are being formed.
Don’t Forget
Only the average reaction
rate can be found using this
formula as the reaction rate is
constantly changing.
This can be expressed by the formula;
change in quantity of reactant or product
Reaction rate =
change in time
Example During a reaction 0⋅30 grams of gas were produced in the first 60 seconds.
Calculate the average rate of reaction for the first 60 seconds.
Reaction rate =
0⋅30
= 0⋅005 grams per second, g/s
60
How to measure reaction rate 1
The reaction between marble chips (a form of calcium carbonate, CaCO3) and dilute
hydrochloric acid, HCl, is useful to show how the rate of a reaction can be measured
when the reaction produces a gas.
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
In this reaction the gas carbon dioxide, CO2, is produced. If appropriate apparatus is
used, the mass or volume of the gas can be measured as the reaction proceeds.
Measuring the volume of gas produced
measuring cylinder
gas syringe
conical flask
reaction
mixture
Using this apparatus the
volume of carbon dioxide
produced is measured
with the syringe or the
measuring cylinder at
suitable time intervals.
ONLINE
water
Measuring the mass of gas produced
cotton wool
Don’t Forget
Although this technique
measures the decreasing mass
of the flask and its contents, it
can also be used to determine
the increasing mass of the gas
produced if the total starting
mass of the chemicals and
apparatus is known.
16
reaction mixture
50.58
g
Using this apparatus the carbon dioxide
gas produced will escape from the flask
and the mass of the flask and its contents
will decrease. The change in mass can be
recorded at suitable time intervals. The
loose plug of cotton wool will not prevent
the gas from escaping but it will stop any
acid spray escaping as the mixture fizzes.
Sample pages
http://www.s-cool.co.uk/
gcse/chemistry/rates-ofreaction/revise-it/the-rateof-a-chemical-reaction
Sample pages
The balanced formula equation for the reaction is
Sample pages
There are several methods of monitoring the rate of a reaction. They all rely on
measuring how much of a reactant is used up or how much of a product is formed in a
given period of time.
Sample pages
Chemical Changes and StructurES
Online test
How to measure reaction rate 2
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tests
Not every reaction will generate a gaseous product. Different methods are therefore
required to measure reaction rate.
If a reaction involves an acidic or alkaline
solution, the pH of the reaction mixture
reaction mixture
will change as the reaction proceeds. This
change can be measured with a pH meter
at suitable time intervals.
Other factors used to determine the rate
of a reaction include measuring changes in
pressure, concentration and conductivity.
glass pH probe
pH meter
6.5
stirrer
The importance of units
The concept of units is crucial when measuring any given quantity in chemistry. A numerical
answer without a unit tells us nothing about what has actually been measured. Consider the
phrase, “I measured out 20 of water”. 20 what? It is much clearer to report “I measured out
20 g of water” as this correctly identifies it was the mass of water being measured.
Don’t Forget
A reaction rate of 4 g/s will
be ten times faster than a
reaction rate of 0⋅4 g/s
Changes in mass
If a gas is produced in a reaction and the changes in mass are recorded at regular intervals then:
Reaction rate =
change in mass
change in time
If the time was measured in seconds, the equation
for units will be:
Reaction rate =
g
s
The unit for reaction rate is grams per second and this would be written as g/s
Changes in volume
If the volume of gas produced is measured at
regular intervals then:
Reaction rate =
change in volume
change in time
If the time was measured in minutes,
the equation for units will be:
Reaction rate =
cm3
min
The unit for reaction rate is cubic centimetres per minute and this would be written as cm3 /min
Don’t Forget
A reaction rate of 5 cm3 /min
will be twice as slow as
a reaction with a rate of
10 cm3 /min
Do not lose marks by missing
out units in chemistry
calculations. The units of
rate depend on two things.
1. Was mass or volume
or some other factor
measured?
2. Was the time measured
in seconds or minutes,
etc?
THINGS TO DO AND THINK ABOUT
1. The apparatus shown was used to monitor the rate of the reaction between
magnesium and dilute hydrochloric acid.
(a) Suggest the name of an essential
piece of equipment, which is not
shown in the diagram, without
which it would not be possible to
hydrogen
measure the rate of the reaction.
(b)In the first three minutes of the
reaction 50 cm3 of hydrogen were
dilute hydrochloric acid
collected. Calculate the average
magnesium ribbon
rate of reaction during the first
three minutes.
2. Many chemicals have colour. If a coloured chemical was made into a solution and then
allowed to react in such a way that the colour faded as the reaction progressed, then
the rate of a reaction could be determined by measuring the intensity of the colour at
regular time intervals. Chemists would make use of an analytical instrument called a
colorimeter to help measure the change in colour intensity. Find out all you can about
how this might be carried out.
17
Nature’s Chemistry – Homologous Series – Alkanes
Homologous Series – Alkanes
Hydrocarbons
ONLINE
http://www.rsc-oilstrike.org/
Hydrocarbons are compounds that contain carbon and hydrogen
only. They are the main compounds present in fossil fuels such as
natural gas and crude oil. They can be obtained from crude oil by
fractional distillation.
Hydrocarbons can be divided into different subsets, sometimes called families. This
course considers three of these families – alkanes, alkenes and cycloalkanes.
Most of the compounds found in crude oil based fuels such as petrol belong to the
alkane family.
One member of this family is butane. You may have heard of butane before as it is a
common fuel used for portable heaters, barbecuing, and caravanning.
In three dimensions butane can be represented as shown in the diagram on the left.
The carbon atoms are shown in black and the hydrogen atoms are shown in white.
This means that butane has 4 carbon atoms and 10 hydrogen atoms and so its formula
can be written C4H10.
All of the carbon atoms are joined together by single covalent bonds and so butane is a
saturated hydrocarbon.
As well as representing the three dimensional structure of alkanes as shown above, there
are three other types of formulae that can be used to represent hydrocarbons.
Molecular Formula
This is the simplest representation of the compound showing only the atoms present in
the compound and the numbers of each type.
The molecular formula for butane is C4H10
Full Structural Formula
The full structural formula for butane is shown here.
This shows all the atoms and all the bonds in the molecule.
Don’t Forget
Online test
Naming of alkanes up to 8
carbons swapping between
three different formula
types http://cloud4test.com/
hello/tests
18
H C C C C H
H H H H
Shortened Structural Formula
The shortened structural formula for butane can be written in two ways
CH3-CH2-CH2-CH3 or CH3CH2CH2CH3
This is an abbreviated version of the full structural formula.
Naming Straight Chain Alkanes
Prefix Number of Carbons
Meth
1
Hydrocarbon molecules are named according to an
internationally agreed system. A set of rules are used to
produce a systematic name for each substance. In this
system a prefix is used which lets other chemists know
how many carbons are in the compound.
The prefix for butane is ‘but’ and this means four.
Eth
2
Prop
3
But
4
Pent
5
Hex
6
Hept
7
Oct
8
Sample pages
You need to know these
prefixes and their meanings
but you can also find the
names of the first 8 alkanes
listed in numerical order of
carbon atoms in the data book.
H H H H
Sample pages
Different Formulae
Sample pages
Alkane Family
Sample pages
Nature’s Chemistry
H
Naming Branched Chain Alkanes
The alkanes that we have considered up to now have been straight chain alkanes with all of
the carbon atoms joined in a continuous chain. Look at the compound shown on the right.
H
H C H
H C
This alkane has a continuous chain of five carbon atoms (shown in red) and so it is based
on pentane. Both of the branches contain only one carbon – CH3 and this is called a methyl
group. A branch with two carbon atoms in it would be an ethyl group – CH2CH3.
C
H
Pick out the longest continuous chain of carbons. This forms the basis for the name.
In the example to the right, the longest chain contains the five carbon atoms that are
shown in red. In this example the name will be based on pentane.
• Number this carbon chain as shown on the right – this will allow us to identify where
in the chain the branch appears. The chain is always numbered starting from whichever
end is closer to the branch. In this case, numbering would start at the left end of
the chain.
• Identify the branches and arrange them in alphabetical order. Ethyl would come before
methyl and so on. If there are two or more of the same type of branch this is shown in
the name by using di, tri, tetra and so on. We also need to indicate, by using a number,
which carbon of the main chain the branches are joined to.
In the example there are two methyl branches, one on carbon 2 and one on carbon 3
and so these would be named as 2,3-dimethyl.
Add
2,3-dimethyl to the base name and so this compound would be 2,3-dimethylpentane.
•
You will also need to be able to write a structural formula for a branched chain alkane
from its name.
For example, 3-ethyl-2-methylpentane:
The base of the name is pentane so five carbons can be written in a continuous chain. A
methyl (-CH3) group will be attached to the second carbon and an ethyl (-CH2CH3) group
will be attached to the third carbon. All of the carbon atoms need to have four bonds and
so hydrogen atoms are written in.
H
C C H
H H C H H H
CH3
The shortened structural
formula for this
compound can be
written:
H C H
Or
H
CH3 CH CH CH2 CH3
CH2
CH3
(a) the full structural formula
(b)the shortened structural formula
(c) the molecular formula
1.heptane
2.propane
3.2-methylpropane
4.2,2,4-trimethylpentane
5.3-ethyl-4-methylheptane
C H
H
H
Full structural formula
CH3 CH CH CH2 CH3
CH3
Shortened structural formula
C
C1 C2 C3 C4 C5
C
Numbering the carbon chain
Numbers in a name are
separated by commas (,)
and numbers and words are
separated by hyphens (–)
Online test
Naming of branched chain
alkanes up to 8 carbons
swapping between three
different formula types
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tests
CH3CH(CH3)CH(CH2CH3)CH2CH3
VIDEO LINK
Link to Scholar animation –
Naming Alkanes
THINGS TO DO AND THINK ABOUT
For the following compounds write:
C
Don’t Forget
Writing Structural Formulae
H H
H
CH3
•
•
HH C H H
H C
C
C
H
HH C HH
To name branched chain alkanes we need to follow a set of internationally agreed
set of rules:
H
H
C
Name the following alkanes
6.
H
H C H
H
H C H
7.
H
H
H
H C
C
C
C
C H
H
H
H
H
H
CH3
CH3 CH CH2 CH2 CH3
8. CH3CH2CH(CH2CH3)CH2CH3
19
Nature’s Chemistry – Homologous Series – Alkenes
Sample pages
Nature’s Chemistry
Homologous Series – Alkenes
Don’t Forget
Alkenes are another family of hydrocarbons. They
are obtained during cracking of crude oil fractions
and they can be used to make ethanol (alcohol) - and
polymers (plastics) - two very important products in
modern society.
Alkenes differ from alkanes in that they contain a
carbon-to-carbon double covalent bond (C=C) and
so are unsaturated molecules. The C=C double
bond is the functional group – the group of atoms
in a molecule that are responsible for the chemical
reactions that the molecule will undergo.
ALKENE FAMILY
Don’t Forget
You will need to be
able to name and
write formula for the
first eight members
of the alkene family.
Alkene
Full Structural
H
H
Formula H C C H
Ethene
Propene
But-1-ene
Pent-1-ene
H
H
H
H
H
H
H
H
H
H
H
C
C
H
C
H
H
C
H
H
H
H
H
C
C
H
C
H
H
C
H
H
H
H
H
H
H
H
H
C
C
H
C
H
H
C
H
H
H
H
H
H
HH
H
H
C
C
H
C
H
H
C
H
H
H
H
H
C
C
H
C
H
H
C
H
H
H
H
H
C
C
H
C
H
H
C
H
H
C C H
C C H
H
C C H
H
H
C C H
C C H
C C H
H
H
H
C C H
H
H
H
H
C C H
C C H
C C H
H
H
H
C C H
H
H
H
H
C C H
C C H
C C H
H
H
H
C C H
H
H
H
Shortened Structural
Formula
Molecular
Formula
CH2CH2
or CH2=CH2
C2H4
CH3CHCH2
or
CH3-CH=CH2
C3H6
CH3CH2CHCH2
or
CH3-CH2-CH=CH2
C4H8
CH3CH2 CH2CHCH2
C5H10
or
CH3-CH2-CH2-CH=CH2
Straight chain alkenes
If we look at pentene we can see that the C=C can be in two different positions.
A
B
H H H
H
H C C C C C
H
H H H H
and
H H
H
H C C C C C H
H H H H H
Naming straight chain alkenes
The two structures of pentene shown above have slightly different properties. The
boiling point of pent-1-ene is 30°C and the boiling point of pent-2-ene is 36°C and so
they cannot both be called pentene.
cont
20
Sample pages
In the table the carbon-to-carbon double bond is always positioned at the end of
the chain. For butene and pentene the C=C could be positioned between different
carbon atoms.
Sample pages
Alkenes contain a carbon-tocarbon double bond. The first
member of the alkene family
is ethene as there must be at
least two carbon atoms in the
molecule.
Like the alkanes, the alkenes belong to a hydrocarbon family. Take a look at the table
containing formulae of the first four members of the alkene family.
Sample pages
Many fractions are obtained
during distillation of crude
oil and there is a very high
demand for some fractions
whereas some fractions
are less useful. To meet the
demand, fractions containing
large, less useful hydrocarbon
molecules can be cracked into
smaller molecules. Alkene
molecules are also made in this
process.
Alkenes – An Introduction
To name these hydrocarbon molecules, we need additional rules:
•
The carbon atoms in the chain are numbered beginning with the end carbon nearest
to the double bond. The two molecules above would be numbered as shown below.
A C5–C4–C3–C2=C1
•
The position of the C=C bond is given by the first carbon in the double bond. In
molecule A this is C-1 and in molecule B this is C-2.
•
The number is inserted into the name using hyphens (-) to separate the words and
numbers.
A is given the name pent-1-ene
B is given the name pent-2-ene
and
Don’t Forget
If we consider this pentene
molecule and use the naming
rules we arrive at the name
pent-2-ene. It is the same
as molecule B just flipped
round.
B C5–C4–C3=C2–C1
1
•
•
3
4
5
H H H H H
To name branched chain alkenes we need to follow a set of internationally agreed set of
rules again.
H
Look at this
CH3
H C H
compound.or
CH2
HH C H
H
CH3 CH C CH2
H C
C
C
C
H
CH2
H
H H C H
CH3
H C H
CH3CH(CH2CH3)C(CH2CH3)CH2
H
•
2
H C C C C C H
Naming branched chain alkenes.
Full
H H
H
structural formula
Online test
Naming straight chain
alkenes from full and
shortened structural
formulae for alkenes up to
octane - http://cloud4test.
com/hello/tests
Online test
Shortened structural formula
Pick out the longest continuous chain of carbons
containing the double bond. This forms the basis for
the name.
In the compound above the longest continuous chain
containing the C=C double bond has five carbons in it.
This is highlighted in the structure below. The name of
this alkene will be based on pentene.
Notice that the longest chain contains six carbons in it
but this would not include the C=C double bond.
Naming of branched chain
alkenes from full and
structural formulae up to 8
carbons - http://cloud4test.
com/hello/tests
H
H C H
HH C H
H C
H
C
C
C
H H C H
H
H
Don’t Forget
H C H
H
Number the chain starting at the end nearest to the C=C
double bond. In this example this means that numbering
will start at the right hand end of the chain.
The first carbon of the C=C is C1 and so the name is based
on pent-1-ene.
C
The position and names of the branches are now added
to the name. In this example there is a methyl branch on
C3 and an ethyl branch on C2.
The systematic name for the above branched chain
alkene is 2-ethyl-3-methylpent-1-ene.
C
5
C
4
C
3
The branches are listed in
alphabetical order when
using systematic naming.
2
C
1
C
VIDEO LINK
C
Link to Scholar animation
– Naming branched chain
alkenes
C
THINGS TO DO AND THINK ABOUT
For the following alkenes write: (a) the full structural formula
(b) the shortened structural formula
(c) the molecular formula
1.hex-3-ene
2.2-methylbut-2-ene
3.4-ethylhex-2-ene
4.4,5-dimethylhept-3-ene
5.3-ethyl-2-methylpent-2-ene
Name the following alkanes
6.
7.
H
H C H
H
H C H H H C H H
H
C H
C
C
C C
H
H
H C H H
CH3 CH3
CH3 C
C CH3
8. CH3CH2CH(CH2CH3)CHCH2
H
21