Lab Session 3- Intermolecular forces investigation

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: C₂H₅OC₂H_5. 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/