The strength of the intermolecular forces is determined by the magnitude (size) of the positive and negative charges on the molecules: the larger the positive and negative charge, the stronger the intermolecular forces and the stickier the molecules are.

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Molecules with permanent positive and negative areas, like the ones shown above, are called polar molecules. These tend to be the stickiest types of molecules. Other molecules don’t have permanent positive and negative areas and are called nonpolar molecules. They still have some attraction between them, though, because the random movement of the electrons create temporary positive and negative areas that flicker on and off.

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The purpose of this lab is to investigate the stickiness of three unknown substances (A,B, and C) to determine which ones are held together by the strongest intermolecular forces.

Name____________________________________________________Date________________Period_____

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Sticky Molecules: Intermolecular Forces

Warm-Up

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1. What causes molecules to stick together?

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• What is a polar molecule?

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• What is a nonpolar molecule?

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• Which molecules tend to be more sticky, polar or nonpolar?

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• How can nonpolar molecules stick together if they don’t have permanent positive or negative areas?

Nonpolar molecules still have some stickiness due to the temporary positive and negative areas that flicker on and off.

Introduction

Molecules stick together, and some molecules stick together more than others. This stickiness, or attraction, is the result of intermolecular forces between molecules. Intermolecular forces arise due to the presence of positive and negative areas on different molecules. In the example to the right, the positive area of one molecule and the negative area of another molecule attract because they are oppositely charged.

Polar Molecule

Nonpolar Molecule

(no charged areas)

Nonpolar Molecule

(temporary charges)

Procedure

Evaporation Rate: Determine the evaporation rate for each of the unknown substances (A,B, and C).

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1. One at a time, place one drop of each substance on a brown paper towel and use the stopwatch to determine how long each takes to completely evaporate. Record your time in Data Table 1 under ‘Time 1.’
2. If it takes longer than 120 seconds (2 minutes), just write ‘120’ in the data table.
3. Get the times from two other groups and record their results under ‘Time 2’ and ‘Time 3.’
4. Average the results together.

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Drop Size: Determine the size of one drop of each of the unknown substances (A,B, and C).

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1. One at a time, place one drop of each substance on a piece of wax paper.
2. Measure the diameter, in millimeters (mm), of each drop. Record your results in Data Table 2 under ‘Diameter 1.’
3. Get the diameters from two other groups and record their results under ‘Diameter 2’ and ‘Diameter 3.’
4. Average the results together.

Data Table 2: Drop Diameters

Drops on a Penny: Determine the number of drops of each substance that can fit on a penny.

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1. Clean the surface of three pennies using steel wool and place them, head side up, on a brown paper towel.
2. For each substance, determine how many drops can be placed on a penny before it overflows down the side. Record your results in Data Table 3 under ‘Drop Count 1.’
3. Get the drop count from two other groups and record their results under ‘Drop Count 2’ and ‘Drop Count 3.’
4. Average the results together.

Data Table 3: Drop Counts

Data Table 1: Evaporation Rates

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Capillary Action: Determine how far each substances moves up a capillary tube.

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1. Fill three separate small glass vials with each liquid to a height of approximately 1 centimeter (cm). Adjust the heights to make sure they are the same height in each vial.
2. Place a capillary tube (small glass tube) in each for 5 seconds.
3. After 5 seconds, remove the capillary tube and measure, in millimeters (mm), how high the liquids moved up each capillary tube. Record your answer in Data Table 4 under ‘Height 1.’
4. Get the heights from two other groups and record their results under ‘Height 2’ and ‘Height 3.’
5. Average the results together.

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Data Table 4: Capillary Tube Heights

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Miscibility: Determine the miscibility of the substances with each other.

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1. Add about 1 centimeters worth of liquid ‘B’ into the vial containing ‘A’.
2. Place the cap on it and shake well. Record is they are miscible or immiscible in the table below.
• Miscible means they do mix together and do not form layers.
• Immiscible means they do not mix together and do for layers.
3. Now add 1 centimeters worth of ‘C’ into ‘B’ and 1 centimeters worth of ‘A’ into ‘C.’ Cap them, shake them, and record your results below.
4. Get a bead of iodine (I₂) from your instructor and place it in the vial with B and C in it. Cap it and shake it well. Record which layer(s) the iodine in soluble in. (Note: B floats on C even though B started on the bottom).
5. Get a small sample of copper (II) chloride (CuCl₂) from your instructor and place it in the same vial. Cap it and shake it well. Record which layer(s) the copper (II) chloride is soluble in.

Data Table 5: Miscibility and Solubility

B

A

C

C

Analysis

1. Why do you think it is important to get the results from other groups and average them together?

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Evaporation Rate

2. How do you think evaporation rate and the strength of intermolecular forces (stickiness) are related? In other words, do you think a faster evaporation rate would indicate strong or weak intermolecular forces? Why? Use complete sentences.

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• Based on your results from Data Table 1, list the substances (A,B, and C) in order from strongest intermolecular forces to weakest intermolecular forces.
• ______________________ (strongest)
• ______________________
• ______________________ (weakest)

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Drop Diameter

4. How do you think drop diameter and the strength of intermolecular forces (stickiness) are related? In other words, do you think a larger drop diameter would indicate strong or weak intermolecular forces? Why? Use complete sentences.

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• Based on your results from Data Table 2, list the substances (A,B, and C) in order from strongest intermolecular forces to weakest intermolecular forces.
• ______________________ (strongest)
• ______________________
• ______________________ (weakest)

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Drops on a Penny

6. How do you think the number drops that fit on a penny and the strength of intermolecular forces (stickiness) are related? In other words, do you think a large number of drops would indicate strong or weak intermolecular forces? Why? Use complete sentences.

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• Based on your results from Data Table 3, list the substances (A,B, and C) in order from strongest intermolecular forces to weakest intermolecular forces.
• ______________________ (strongest)
• ______________________
• ______________________ (weakest)

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When molecules of the same substance stick together, cohesion occurs. When molecules of different substances stick together, such as a drop of liquid to a solid surface, adhesion is taking place. Both cohesion and adhesion occur due to intermolecular forces of attraction. Cohesive forces tend to be stronger than adhesive forces.

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8. What is one example of cohesion you observed in the lab? ____________________________________
9. What is one example of adhesion you observed in this lab? ____________________________________

Capillary Action

Capillary action occurs when a liquid creeps up the sides of a thin tube, in apparent defiance of gravity. Capillary action occurs due to the adhesion of the substance to the tube and the cohesion between the molecules in the column of liquid.

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10. How do you think the height in a capillary tube and the strength of intermolecular forces (stickiness) are related? In other words, do you think a larger height would indicate strong or weak intermolecular forces? Why? Use complete sentences.

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• Based on your results from Data Table 4, list the substances (A,B, and C) in order from strongest intermolecular forces to weakest intermolecular forces.
• ______________________ (strongest)
• ______________________
• ______________________ (weakest)

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Miscibility and Solubility

12. Why do you think substances ‘B’ and ‘C’ were immiscible, yet ‘A’ was miscible in ‘B’ and ‘C’? Explain in terms the strengths of their intermolecular forces.

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• One of the substances was water, which contains only O-H bonds. Another substance was acetone, which contain a C=O bond. The other substance was hexane, which contains only C-H bonds. Based on the electronegativity values for O, H, and C, which substances do you think were water, acetone, and hexane?

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• _______ water
• _______ acetone
• _______ hexane
• Draw the Lewis structure for iodine (I₂) and the write the ions in copper (II) chloride.

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• Explain why iodine was only soluble in ‘B’ and copper(II) chloride was only soluble in ‘C’ using the ‘like dissolves like’ principle.

Hexane

Acetone

Water