Objective
Relate the intensity of the intermolecular forces
to a measurable property. Associate
the intensity of the intermolecular forces to structural characteristics of the molecules.
Task: What is the relationship between the evaporation rate
of a substance and its structure and Inter molecular force?
Theoretical background
Many physical properties are related
to the types and strength of intermolecular forces found in a chemical
substance. In this lab session we will look at the rate of evaporation, surface
tension and viscosity.
Vaporization:
According to the Encyclopaedia Britannica;
Vaporization is the “conversion of a
substance from the liquid or solid phase into the gaseous (vapour) phase”
(Encyclopedia Britannica, 2015).
Moreover, the forces that hold molecules
together in a substance are called Inter Molecular Forces and how the substance
is held (the force of these forces) will determine its phase. Therefore, a
solid will have greater IMF’s than a liquid and a gas. In other words; when
energy was applied to these three; the first one to overcome its IMF’s would be
the gaseous substance, as its arrangement is weaker.
However, when talking about vaporization we
refer to the transformation of a solid or liquid into a gas. According to the
previous point, a liquid will normally vaporize quicker than a solid due to its
structure. Vaporization will occur once thermal energy overcomes IMF’s of a
liquid or solid. At the same time; vaporization is endothermic as it absorbs
energy in the form of heat on order to break bonds inside a substance.
The rate of vaporisation
The rate of vaporization (the change of a
substance into gas) is determined by its surface area, IMF’s and temperature.
The greater the temperature and surface area, the greater the rate of
vaporisation will be. Moreover the heat of vaporization “is the heat required to
vaporize one mole of a liquid” (Wps.prenhall.com, 2015).
Types of Vaporization
Depending on ---, there are two types of
Vaporization: evaporation and boiling. In this lab session we will be studying
the first one; evaporation. Both need energy in order to occur (kinetic theory:
“When an object is heated the motion of
the particles increases as the particles become more energetic” (Le.ac.uk,
2015)).
Boiling occurs when the pressure of a
solution overpasses the atmospheric pressure and as a result, the liquid starts
to boil and bubbles start to appear due to the conversion of all liquid
molecules into gas. On the other hand, evaporation
happens because in a liquid some particles have a greater energy than others.
These “more energetic particles” normally result to have enough energy to
escape from the surface of the liquid by overcoming surface pressure and IMF’s.
According to Maxwell-Blotzmann distribution, a small amount of molecules inside
a liquid will always have enough energy to vaporize. Therefore, water in a wet
cloth or in a pool will always tend to vaporize. All in all, the main
difference is that evaporation only occurs on the surface of the liquid whilst
boiling affects the whole mass.
Evaporation process and IMFs
There are two main factors influencing the rate
of evaporation of a substance; temperature
and volatility, which is “the ease at which a liquid evaporates”. At the same time; Chemicals with
strong intermolecular forces will be less volatile than those with weak forces
and hence, will take longer to evaporate.
As said before, IMFs are those holding
molecules together. The type of force present in the substance will determine
how volatile a substance is. Furthermore,
there are three types of IMF’s: London dispersion forces, permanent dipole and
hydrogen bonding (from weaker to stronger). A substance containing temporary
induced dipoles (London dispersion) will be more susceptible to evaporation
than another containing permanent dipoles (this is because permanent dipoles
are stronger than induced ones as they have a greater attraction). Finally, the
less volatile substance will be the one having hydrogen bonding as this type of
force occurs with the binding of hydrogen to the three of most electronegative
substances on the periodic table: Oxygen, Fluorine and Nitrogen. It is
important to highlight that some substances usually contain more than one force,
which in turn will mean a stronger arrangement of particles.
Molecular mass, polarity and hydrogen
Substances with similar molecular mass will
have similar London forces as the mass of a substance is determined by its
number of subatomic particles and as the number of protons is equal to the
number of electrons; the higher the mass, the higher the number of electrons
and the more induced poles the substance will have. Thence, the greater the
mass, the less volatile a substance will be. Finally, all substances have this
type of force.
Moreover the polarity of a substance also plays
an essential role in kinetic theory. Polarity occurs when there is a difference
between the poles of a substance; which will be temporary; one positive and
another negative. As a result, all polar substances exert permanent
dipole-dipole dipole forces. At the same time, it is important to highlight
that similar types of chemicals will have similar dipole-dipole forces because
of their arrangement. Finally, all
substances containing hydrogen bonding will be very non-volatile and have a
high evaporation point.
In this session we have studied group 1, which
are chemicals with similar molecular masses.
GROUP 1: methyl
acetate, diethyl ether, pentane, butanone, butanol, propionic acid. All of
these compounds contain carbons and have similar intermolecular forces so any
diversity in evaporation rate must be due to hydrogen bonding or permanent
dipole-dipole forces.
·
Pentane: C5H12. Pentane is a hydrocarbon which
belongs to the alkane group; it is
therefore a dehydrated alcohol (removed H20). They contain a carbon-carbon
double bond and therefore are non-polar (electronegative values = C (2.5) and H
(2.1) substances containing the weakest force: London dispersion. Pentane’s
boiling point is 36ºC and hence its evaporation rate very low.
·
Methyl acetate: CH3COOCH3. Belongs to the family of esters; which are ions formed when a hydrogen is removed from
acetic acid. This chemical does not hydrogen-bond, as the hydrogen is only bonded
to carbon. Furthermore, it is weakly polar due to the slight difference between
positive and negative poles (polarity index 2, 5). So it only has the weakest
force, Van der Waals. Its boiling point is therefore around 56.9 °C and its
evaporation rate low.
·
Diethyl ether: C2H5OC2H5. Belongs to the ether family and it is formed by two carbon groups connected to a
single oxygen. (R-O-R). Ethers do
not hydrogen-bond to each other because of their lack of O- H bonds (comparing
to alcohols). Ethers are slightly polar as their polarity index is 2, 8 and
therefore have low boiling points. Diethyl ether is a volatile component used
as an anaesthetic. According to toulen’s evaporation rate of Diethyl ether is
4, 5, low.
·
Butanone: CH3C (O) CH2CH3. Belongs to the ether family and it is also called methyl ethyl ketone. Its
polarity index is 4, 7, so it does have permanent dipole-dipole forces. In
addition, butanone it has Van der Waals forces but does not contain the last
and strongest force, hydrogen bonding. All in all, it has a boiling point of 79.6
ºC and therefore a medium
evaporation rate.
·
Butanol: C4H9OH. It is a primary school alcohol with a 4- carbon structure. It
contains first of all, as all chemicals, London dispersion forces. It is medium
polar and contains permanent dipole-dipole forces, as its polarity index is 4.
To end, as it is an alcohol, it has hydrogen bonding. We are able to see the
H-O bond in the picture below. As a consequence, its boiling point is 117.6 ºC
and its evaporation rate high.
·
Propionic acid: C3H6O2. It is a carboxylic acid which is an organic compound containing(C (O) OH).
It has a very high boiling point, the highest actually, 141.15 °C. This means
that its evaporation rate is very high
too. It has both Van der Waals forces and hydrogen bonding, as we can see in
its structure. Moreover, it contains permanent dipole-dipole forces as it is a
polar molecule because the carboxylic acid group (COOH) on the end is
electron-rich. It has the three Intermolecular Forces, so therefore, it´s
really difficult to break down.
Hypothesis
According to all mentioned above, my research
suggests that the more Intermolecular Forces a chemical has, the less volatile
it is and the highest the evaporation rate is. Before actually doing the
experiment we could have predicted what would happen. Alkanes have less
intermolecular forces than esters (because they only have Van der Waals
forces); esters have less intermolecular forces than ethers (because they only
have London dispersion force and are weakly polar); ethers have less
intermolecular forces than alcohols (because they have Van der Waals and medium
permanent dipole-dipole forces); alcohols have less intermolecular forces than
carboxylic acids (because they contain Van der Waals forces, medium permanent
dipole-dipole forces and hydrogen bonding); and carboxylic acids contain all
three intermolecular forces. Alkanes are therefore the most volatile chemicals
and alcohols and carboxylic acids the less. Summing it up, carboxylic acids and
alcohols have the highest evaporation rate and alkanes the lowest.
Results table
PENTANE
TIME (seconds)
|
TEMPERATURE (°C)
|
0
|
21,80
|
3
|
20,67
|
6
|
17,24
|
9
|
14,03
|
12
|
11,50
|
15
|
9,15
|
18
|
7,34
|
21
|
5,83
|
24
|
4,80
|
27
|
4,27
|
30
|
4,08
|
33
|
4,07
|
36
|
4,36
|
39
|
4,81
|
42
|
5,37
|
45
|
5,95
|
48
|
6,54
|
51
|
7,10
|
54
|
7,67
|
57
|
8,22
|
60
|
8,76
|
63
|
9,29
|
66
|
9,82
|
69
|
10,34
|
72
|
10,82
|
75
|
11,26
|
78
|
11,67
|
81
|
12,04
|
84
|
12,38
|
87
|
12,69
|
90
|
12,99
|
93
|
13,28
|
96
|
13,53
|
99
|
13,78
|
102
|
14,00
|
105
|
14,22
|
108
|
14,42
|
111
|
14,60
|
114
|
14,78
|
117
|
14,95
|
120
|
15,10
|
METHYL ACETATE
TIME (seconds)
|
TEMPERATURE (°C)
|
0
|
22,87
|
3
|
22,77
|
6
|
22,29
|
9
|
21,48
|
12
|
19,75
|
15
|
18,13
|
18
|
16,84
|
21
|
15,73
|
24
|
14,72
|
27
|
13,81
|
30
|
12,97
|
33
|
12,26
|
36
|
11,62
|
39
|
11,17
|
42
|
10,80
|
45
|
10,43
|
48
|
10,10
|
51
|
9,75
|
54
|
9,45
|
57
|
9,15
|
60
|
8,86
|
63
|
8,61
|
66
|
8,45
|
69
|
8,28
|
72
|
8,08
|
75
|
7,85
|
78
|
7,75
|
81
|
7,65
|
84
|
7,54
|
87
|
7,47
|
90
|
7,41
|
93
|
7,40
|
96
|
7,38
|
99
|
7,33
|
102
|
7,39
|
105
|
7,56
|
108
|
7,68
|
111
|
7,86
|
114
|
8,05
|
117
|
8,83
|
120
|
9,18
|
DIETHYL ETHER
TIME (seconds)
|
TEMPERATURE (°C)
|
0
|
22,39
|
3
|
21,99
|
6
|
21,18
|
9
|
18,35
|
12
|
14,93
|
15
|
12,20
|
18
|
10,20
|
21
|
8,80
|
24
|
7,70
|
27
|
6,83
|
30
|
6,05
|
33
|
5,34
|
36
|
4,68
|
39
|
4,13
|
42
|
3,64
|
45
|
3,24
|
48
|
3,22
|
51
|
3,57
|
54
|
4,15
|
57
|
4,83
|
60
|
5,61
|
63
|
6,39
|
66
|
7,17
|
69
|
7,92
|
72
|
8,60
|
75
|
9,26
|
78
|
9,84
|
81
|
10,38
|
84
|
10,82
|
87
|
11,22
|
90
|
11,61
|
93
|
11,96
|
96
|
12,27
|
99
|
12,57
|
102
|
12,84
|
105
|
13,09
|
108
|
13,35
|
111
|
13,60
|
114
|
13,83
|
117
|
14,07
|
120
|
14,40
|
BUTANONE
TIME (seconds)
|
TEMPERATURE (°C)
|
0
|
22,48
|
3
|
22,12
|
6
|
21,56
|
9
|
20,62
|
12
|
19,75
|
15
|
19,00
|
18
|
18,37
|
21
|
17,79
|
24
|
17,22
|
27
|
16,73
|
30
|
16,32
|
33
|
15,99
|
36
|
15,68
|
39
|
15,37
|
42
|
15,08
|
45
|
14,77
|
48
|
14,43
|
51
|
14,13
|
54
|
13,81
|
57
|
13,56
|
60
|
13,34
|
63
|
13,12
|
66
|
12,90
|
69
|
12,72
|
72
|
12,62
|
75
|
12,49
|
78
|
12,37
|
81
|
12,28
|
84
|
12,23
|
87
|
12,14
|
90
|
12,04
|
93
|
11,99
|
96
|
11,94
|
99
|
11,91
|
102
|
11,85
|
105
|
11,78
|
108
|
11,68
|
111
|
11,62
|
114
|
11,61
|
117
|
11,60
|
120
|
11,55
|
BUTANOL
TIME (seconds)
|
TEMPERATURE (°C)
|
0
|
21,56
|
3
|
21,29
|
6
|
21,29
|
9
|
21,20
|
12
|
21,15
|
15
|
21,09
|
18
|
21,05
|
21
|
20,99
|
24
|
20,93
|
27
|
20,88
|
30
|
20,84
|
33
|
20,79
|
36
|
20,75
|
39
|
20,68
|
42
|
20,61
|
45
|
20,54
|
48
|
20,45
|
51
|
20,39
|
54
|
20,33
|
57
|
20,27
|
60
|
20,21
|
63
|
20,16
|
66
|
20,11
|
69
|
20,07
|
72
|
20,03
|
75
|
19,98
|
78
|
19,94
|
81
|
19,90
|
84
|
19,87
|
87
|
19,83
|
90
|
19,78
|
93
|
19,70
|
96
|
19,67
|
99
|
19,61
|
102
|
19,55
|
105
|
19,52
|
108
|
19,48
|
111
|
19,44
|
114
|
19,40
|
117
|
19,36
|
120
|
19,34
|
PROPIONIC ACID
TIME (seconds)
|
TEMPERATURE (°C)
|
0
|
22,04
|
3
|
21,97
|
6
|
21,96
|
9
|
21,95
|
12
|
21,93
|
15
|
21,87
|
18
|
21,82
|
21
|
21,80
|
24
|
21,75
|
27
|
21,73
|
30
|
21,70
|
33
|
21,64
|
36
|
21,58
|
39
|
21,56
|
42
|
21,52
|
45
|
21,46
|
48
|
21,41
|
51
|
21,40
|
54
|
21,38
|
57
|
21,37
|
60
|
21,35
|
63
|
21,34
|
66
|
21,31
|
69
|
21,30
|
72
|
21,29
|
75
|
21,26
|
78
|
21,25
|
81
|
21,21
|
84
|
21,20
|
87
|
21,12
|
90
|
21,06
|
93
|
21,03
|
96
|
21,00
|
99
|
20,99
|
102
|
20,93
|
105
|
20,91
|
108
|
20,88
|
111
|
20,87
|
114
|
20,83
|
117
|
20,81
|
120
|
20,76
|
DIFFERENT EVAPORATION RATE DUE TO DIFFERENT
INTERMOLECULAR FORCES
Independent variable- Chemical
|
Dependent variable- Time to reach evaporation
point(s)
|
Pentane
|
36 seconds
(21,80- 4,36°C)
|
Methyl acetate
|
99 seconds
(22,87-7, 33°C)
|
Diethyl ether
|
48 seconds
(22,39- 3,22°C)
|
Butanone
|
+120
seconds (22,48-11,55°C …)
|
Butanol
|
+120
seconds (21,56 – 19,34°C …)
|
Propionic acid
|
+120
seconds (22,04 – 20,76°C …)
|
Graphs
Pentane´s evaporation
rate
Methyl acetate´s
evaporation rate
Diethyl ether´s
evaporation rate
Butanone´s evaporation
rate
Butanol´s evaporation
rate
Propionic acid´s
evaporation rate
Conclusion
To begin with, all six graphs convey that my
hypothesis was correct. The first graph shows pentane´s evaporation rate and we
are able to prove how weak its intermolecular forces are due to the line
formed. The substance evaporates within a very short period of time and then
goes back to normality very quickly. This clearly shows its low evaporation
rate. Moreover, methyl acetate´s graph suggests that it doesn´t have a lot of
intermolecular forces as it evaporates in a relative small amount of time.
However, the maximum drop of temperature of this chemical must have occurred
earlier. Furthermore, the third graph conveys the low evaporation rate of
Diethyl ether, whose maximum drop of temperature occurs in the second 47. This
happens because as it an ether and has small intermolecular forces, it does not
need much energy to evaporate. In addition, Butanone which is also an ether
takes longer to reach a low temperature and it does not actually reaches
evaporation. This is because even though both are ethers, Butanone has a bigger
polarity, therefore more permanent dipole-dipole forces and consequently needs
much more energy to evaporate. As well, Butanol does not evaporates and we are
able to see it because in the graph it does not heat up due to the big amount
of energy it needs to reach boiling point and evaporate as its intermolecular
forces are quite strong. Finally, the last chemical, propionic acid shows a
nearly perfect straight line. This is because it needs too much energy to
overcome its intermolecular forces and evaporate.
Below we are able to see the differences in
each chemical´s graph in relation to its intermolecular forces.
Pentane Methyl acetate
Diethyl ether Butanone
Butanol Propionic acid
Evaluation of Results
Even though most results are
coherent and match with the expected ones, there is a graph which shocks a bit.
It´s the methyl acetate´s one. As this chemical is an ester with only Van der
Waals forces, it shouldn´t need so much energy and should have evaporated in a
smaller period of time, within 50 seconds approximately, not a minute 40
seconds. In the table of results we are able to see this incoherence. Methyl
acetates takes 99 seconds to evaporate, howbeit diethyl ether, which has more
permanent dipole-dipole forces, takes 48 seconds. This might have been caused
by an error in the procedure.
Evaluation of method
During the experiment we found some
errors within the method that could have influenced in the results. First of
all, as we are carrying out the procedure without controlling the atmospheric
temperature, the temperature might vary from one chemical to another and this
can cause incoherent results regarding evaporation rate. We can notice with the
tables of results that each chemical starts to evaporate at a different
temperature, which is insane. A solution to this may be performing it in a
place completely isolated and with a constant temperature. Moreover, the fact that
we were not given the concentration of each of the chemicals might influence in
the results. If a chemical was more concentrated than another it means that
there are more intermolecular forces even though the molecular mass was the
same. To solve this we should have bottle of chemicals with the same
concentration. Furthermore, when we dip the probe in the tube some of the
papers got more chemical than others and this might cause problems too due to
the difference in quantity. A solution to this could be timing exactly the
seconds the probe needs to be inside the beaker.
References
-
Encyclopedia Britannica,. (2015). vaporization
| phase change. Retrieved 18 January 2015, from http://global.britannica.com/EBchecked/topic/623152/vaporization
-
Le.ac.uk,. (2015). Particle
Theory - changes of state. Retrieved 18 January 2015, from https://www.le.ac.uk/se/centres/sci/selfstudy/particle02.html
-
Niroinc.com,. (2015). Evaporation
Process Principles. Retrieved 18 January 2015, from http://www.niroinc.com/news_archives/evaporation_process_principles.asp
- Wps.prenhall.com,. (2015). Liquids, Solids, Intermolecular
Forces. Retrieved 18 January 2015, from http://wps.prenhall.com/esm_tro_chemistry_1/77/19899/5094337.cw/
