2516 Intermolecular Forces of Liquids

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.