To investigate the relationship between the molecular mass of a substance and its boiling point.
Research question
What is the relationship between the boiling point of a substance with the strength of its intermolecular force.
Hypothesis
The bigger the molecular mass, the higher the boiling point of a substance and it will take longer for that substance to reach that boiling point. The substances that we tested are water (H2O), methanol (CH3OH), absolute ethanol (C2H5OH), propanol (C3H7OH) and diethyl ether ([(C2H5)]2O)). The alcohols have dipole-dipole and Van der Waals force (Clark), diethyl ether has only dipole-dipole force (Andreska) but it is a hydrogen bond acceptor ("Hydrogen Bonding"), while water has hydrogen bond (a special type of dipole-dipole) (Ophardt). Hydrogen bond is the strongest bond ("Hydrogen Bonding") and hence it is hypothesized that water will have higher boiling points than other substances. In the case of diethyl ether, however, there is a weak dipole-dipole attraction (Andreska) as well as dispersion force. The OH tails in alcohols give them a strong polar end (Clark), but the carbon ends mean they will have weak dispersion force (Clark). However, their large molecular mass also means that their boiling point is increased, albeit not as high as water’s. This means diethyl ether will have the lowest boiling point if the hypothesis is taken to be true.
Order of boiling hypothesize (from lowest to highest): diethyl ether, methanol, absolute ethanol, propanol and water.
Background research
Intermolecular force is the attraction or repulsion forces between neighbouring molecules or atoms ("Intermolecular Force"). There are three types of intermolecular force: Van der Waals (or dispersion force), dipole-dipole attraction and hydrogen bonding ("Intermolecular Forces").
In the area surrounding an atom or a molecule, there is an electron cloud with a particular charge density (Rashe et al.). Electrons are not stationary all the time so they often move around, thus shifting the dipole (Rashe et al.). This means that even though the molecule itself is non-polar since there is no permanent dipole , the constant shifting of electrons induce a temporary charge within the molecular species, thus the dispersion force (Rashe et al.).
The higher molecular mass of a particle, the stronger its dispersion force because there are more electrons, thus the dipole shifting occurs less because one end of the molecule becomes slightly more electronegative than the other(Rashe et al.).
When molecules become polar, melting points and boiling points increase because the dipole-dipole attraction means that the intermolecular force is relatively stronger than of Van der Waals force, so more energy is required to break this polar bond (Rashe et al.). A dipole-dipole force means a particle has a permanent polar because one species of the molecule is more electronegative than the other (Rashe et al.). For examples, hydrochloric acid (HCl) is has a dipole-dipole intermolecular attraction because the Cl- species is more electronegative than the H+ species, so electrons tend to cloud around the Cl- species more, creating a permanent dipole.
Hydrogen bonding is a special type of dipole-dipole attraction ("Intermolecular Forces"). A hydrogen bond formed between the hydrogen element and a highly electronegative element such as Nitrogen, Oxygen and Fluorine ("Intermolecular Forces"). This is because hydrogen loses the only electron it has and becomes a positive species, while the more electronegative elements grab that electron and becomes negative species ("Intermolecular Forces").
An alcohol will have a hydrogen bonding due to the OH tail of the molecules (Clark). It has dispersion force as well as dipole-dipole interaction (Clark). However, as the alcohols become bigger, meaning that more carbons are being added to the change, the dispersion force in fact increases while the hydrogen bonding force stays the same (Clark). As shown in figure 1, there are more carbons so the dispersion force becomes larger (Clark)
Figure 1
Variables
Types of variables
What is it?
Explanation
Independent
Types of covalent molecular substance that are liquid (diethyl ether, methanol, absolute ethanol, propanol and water)
All of these substances have hydrogen bonding, dipole-dipole attraction and Van der Waals force, which means the type of intermolecular force between them is kept constant. The only difference these substances would have is their molecular mass, and perhaps the strength of hydrogen bonding and Van der Waals forces between each molecules.
Five substances are chosen because an in depth analysis can be made using them. Water is a classic example of a strong hydrogen bond and it is universally known that at lab condition, water boils at 100 °C. Methanol, ethanol and propanol are alcohols with boiling points known from secondary data to be less than 100 °C (Clark), which is the upper limit of a standard laboratory thermometer (the maximum temperature the thermometer could reach is 110 °C. Any temperature higher than that, the thermometer would break). Diethyl ether is chosen because it has an oxygen and hydrogen species. However, the oxygen and hydrogen are not covalently bonded with each other, thus making diethyl ether a hydrogen bond acceptor rather than a full-fledged hydrogen bond.
Dependent
Boiling point (°C)
An indication for strength of covalent bond would be the temperature in which the liquid begins to boil. To measure this, a thermometer with an upper limit of 110 °C is used because all of the substances tested boiled below this limit, according to secondary data. To measure the boiling point, a reading of temperature is taken every 30 seconds, so that when it reaches the stage where the liquid starts bubbling and the temperature stays the same even though many seconds have passed, then that point in temperature is taken as the experimental boiling point.
Controlled
Same amount of liquid used (30 mL)
A larger body of liquid requires more time to be heated up, and similarly, a small body of liquid requires less time to be heated up. If the amount of liquid used for each substance is not the same, it means that the rate of evaporation of each liquid cannot be determined because the same amount of liquid is not evaporated off.
Same thermometer used
Different thermometers have different systematic error, so in order to keep the systematic error the same for all trials in this experiment, the same thermometer should be used to avoid varying systematic errors. A data logger is recommended if the resources are available, however, as it gives a more precise and accurate temperature reading than an average thermometer
Same heating plate being used with the same heating setting
A heating plate is used instead of a Bunsen burner because alcohols are flammable. This means that the same heating plate should be used because different heating plates use a slightly varying power capacity, which in turn leads to its systematic error. A heating plate would have had 6 settings with varying heating capacity, so one setting should be selected to ensure that the heating rate is the same throughout the experiment.
Same stopwatch being used
To measure time, a stopwatch is used. However, like the other equipment, each stopwatch has a different systematic error because not all stopwatches are identical. Therefore, the same stopwatch should be used throughout the experiment to keep the systematic errors constant.
Equipment
• 30 mL of distilled water
• 30 mL of methanol
• 30 mL of absolute ethanol
• 30 mL of propanol
• 30 mL of diethyl ether
• 5 x 50 mL beaker
• 1 x 110 °C thermometer
• 1 x heating plate
• 1 x stopwatch
• 1 x 4 mL pipette
• 1 x 100 mL beaker
• 1 x pipette used in titration that has a 25 mL marking and a rubber squeezer to suck the liquid up
• 1 x heat proof glove
Risk assessment
1. Alcohols are flammable. The vapour, when evaporated, can still catch flame and burn in the air. Therefore a heating plate should be used to heat up the liquid rather than Bunsen burner. A heating plate means that the heating can be moderated as well, leaving no room for unpredictable physical change. Since alcohols are flammable and there are many experiments occurring around the room, the experiment should be conducted in the fume cupboard so the vapour does not escape into open air.
2. Diethyl ether has a very strong smell and it may cause nasal irritation because of its strong smell. According to secondary data, diethyl ether boils at a low temperature, meaning it can evaporate in the air quickly. To avoid this, place the chemical inside a fume cupboard as well
3. Heating plates are hot. The metal plate, when heated, takes a very long time to cool down so the surface of the plate is very hot and it can cause burn if touched. Avoid touching the heating plate after the experiment is finished. Unplug the electrical cord and allow the plate to cool down before packing away. When a beaker is finished being heated, remove it from the heating plate using a heat proof plastic holder to avoid being burned.
4. Thermometer can only go up to 110 °C. Any temperature higher than that will cause the thermometer to break. Therefore, all the substances that are selected to be tested in this experiment all have their boiling points below this upper limit, according to secondary data.
Method
1. Pour a large amount of distilled water into a clean 100 mL beaker until it fills about half of that beaker.
2. Obtain 25 mL of distilled water from the 100 mL beaker by using the titration pipette and put the water inside the 50 mL beaker. Use the smaller 4 mL pipette to add an additional 5 mL into the beaker.
3. Measure the initial temperature of the water inside the beaker using a thermometer. This means wait until the temperature on the thermometer stops dropping and take the final reading. Record it.
4. Turn on the heating plate inside the fume cupboard and turn the setting to 3. Place the 50 mL beaker of distilled water on with the thermometer inside it. Start the stopwatch
5. Read the temperature on the thermometer every 30 seconds and record it.
6. Keep doing so until the distilled water inside the beaker starts bubbling. Record any qualitative observation and mark the time interval where it starts bubbling with a *.
7. Keep heating the beaker up until constant temperature is reached. This is when the liquid starts boiling. Leave the beaker on the plate for 2 more minutes and record any loss of liquid or any physical changes inside the beaker (bubbling vigorously, steam coming out).
8. Continue with the heating for 3 more 30 seconds intervals (1 minute 30 seconds more).
9. Turn off the heating plate and take the beaker off using a heat proof glove.
10. Repeat step 1 – 8 three more times to ensure reliability.
11. Repeat step 1 – 9 for methanol.
12. Repeat step 1 – 9 for ethanol.
13. Repeat step 1 – 9 for propanol.
14. Repeat step 1 – 9 for diethyl ether.
Results
Raw data
Table 9.1 showing the time taken for different types of liquid to boil, measure at every 30 seconds time interval to indicate the rate of evaporation
Time (s)
Temperature (°C)
Water
Propanol
Absolute ethanol
Methanol
Diethyl ether
0
26
26
22
18
19.5
30
26
33
29
25
32.5
60
26.5
42
40
37.5
35
90
28.5
49
49
47
35.5
120
31.5
55
60
57
36
150
36
61
72
62
36
180
42
64.5
74
64.5
36
210
49
67.5
74.5
64
36
240
56
70
74.5
64.5
36
270
63
71
74.5
64.5
300
69.5
76
74.5
64.5
330
75
82
64.5
360
80
89.5
390
84
91
420
87
91
450
91
91
480
90.5
91
510
91
540
91
570
91
600
93.5
630
95.5
660
97
690
99.5
720
99.5
750
100
780
100
810
100
840
100
Observation
Roughly 30 mL of water is still in the beaker, with little amount evaporated away even when it has reached its boiling point. The water begins to give off vapour at around 85 °C
Bubbles start appearing around 89°C and bubble appears to be more vigorous around 89.5°C. Boiling occurs at 91°C. When the heating plate is turned off, the remaining volume is roughly around 18 – 19 mL.
Ethanol bubbles for a little bit before bubbling vigorously at 2 minutes and 30 seconds and keeps evaporating until it reaches its boiling point. Its final volume is decreased roughly by half when the heating plate is turned off, at 15 mL
Bubbles start appearing around 2 minutes 30 seconds and it bubbles vigorously before reaching its boiling point. The final volume, when the heating plate is turned off, is 20 mL
Bubbles start appearing around 30 seconds after being heated. Some of the alcohol evaporate off at 2 minutes and 20 seconds. Only 1/3 of the alcohol is left. Roughly 3 minutes since boiling, all of the alcohol evaporated off completely.
Processed data
Table 9.2 showing time vs temperature rise of 30 mL of water
Time (s)
Temperature (°C)
0
26
30
26
60
26.5
90
28.5
120
31.5
150
36
180
42
210
49
240
56
270
63
300
69.5
330
75
360
80
390
84
420
87
450
91
480
90.5
510
91
540
91
570
91
600
93.5
630
95.5
660
97
690
99.5
720
99.5
750
100
780
100
810
100
840
100
Figure 2
Table 9.3 showing time vs temperature rise of 30 mL of propanol
Time (s)
Temperature (°C)
0
26
30
33
60
42
90
49
120
55
150
61
180
64.5
210
67.5
240
70
270
71
300
76
330
82
360
89.5
390
91
420
91
450
91
480
91
Figure 3
Table 9.4 showing time vs temperature rise of 30 mL of absolute ethanol
Time (s)
Temperature (°C)
0
22
30
29
60
40
90
49
120
60
150
72
180
74
210
74.5
240
74.5
270
74.5
300
74.5
Figure 4
Table 9.5 showing time vs temperature rise of 30 mL of methanol
Time (s)
Temperature (°C)
0
18
30
25
60
37.5
90
47
120
57
150
62
180
64.5
210
64
240
64.5
270
64.5
300
64.5
330
64.5
Figure 5
Table 9.6 showing time vs temperature rise of 30 mL of diethyl ether
Time (s)
Temperature (°C)
0
19.5
30
32.5
60
35
90
35.5
120
36
150
36
180
36
210
36
240
36
Figure 6
Table 9.7 showing the relationship between molecular mass and boiling point
Substance
Molecular Mass (gMol-1)
Boiling point (°C)
H2O
18
100
CH3OH
32
64.5
C2H5OH
46
74.5
C3H7OH
60
91
[(C2H2]2O)
74
36
Conclusion
This experiment seeks to investigate the relationship between the molecular mass of a substance and its boiling point. However, it is hypothesized that the factor affecting the boiling point of the substance is not necessarily just the molecular mass, and table 9.7 seems to support this hypothesis. However, the situation is not as simple as it presents itself in the data table.
The boiling point of a substance depends on the strength of their intermolecular force (Rashe et al.). However, the mass of the substance is also important an important indicator of a substance’s intermolecular force (“Intermolecular Force”). This is because the heavier a substance, the stronger its Van der Waals force (“Intermolecular Forces”). In alcohols, we can see that as more carbons are added to the chains, the molecular mass of alcohol substances become heavier (Clark) and as table 9.7 has indicated, propanol has a higher boiling point than ethanol, which in turn has a higher boiling point than methanol.
The substance with highest boiling point is water (H2O) and it also takes the longest time to boil as well. However, it is very strange because water has the lowest molecular mass, as indicated by table 9.7. Then why is it that water has the highest boiling point? To answer this question it is important to consider the nature of bonding of water. Water is made up of hydrogen and oxygen. Hydrogen ion only has a proton so it is very positive, while oxygen is a highly electronegative element. Figure 7 shows how water molecules are bonded together on an intermolecular level.
Figure 7
The intermolecular bond between the slightly negative oxygen polar and the slightly positive hydrogen polar enables this strong hydrogen bond to take places. Hydrogen bond is the strongest intermolecular forces (“Hydrogen Bonding”) and its strength outweighs Van der Waals force and normal dipole-dipole force (Ophardt). Oxygen is especially electronegative so that is why water has such a strong intermolecular force and thus, takes more time to reach its boiling point and its boiling point is a relatively high temperature (100°C) as well.
Alcoholic substances have both Van der Waals force and hydrogen bonding due to the OH tail (Clark). However, the OH tail is a very small part of the intermolecular force that cause the attraction between these alcoholic molecules. This is because the carbon end of the molecule is not attracted to the polar OH so when the molecules are closed together, the main force keeping them together is the dispersion force with some hydrogen bonding from the OH tail (Clark). This explains why even though within the alcohol group, the higher the molecular mass the higher the boiling points as indicated by table 9.7, their boiling points are still lower than water and they take shorter time to boil compare to water.
Diethyl ether is a special case scenario because even though it has an oxygen polar end (Jannis), its bonding is inductive to hydrogen bonding and dipole-dipole bonding from other substance rather than forming these types of bonding based on the atoms they have (Jannis). The carbon-donating hydrogen bonding, however, is relatively a weak polar bond (Jannis). The bond itself is not very stable in the first place so therefore, diethyl ether evaporates very easily (Jannis).
In conclusion, the hypothesis is correct because I hypothesize that the order of boiling point, from lowest to highest, would be diethyl ether, methanol, absolute ethanol, propanol and water.
Evaluation
This experiment is reliable because the method ensures repetition to occur and when repetition is made, the results are shown to be very similar to one another. Therefore, this experiment is reliable because the method ensures repetition is possible and when repeated, the similar results are produced.
Secondary data indicates that at room temperature and pressure, water boils at 100°C. The experiment produces indicates water boiling at 100°C so the result is accurate for this experiment. Propanol boils at 97°C but the experiment indicates that it boils at 91°C. Ethanol boils at 78°C but the experiment says that it boils at 74.5°C . Methanol boils at 64.7°C and the experiment indicates it to boil at 64.5°C , which is surprisingly very close. However, for propanol and ethanol, the reason why the difference between secondary data and experimental data is big because there are heat loss to the environment and because the heating plate automatically turns itself off to prevent overheating. However, the surface in which the cup is placed on is a metal plate anyway so the heat should have been kept and transpired to heat the liquid up. Perhaps it is because the liquid has evaporated off so the heat was carried away as well. To prevent this the beaker could be replaced with a conical flask and a rubber stopper placed on top to stop the heat from escaping.
This is my very first lab report that I wrote in year 10. It's not perfect and there are many things that are wrong. I didn't include things such as significant figures, error analysis, trend analysis and all of those stuff. In fact I didn't score very high on it at all. However, I have since written many more lab reports and believe it or not, I didn't find it very tedious at all. When people think of science, they usually take for granted how much must be written in order to make your investigation as clear as possible, and how important language is in creating biases in your results. To be good at science does not negate the possibility to be good at languages.
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