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2516 Intermolecular Forces of Liquids

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    440582
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    INTERMOLECULAR FORCES OF LIQUIDS

    1.0 INTRODUCTION

    The attractive forces between molecules, the intermolecular forces, are responsible for many of physical properties of a substance. Forces between molecules in a liquid are called intermolecular. Forces within the molecule that keep the molecule together (such as the bonds between atoms) are called intramolecular.

    Evaporation is vaporization that occurs at the surface of a liquid. Evaporation is an endothermic process and the extent of the temperature change is related to the strength of the intermolecular forces of the substance.

    The vapor pressure of a liquid is the pressure of the vapor from evaporation of a liquid above a sample of the liquid in a closed container. The vapor pressure of a liquid depends on the intermolecular forces of the liquid as well as temperature and external pressure.

    The surface tension of a liquid occurs at the interface between the liquid and a gas (or sometimes, another liquid). Surface tension is affected by the intermolecular forces of the material at the interface.

    This experiment measures the evaporation temperature changes for several liquids and surface tension. Differences among these surface tensions and temperature change during evaporation illustrate different intermolecular forces that are present, which are in turn dependent upon the chemical structure of the liquid.

    References and further reading

    OpenStax College. (2015, March 17). Chemistry. OpenStax College. Retrieved from https://openstax.org/details/books/chemistry

    2.0 SAFETY PRECAUTIONS AND WASTE DISPOSAL

    !!Wear your safety goggles!!

    Several liquids will be evaporated during this experiment. Avoid breathing in vapors. Use fume hoods if available as much as possible. Avoid eye and skin contact with liquids.

    This experiment uses digital thermometers that should not be completely submerged in liquids. Only the measuring probe should be submerged. If glass thermometers are used, they are fragile. Mercury-containing thermometers have mostly been replaced due to problems with cleanup in the event of a break. If a mercury thermometer breaks, notify your instructor and the stockroom for appropriate and safe cleanup procedures.

    Dispose of organic liquids in organic waste

    Most of the liquids that are used in this experiment will be evaporated into the atmosphere. Any excess organic liquids need to be disposed of in the organic waste container.

    Solid filter papers, if dry, can be disposed of as regular trash.

    3.0 CHEMICALS AND SolutionS

    Chemical

    Formula

    Approximate amount

    Notes

    Water

    H2O

    Drops

    Use laboratory water

    Hexane

    CH3(CH2)4CH3

    Drops + 2 ml

    Flammable liquid and vapor

    Acetone

    CH3COCH3

    Drops + 2 ml

    Flammable liquid and vapor

    Isopropanol (Rubbing alcohol, also called 2-propanol)

    CH3C(OH)HCH3

    Drops + 2 ml

    Flammable liquid and vapor

    Methanol (methyl alcohol)

    CH3OH

    2 ml

    Flammable liquid and vapor

    Ethanol (ethyl alcohol)

    CH3CH2OH

    2 ml

    Flammable liquid and vapor

    1-Butanol

    CH3CH2CH2OH

    2 ml

    Flammable liquid and vapor

    4.0 GLASSWARE AND APPARATUS

    Item

    Use

    Notes

    Test tubes

    Collecting and storage of samples

    Start with clean, dry tubes. Label properly.

    Pennies

    Thermal sink

    Pennies prior to 1982 were 100% copper. Pennies after 1982 are copper-coated zinc. Differences may be present.

    50-ml graduated cylinders

    Volume measurement

    Keep separate cylinders for NaOH and HCl solutions

    Stopwatch or phone

    Time measurement

     

    Molecular model kits

    Chemical models

     

    Filter paper

    Solvent dispersal

    Use approximately 2.5 cm x 2.5 cm

    Thermometer

    Temperature measurement

    Do not submerge electronic portion

    5.0 PROCEDURE

    Part A: Penny Drops

    1. Clean and dry several pennies. Using a dropper, drop one of the liquids slowly onto the top of a penny laying down on the top of the lab bench. Count the number of drops a penny can hold before the liquid starts to run off the side of the penny for each of the solvents. Be consistent with the size of each drop.

    2. Repeat the process for each liquid on separate pennies.

    3. Complete Data Table 1 in Section 6.0 with other students' data for comparison.

    Part B: Evaporation Temperature Change

    1. In clean, labeled, small test tubes, obtain approximately 2 mL of the following liquids:

    methanol (methyl alcohol)
    ethanol (ethyl alcohol)

    isopropanol

    1-butanol

    hexane

    acetone

    2. Wrap a digital thermometer probe tip with square pieces of filter paper (2.5 cm × 2.5 cm) in a cylinder or test tube. Secured the cylinder with a small rubber band. The paper should be even with the probe end.

    3. Submerge the filter paper in methanol for 45 seconds to soak the filter paper. Monitor the temperature while submerged for 15 seconds to establish the initial temperature. Record the initial temperature in Data Table 2.

    4. Remove the thermometer from the liquid and use tape to secure the thermometer to the lab bench. The thermometer probe tip should extend about 5 cm over the edge of the bench. Continue monitoring the temperature until it reaches a minimum and begins to increase. Readings should be every 15 - 30 seconds. Record data in the table in Section 6.

    5. Subtract the minimum temperature from the initial temperature to determine ∆T, the temperature change during evaporation.

    6. Roll the rubber band up the probe and dispose of the filter paper.

    7. Replace the filter paper and repeat the evaporation with the other liquids. Record their data in the data table.

    Last Name

    First Name

    Partner Name(s)

    Date

    6.0 DATA RECORDING SHEET

    Data Table 1. Penny Drop

    Solvent

    Trial 1

    Trial 2

    Other Students’ Data

    Average

    water (H2O)

                 

    Hexanes (CH3(CH2)4CH3)

                 

    acetone (CH3COCH3)

                 

    Isopropanol (Rubbing alcohol) (CH3C(OH)HCH3)

                 

    Other liquids (if instructor requests) can be included below:

                 
                   
                   
                   

    Data Table 2. Evaporation Temperature Change

    Methanol

    Ethanol

    Isopropanol

    1-butanol

    hexane

    acetone

    Initial Temp:

    Initial Temp:

    Initial Temp:

    Initial Temp:

    Initial Temp:

    Initial Temp:

    Time

    Temp

    Time

    Temp

    Time

    Temp

    Time

    Temp

    Time

    Temp

    Time

    Temp

                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           

    Note: Continue to measure the temperature until it has begun to increase.

    7.0 Data Analysis

    Part A: Penny Drops

    1. Group the liquids used in the penny drop portion on the experiment according to intermolecular forces. Does your data support your grouping? Explain.





    Part B: Evaporation Temperature Change

    2. Plot a graph of ∆T values of the four alcohols versus their respective molar masses. Plot molar mass on the x-axis and ∆T on the y-axis. Use a linear regression to find an algebraic expression to express the data.







    8.0 POST-LAB QUESTIONS AND CONCLUSIONS

    1. Provide a sketch of four water molecules interacting. Label the intermolecular and intramolecular forces.






    2. Provide a sketch of four acetone molecules interacting. Label the intermolecular and intramolecular forces.







    3. Provide a sketch of four hexane molecules interacting. Label the intermolecular and intramolecular forces.





    4. Two of the liquids, hexane and 1-butanol, had nearly the same molar masses, but significantly different ∆T values during evaporation. Explain the difference in ∆T values of these liquids, based on their intermolecular forces.




    5. Which of the alcohols studied has the strongest intermolecular forces of attraction? The weakest intermolecular forces? Explain using the results of this experiment.




    6. Using the language of intermolecular forces, explain the order of the evaporation temperature changes and surface tensions you observed.











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