IB Chemistry HL Name: Yoojin Lee Candidate Number: 002213-067
Candidate Name Candidate Number Date of Practical
:Yoojin Lee :002213-067 :October 17, 2010
Internal Assessment – Rate of Evaporation Research Question How will changing the molar mass of alcohol affect the rate of evaporation, represented by the change in temperature over time, measured using a temperature probe?
Introduction Alcohol1 is an organic compound made out of carbon chain and a hydroxyl functional group. Alcohols differ by CH2 groups and the number of CH2 groups determines the size of the alcohol. Alcohols have general formula CnH2n+1OH. In this investigation, five different alcohols were tested: methanol (CH3OH) ethanol (C2H5OH), propan-2-ol (C3H7OH), butan-1ol (C4H9OH), and pentan-1-ol (C5H11OH), increasing by CH2. Ethanol is commonly used for fuel resources and alcoholic beverages, while propan-2-ol is used as a cleaning fluid. Lastly, methanol, butan-1-ol, pentan-1-ol are used for manufacturing chemicals in industry.2
Evaporation is a change in phase from liquid to gas. The major factor that determines the rate of evaporation is the intermolecular forces of attraction between molecules. For instance, alcohols evaporate faster than water, because water molecules have stronger intermolecular forces of attraction than alcohols; one H2O molecule can form up to four hydrogen bonds, whereas CnH2n+1OH can only form one hydrogen bond, because only one OH group attached at the end of the carbon chain. Moreover, the intermolecular forces of attraction also increase when the relative molecular mass increases. In this experiment, since only alcohols are tested,
1
“Alcohol,” Wikipedia, the free encyclopedia, http://en.wikipedia.org/wiki/Alcohol (accessed January 8, 2011).
2
“N-butanol,” Wikipedia, http://en.wikipedia.org/wiki/N-Butanol (accessed January 8, 2011). 1
IB Chemistry HL Name: Yoojin Lee Candidate Number: 002213-067
the number of carbon chain will determine the relative molecular mass as well as the intermolecular forces of attraction. When the carbon chain is long, the molecule will have larger surface area, resulting in greater Van Der Waals forces between the carbon chains.
As the molecules are converted to gas phase, they absorb heat, cooling the surrounding temperature. Consequently, evaporation reaction is an endothermic process. Thus, in measuring the rate of evaporation, temperature probe is used to measure the decrease in temperature. The faster the rate of evaporation is, the faster the temperature will decrease. Hence, by examining the change of temperature overtime, the rate will be calculated and analyzed. For this investigation, since the evaporation occurs as soon as the alcohol is released, only instantaneous rate will be considered.
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IB Chemistry HL Name: Yoojin Lee Candidate Number: 002213-067
Hypothesis The rate of evaporation is represented by the change in temperature over time. Among methanol, ethanol, propan-2-ol, butan-1-ol, and pentan-1-ol, the rate of methanol evaporation will be the fastest, because it has the lowest relative molecular mass, and thus the intermolecular forces of attraction are the smallest, meaning that the methanol molecules can easily evaporate. On the other hand, the rate of pentan-1-ol will be the slowest, because it has the highest mass and the greatest intermolecular forces of attraction. The intermolecular forces of attraction is directly proportional to the relative molecular mass when the molecules’ properties are the same and the rate of evaporation is inversely proportional to the intermolecular forces. Hence, as the molar mass increases, the rate of evaporation will decrease. However, longer chains will gradually stabilize and evaporate to a limited extent or not evaporate at all as the intermolecular forces of attraction gets stronger. ㅣ
ă…Ł
Figure 1 shows the predicted relationship between the rate of evaporation and the molar mass of the alcohol 3
IB Chemistry HL Name: Yoojin Lee Candidate Number: 002213-067
Variables Variables Independent
Description Molar mass of alcohol
Dependent
Rate of evaporation ă…Ł
Controlled
Method of Measuring Different types of alcohol were chosen: methanol, ethanol, propan-2-ol, butan-1ol, and pentan-1-ol. Each differs by CH2, and thus increasing the relative molecular mass by approximately 14g. Triplicate trials were performed on each type of alcohol to obtain the mean. Rate of evaporation is represented by the change in temperature over time. Temperature was measured using the temperature probe on Logger Pro.
ă…Ł
Size of the sponge
The size has to be identical, because the sponge pieces are directly attached to the probe for absorption of the alcohol sample. The sponge pieces of identical shape and size was prepared using scalpel and ruler. Only well-cut pieces were used for each trial. Type of the sponge For the experiment, a tight sponge was used so that it is easier to cut. Different types of sponge have different absorption rate. Thus, the same type of sponge was used to ensure that the alcohols are absorbed simultaneously. Volume of alcohol For all types of alcohol, 1cm3 of alcohol was released onto the sponge piece. Micropipette was used for accurate measurement and transfer. Temperature In higher temperature, the rate of evaporation will increase. Thus, all trials need to be performed under constant room temperature. Also, the atmosphere has to be windless; otherwise, the rate of evaporation will vary to a great extent. Temperature Probe Since different temperature probes are calibrated slightly differently, in order to prevent random error, the same temperature probe was used throughout the experiment. Alcohol Samples Each alcohol was used from the same bottle to reduce error and each trial was performed immediately to prevent natural evaporation of the liquid. Table 1 shows the independent, dependent, and controlled variables and the methods of measuring 4
IB Chemistry HL Name: Yoojin Lee Candidate Number: 002213-067
Apparatus
Materials Methanol Ethanol Propan-2-ol Butan-1-ol Pentan-1-ol
Temperature probe (Logger Pro) Micropipette (± 0.006cm3) A tight sponge Styrofoam Box Scalpel Ruler Stapler
Procedure 1. The temperature probe was inserted in a Styrofoam box to prevent change in temperature during experimentation. 2. Identical pieces of sponges were cut using scalpel and ruler and were wrapped around the tip of the temperature probe, fixed with staple at the edge.
Figure 2 shows the setup of the experiment 3. Data was collected for a few seconds to ensure that the room temperature was constant. 4. 1cm3 of pentan-1-ol was measured and transferred onto the sponge using a micropipette and the change in temperature was recorded. 5. Steps 2-4 were repeated to obtain the mean for the triplicate trials 6. Steps 2-5 were repeated for remaining butan-1-ol, propan-2-ol, ethanol, and methanol
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IB Chemistry HL Name: Yoojin Lee Candidate Number: 002213-067
Data Collection – Qualitative Data There were no visible changes other than the sponge absorbing the alcohol. During the experiment, the strong smell of alcohol spread as soon as the alcohol was transferred onto the sponge. This shows that the evaporation was surely occurring. All of the alcohol samples emitted strong, pungent smell.
Data Collection – Quantitative Data Temperatures of Alcohols with Different Molar Masses (M)/ ℃ Time, t/ sec
Methanol (M=32.0g/mol)
Ethanol (M=46.1g/mol)
Propan-2-ol (M=60.1g/mol)
Butan-1-ol (M=74.1g/mol)
Pentan-1-ol (M=88.2g/mol)
1st
1st
1st
1st
1st
2nd
3rd
2nd
3rd
2nd
3rd
2nd
3rd
2nd
3rd
0.00
16.9 16.5 18.0 25.5 25.0 25.1 25.1 25.7 25.5 25.9 25.5 25.3 18.3 18.0 18.3
30.0
17.0 16.6 17.9 25.3 24.9 25.0 25.0 25.5 25.3 25.7 25.4 25.2 18.4 18.1 18.3
60.0
15.4 16.0 16.1 23.9 23.8 23.4 23.9 24.0 24.3 24.9 25.1 25.1 18.0 17.8 17.9
90.0
12.5 12.4 12.7 22.2 22.0 21.8 22.9 22.8 23.0 25.1 25.2 25.1 17.9 17.8 17.8
120
10.2 9.51 10.2 20.8 20.7 20.5 22.1 22.1 22.1 25.2 25.3 20.8 18.0 17.9 17.8
150
8.52 7.55 8.33 19.7 19.7 19.4 21.5 21.5 21.4 25.3 25.3 25.3 13.1 17.9 17.8
180
7.28 6.21 6.98 18.9 18.8 18.5 21.0 21.1 20.8 25.4 25.3 25.3 18.2 18.0 17.9
210
6.38 5.29 6.00 18.3 18.2 17.8 20.6 20.7 20.3 25.4 25.3 25.3 18.2 18.0 17.9
240
5.76 4.66 5.25 17.8 17.7 17.3 20.2 20.3 19.9 25.4 25.3 25.3 18.2 18.0 18.0
270
5.31 4.24 4.65 17.5 17.3 17.0 19.9 20.1 19.6 25.4 25.2 25.2 18.3 18.1 18.0 Table 2 shows condensed raw data for the experiment extracted from Logger Pro.
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IB Chemistry HL Name: Yoojin Lee Candidate Number: 002213-067
Graph 1 shows the raw data for the effect of changing the molar mass of alcohol on the rate of evaporation 7
IB Chemistry HL Name: Yoojin Lee Candidate Number: 002213-067
Data Processing The absolute value of the gradient of Graph 1 represents the change in temperature over time. Thus, it represents the rate of evaporation. The processed data is shown in Table 3 below. Rate of Evaporation, r/ ℃ s-1 Molar Mass of the Alcohol, M/ g mol-1
Trials Mean(a)
Mean ± SD(b)
1
3
3
32.0
0.174
0.129
0.177
0.160
0.027
46.1
0.146
0.119
0.140
0.135
0.014
60.1
0.0669
0.0703
0.0683
0.0685
0.0017
74.1
0.0163
0.0179
0.0182
0.0175
0.0010
88.2
0.0150
0.0134
0.0156
0.0143
0.0017
Table 3 shows the rates of evaporation for different molar masses of the alcohol. Mean: average of triplicate trials for each set. (b) SD: standard deviation for triplicate trials. (a)
Sample Calculations ㅣ
ㅣ
Calculation of the mean rate of 32.0g mol-1 (methanol) from the triplicate trials. Mean ( ) = =
s-1
0.160
Calculation of the standard deviation of 32.0g mol-1 (methanol) from the triplicate trials Standard deviation =
= = 0.027
s-1
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IB Chemistry HL Name: Yoojin Lee Candidate Number: 002213-067
Data Presentation
Effect of Changing the Molar Mass of the Alcohol on the Rate of Evaporation
y = -0.0029x + 0.254 R² = 0.94
0.2 (a) 0.16
Rate of Evaporation, r/ ℃/s
0.15
0.135
0.1 0.0685 0.05 0.0175
0.0143
0 0 -0.05
(a)
10
20
30
40
50
60
70
80
90
Molar Mass, M/ g/mol
Graph 2 shows the processed data of average rates of evaporation against the number of the carbon chain. Vertical error bar shows the standard deviation of the triplicate trials for the rate of evaporation. 9
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IB Chemistry HL Name: Yoojin Lee Candidate Number: 002213-067
Conclusion The data suggests that as the molar mass of the alcohol increases, the rate of evaporation decreases and that my hypothesis is valid. The linear regression and the R2 value show that there is a negative correlation between the rate of evaporation and the molar mass of the alcohol. However, it cannot be proved that the correlation is always true for other types of alcohol. The rate of evaporation will eventually reach 0 and stop decreasing. When there are many carbon chains, an addition of another carbon chain will not make any difference, because the intermolecular forces of attraction is already too strong to let the molecules easily evaporate. At some point, the alcohol is no longer in liquid phase, but in a solid phase due to strong intermolecular forces of attraction. Thus only for the five sets of alcohol, methanol, ethanol, propan-2-ol, butan-1-ol, and pentan-1-ol, the linear regression is applied. Although, the trend of the results does not portray the overall relationship between the rate of evaporation and the number of the carbon chain in alcohol, it still supports the fact that the relationship is inversely proportional.
Evaluation The experiment is justifiable because reliable triplicate trials were obtained. Although the standard deviation of ethanol is relatively wide compared to other types of alcohol, it nevertheless shows a negative correlation, because the standard deviation does not overlap with other variables. The standard deviations of methanol and ethanol are greater than others, because they evaporated so rapidly but not consistently, since the instantaneous slope of the regression varied a lot. Perhaps, this is due to human error when releasing the alcohol onto the sponge. Hence, both systematic and random errors were present.
Since this experiment dealt with small amount of alcohol to prevent entire soakage of the sponge piece, a constant atmospheric state was vital. For instance, a sudden blow of air could 10
IB Chemistry HL Name: Yoojin Lee Candidate Number: 002213-067
have increased the rate of evaporation to a great extent.
One of the biggest errors came from the temperature when the experiments were performed. Due to the limited types of alcohols provided, ethanol, propan-2-ol, and butan-1-ol were tested during the fall while methanol and pentan-1-ol were tested during the winter. Hence, the room temperature differed by around 7℃, which could have made a huge different in data collection.
Limitations and Improvements Limitations
Improvements
When cutting the sponge in identical pieces, it inevitably involved human errors. Although it was measured as accurately as possible with a ruler and cut with a scalpel, the cross section was not perfectly smooth, which could have resulted in different thickness. This could have affected the results to a great extent, because the experiment dealt with a small amount of alcohol and when the thickness was not leveled well, the probe could not have sensed the change in temperature properly.
In order to improve, more mechanical methods are needed to produce identical pieces of sponges. For example, using a proper paper cutter with a lever and a grid could produce the pieces more accurately.
The alcohol solutions were released on different parts of the sponge at different pace. Again, this is also an inevitable human error. Although micropipette was used to adeptly transfer the solution, the results would vary. Releasing slowly to make sure the sponge is well soaked will produce inaccurate instantaneous rate while releasing fast will result in alcohol solutions dripping from the sponge.
If the experiment was done in larger scale, then such minor errors could be negligible. Using a larger sponge piece with greater amount of alcohol solution will yield more consistent results.
Testing a greater variety of alcohols such as Only a limited number of different alcohols decanol or dodecanol will yield more holistic was available to be tested, which yielded a results on the relationship between the rate of result that could not be applied to a wider evaporation and the molar mass of the variety of alcohols. alcohol. Table 4 shows the limitations and the improvements 11
IB Chemistry HL Name: Yoojin Lee Candidate Number: 002213-067
Bibliography 1
“Alcohol.” Wikipedia, the free encyclopedia. http://en.wikipedia.org/wiki/Alcohol (accessed January 8, 2011).
2
“N-butanol.” Wikipedia. \http://en.wikipedia.org/wiki/N-Butanol (accessed January 8, 2011).
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