Intermolecular Attractions Part 1

Transcription

1 Intermolecular Attractions Part 1 Purpose: To investigate and describe the intermolecular forces acting between molecules in various types of molecular substances. Materials: Dropper Bottles of water, ethanol, 4- polyethylene plate, 4- glass plate slides, micro-wells, ruler, digital thermometer and stopwatch. Procedure: Data: 1. Place 1 drop of liquid on the polyethylene plate and on the glass plate. Measure the drops width and height on the glass plate and the polyethylene plate using ruler provided. 2. Determine which chemical has the greatest rate of evaporation. 3. Repeat step one but place the identical plate over the drop. Compress the liquid between the glass plate slides and the polyethylene slides. Try separating the plates and determine which needs more force to separate. 4. Obtain new plates and hold vertically at an angle. Place one drop of each liquid at the top of the plate and determine the amount of time it takes for the drop to reach the bottom. Note the pattern or path that the liquid takes to reach the bottom. 5. Count the # of drops it takes to fill a micro-well for each liquid. 6. Determine the temperature of each liquid using a thermometer. 7. Take half the number of drops it takes to fill a micro-well with water and place them into a new micro-well. Determine its temperature. Take half the number of drops of ethanol it takes to fill a micro-well and add them to the water. Note the height of the liquid in this vial. Did the liquid fill the well? Determine the temperature change of this mixture. Was the formation of the mixture an endothermic process or an exothermic process? 8. Take a capillary tube and place it in each well. Determine the liquid level rise in each capillary tube including the mixture. 9. Place one drop of each liquid on your finger. Describe what you feel.

2 Questions 1. Which of the substances had the drop that was least attracted to the surface? How could you tell that this was true? 2. Which of the drops was the flattest and widest? What does this mean about the attraction of the molecule to the surface? 3. Which took the longest time to evaporate? What does this imply about the attraction of the molecules to each other? 4. Which took the most drops to fill the well? What does that mean about the size of those drops? What does that mean about the attraction of those molecules to each other? 5. When you compare the size of the molecules of water and Ethanol, which is bigger? How does this compare to the size of the drops? 6. When the drops run down the plate which liquid seem to spread on what surface? 7. Can you determine whether the polyethylene is made of polar or non-polar molecules? 8. The glass capillary tube contains ions, based on this fact which molecule shows the greatest Ion Dipole attraction. 9. Can you rank these substances from weakest intermolecular attraction to strongest intermolecular attractions? 10. When you compressed the plates, which plate seemed the most difficult to separate? 11. When you placed the liquids on your finger, which seemed to cool your finger and why?

3 Intermolecular Attractions Part 2 Purpose: To investigate and describe the intermolecular forces acting between molecules in various types of molecular substances. This lab centers on the polarity of a molecule and the ability of the molecule to dissolve various solids. Materials: Dropper Bottles of water, and mineral oil, a granule of solid iodine, crystals of an ionic solid (Copper (II) nitrate), wooden splint to use as a scoopula, and a small Petri dish. Procedure: 1. Fill the bottom of a small plastic Petri dish with mineral oil. Note how the oil interacts with the dish. Place 5 drops of water into the middle of the Petri dish with the oil. Note how the two chemicals interact and draw a picture. 2. Place a small gradual of iodine in the water and into the oil. Place a small gradual of the ionic salt in the water and the oil. Determine the solubility of the iodine and the ionic salt. Observations: 1 Iodine and Water 2 Iodine and Oil 3 Ionic Solid and Oil 4 Ionic Solid and Water Observations Observations Observations Observations Sketch Sketch Sketch Sketch

4 Questions 12. What happened to the Iodine solid in the Water? Did it dissolve? Why do you think this happened? Think about the polarity of the iodine vs. the polarity of the water? 13. What happened to the ionic solid in the water? Did it dissolve? Why do you think this happened? Think about the charges in the ionic solid vs. the polarity of the water? 14. What is happening at the interface between the oil and the water? Was the Iodine able to dissolve in the oil? Explain. Did you notice any color changes? What do you think happened? Conclusion: Can you divide the substances up into polar (charged) and nonpolar based on how they interacted? Clue iodine has the formula I 2, what do you think that means about the bond and the polarity of the molecule? Draw the structure of Iodine and Copper (II) Chloride.

5 Intermolecular attractions-part 3 Floating Oil Droplet Experiment Purpose: Given various types of substances, investigate and describe how intermolecular forces affect physical properties of mixtures such as solubility and density. Materials: Dropper Bottles of water, 2-Propanol, Iodine mineral oil solution and 8-dram vial with cap. Procedure: 1. Fill the vial half way with 2-Propanol and cap. Shake the vial to ensure that the walls of the vial are coated with 2-Propanol. 2. Slowly add four drops of oil into the vial. Note the appearance and location of the drop. 3. Slowly add drops of water to the vial making sure that the drops run down the wall of the vial. Keep adding the drops of water till you note a change in location of the oil droplets. Conclusion Questions: 1. When you initially drop the oil into the Propanol what was the shape and number of drops that appear? 2. Are the drops of oil soluble in the Propanol? 3. As you added the water to the mixture what did the water do to the density of the Propanol? 4. Was the water soluble in the Propanol? What structural features would make the water soluble or insoluble in the Propanol? 5. The purple drops eventually fades in time. Why do you think that could happen?

6 Teacher Post Lab Notes Drop Curvature, Diameter and Evaporation Rate You ve probably noticed the interesting behavior that can take place at the surfaces of liquids. To minimize energy, most fluids assume the shape with the smallest surface area. A sphere is the shape with the minimum surface area for a given volume. Soap bubbles also tend to form themselves into shapes with minimal surface area. Water is not attracted to polyethylene (there is no adhesion between the drop and the polymer). Each molecule in the water drop is attracted to the other water molecules in the drop. This causes the water to pull itself into a shape with the smallest amount of surface area, a bead (sphere). All the water molecules on the surface of the bead are holding each other together or creating surface tension. However the water is attracted to the glass because of the ions encapsulated in the glass. The ion dipole attraction makes the water droplet spread on the surface and also creates an interesting attraction between the two glass plates that are compressed.

7 Drop Size The larger the drop the greater the intermolecular attractions the less drops it takes to fill up the micro vial. The surface tension of the liquid drop is directly related to the volume and density of the liquid you are testing. Water droplets will be larger in volume because of its strong intermolecular attractions.

8 Capillarity For a liquid to rise in a glass capillary it has to wet the inner surface of the capillary. Water and many organic liquids wet the glass surface well when it is clean. However if the liquid does not wet the surface of the capillary at all, what is known as the capillary depression will take place. Liquids that have an adhesion to glass will create a large meniscus inside the capillary thus reducing pressure at the surface of the liquid. This decrease will make the liquid rise up the capillary until the internal pressure is equivalent to the external pressure. Glass contains ions encapsulated into the silicon dioxide. Polar molecules have an affinity to the ions. If the molecules are light and have a strong dipole moment they will rise rapidly.

9 What energies are involved in dissolution? 1. Breaking intermolecular attractions or bonds of solute and solvent is an Endothermic process. 2. Formation of new bonds or intermolecular attractions) between solute and solvent is an Exothermic process. Note: When the strength of attractive forces increases the potential energy of the system decreases and visa versa Note: Note that when you increase the entropy of the system you favor disorder that follows the 2nd law of thermodynamics that systems have the tendency to be disordered. This law drives the formation of solutions even though they may have positive heats of solutions. Processes requiring large amounts of energy tend not to occur. Density and Intermolecular Attractions in Solutions If the formation of a solution expand the particles in the solute and solvent; thus there is a decrease in attractive forces, there will be an increase in potential energy of solution. The heat of solution will be an endothermic process. If the formation of solution allows the particles of the solute and solvent to come closer because of an increase of attractive forces between solute and solvent ten the heat of solution should be an exothermic process. Ideal solutions have no net heat of solution. Deviations from ideal behavior o o o Positive deviation - would indicate repulsion between solute and solvent relative to initial attractions between original solute and solvent molecules. Negative deviation - attraction Ideal, no intermolecular interaction between solvent and solution.

10 Like Dissolves Like An easy way to demonstrate that chemicals with similar chemical or molecular polarity will disperse into chemicals with similar chemical or molecular polarity is simply the fact that oil and water do not mix. Using this fact that oil and water do not mix you can create an interface between the oil and the water that will be useful to observe how other chemicals placed into this heterogeneous mixture will interact. Note the interesting bubble curvature that the water creates. Place a small crystal of iodine into the water and a small crystal of iodine into the oil. Note the solubility. The water has the tendency to force the iodine to the side of the drop at the interface. The water in attracted to the charged ions in the ionic compound thus it is able to dissolve in the water. Solubility Iodine is only slightly soluble in water, but very soluble in many common organic solvents and aqueous iodine solutions. Solubility: Slight water solubility (0.03 g/100 g 20C). Interesting is the fact that water can induce a dipole moment in iodine making it slightly soluble in water.