Lab 11: Evaporation and Intermolecular Attractions
- Page ID
- 514173
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)PURPOSE
- To investigate the relationship between intermolecular forces and the evaporation rate by measuring temperature changes.
- To compare the strength of dispersion forces and hydrogen bonding in alkanes and alcohols based on their molecular structure and evaporation behavior.
- To predict and analyze the temperature change (∆T) of different substances based on their intermolecular forces and molecular weight.
INTRODUCTION
Evaporation is a phase change in which molecules transition from the liquid state to the gaseous state. This process is endothermic, meaning it requires energy in the form of heat from the surrounding environment. As a result, the temperature of the remaining liquid and the surrounding surface decreases. The extent of this temperature drop depends on the strength of intermolecular forces present in the liquid.
Intermolecular forces dictate many physical properties of substances, including viscosity, boiling point, and evaporation rate. The three main intermolecular forces are London dispersion forces, dipole-dipole interactions, and hydrogen bonding. London dispersion forces are present in all molecules and become stronger as molecular weight increases. Dipole-dipole interactions occur between molecules with permanent dipoles. In contrast, hydrogen bonding—a powerful form of dipole interaction—occurs in molecules containing hydrogen directly bonded to nitrogen (N), oxygen (O), or fluorine (F).
In this experiment, temperature probes will be used to measure the temperature change caused by the evaporation of different organic liquids, including alkanes and alcohols. Alkanes, which consist only of carbon and hydrogen, exhibit only dispersion forces, while alcohols, which contain an –OH (hydroxyl) functional group, experience both dispersion forces and hydrogen bonding. By comparing the temperature changes of these substances during evaporation, the strength of their intermolecular forces can be assessed. Additionally, the molecular structure, molecular weight, and hydrogen bonding capability of each liquid will be analyzed to predict and interpret the experimental results.
SAFETY PRECAUTIONS
- Work in a well-ventilated area or under a fume hood to avoid inhaling vapors from volatile and flammable liquids.
- Keep all liquids away from open flames or heat sources, as they are highly flammable.
- Avoid direct skin contact with the substances. Use gloves if necessary, and wash your hands thoroughly after handling chemicals.
EQUIPMENT AND CHEMICALS NEEDED
| Equipment | Chemicals |
|---|---|
| LabQuest | methanol (methyl alcohol) |
| 2 Temperature Probes | ethanol (ethyl alcohol) |
| (Glass thermometers can also be used with caution.) | 1-propanol |
| 6 pieces of filter paper (2.5 cm × 2.5 cm) | 1-butanol |
| 2 small rubber bands | n-pentane |
| masking tape | n-hexane |
EXPERIMENTAL PROCEDURE
1. Plug Temperature Probe 1 into Channel 1 and Temperature Probe 2 into Channel 2 of the LabQuest device.
2. Roll the filter paper into a cylinder around the probe tip, ensuring it is flush with the end. Secure the filter paper with a rubber band.
3. Pour a small amount of ethanol into a test tube, just enough to cover the filter paper on the probe. Pour a small amount of 1-propanol into a separate test tube.
4. Place Probe 1 in the ethanol test tube and Probe 2 in the 1-propanol test tube, and allow the probes to remain in the liquids for at least 30 seconds before beginning data collection.
5. Begin recording temperature data by selecting the green arrow on LabQuest. Monitor the temperature readings for 15 seconds to establish initial temperatures.
6. Simultaneously remove both probes from the liquids and secure them with tape so that the probe tips extend 5 cm beyond the edge of the table. Data collection will continue for 3 minutes. If the temperature graphs continue to decrease after 3 minutes, restart data collection by selecting the green arrow and tapping "Append" to continue.
7. If both temperature graphs have plateaued or started increasing, stop data collection by selecting the red square.
8. Examine the graph to find the maximum temperature (Tmax) and minimum temperature (Tmin) for each probe. Record Tmax and Tmin, then calculate ΔT (temperature change) using the formula: ∆T = Tmax - Tmin
9. Remove the rubber bands and dispose of the used filter paper as instructed by your teacher.
10. Use the ΔT values from ethanol and 1-propanol, along with pre-lab information, to predict the ΔT for 1-butanol and n-pentane. Record your predictions and explain the reasoning based on molecular weight and hydrogen bonding.
11. Repeat using 1-butanol and n-pentane to verify the predictions.
12. Based on the ΔT values of all four substances tested so far, predict the ΔT for methanol and n-hexane. Compare their hydrogen bonding capabilities and molecular weights to those of the previous substances.
13. Repeat the experiment using methanol and n-hexane to validate predictions.
PRE-LAB QUESTIONS
Complete the Pre-Lab table by analyzing the given compounds. The name and chemical formula of each substance are given to you; draw their structural formula. Next, calculate the molecular weight of each molecule, as dispersion forces exist between all molecules and generally increase in strength with increasing molecular weight. Finally, determine whether each compound is capable of hydrogen bonding. Remember that hydrogen bonding occurs only when a hydrogen atom is directly bonded to nitrogen (N), oxygen (O), or fluorine (F) within the molecule. Indicate whether each substance exhibits hydrogen-bonding interactions.
Substance |
Formula |
Structural Formulas |
Molecular Weight |
Hydrogen Bonding? |
|---|---|---|---|---|
|
Ethanol |
C2H5OH |
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|
1-Propanol |
C3H7OH |
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|
1-Butanol |
C4H9OH |
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|
n-Pentane |
C5H12 |
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|
Methanol |
CH3OH |
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|
n-Hexane |
C6H14 |
DATA AND OBSERVATIONS
Substance |
Tmax [°C] |
Tmin [°C] |
∆T (Tmin–Tmax) [°C] |
Predicted ∆T [°C] |
Brief Explanation |
|---|---|---|---|---|---|
|
Ethanol |
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|
1-Propanol |
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|
1-Butanol |
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|
n-Pentane |
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|
Methanol |
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|
n-Hexane |
POST-LAB QUESTIONS
- Although n-pentane and 1-butanol have nearly identical molecular weights, their temperature change (∆T) values differ significantly. Explain this difference based on the strength and type of intermolecular forces present in each substance.
- Among the alcohols tested, which one exhibits the strongest intermolecular forces? Which one has the weakest? Support your answer using the experimental results and your understanding of intermolecular forces.
- Between n-pentane and n-hexane, which one has stronger intermolecular forces? Which one has weaker forces? Justify your answer based on the data collected in this experiment.
- Create a graph plotting the magnitude of ∆T values for the four alcohols against their molecular weights. Use molecular weight on the x-axis and ∆T on the y-axis. Describe any trends observed and explain how they relate to intermolecular forces. (Remember to upload your graph with your lab report.)