INTERMOLECULAR FORCES LAB STATION 1 SURFACE TENSION LEARNING TARGET: You should be able to identify the comparative strength of intermolecular forces through observations of the surface tension in droplets. BACKGROUND: When the molecules in a liquid are attracted to each other, the outer molecules create a membrane like surface. These outer molecules are strongly attracted to each other and can be hard to permeate (or cross). Liquids with stronger intermolecular forces will create a bigger mound because the molecules pull together strongly. The force that pulls the molecules together causes the molecules to pack closer together, reducing the surface area as the droplets mound together. The type of intermolecular force determines the strength of the attraction between molecules. A liquid with very weak intermolecular forces will end up being somewhat flat because the molecules cannot pull together. Molecules with hydrogen bonds have the strongest attraction. Dipole- Dipole bonds are the second strongest intermolecular force Basic dispersion forces are normally the weakest. In this lab you will experimentally determine which of the liquids has the greatest surface tension! MEDICAL APPLICATION: Breathe Easy, you have low Surface Tension As we breathe the lungs need to fill up with air so that tiny pockets of air, called alveoli, can expand allowing the oxygen to pass through the walls of the alveoli. Most of your inner respiratory surfaces are covered in mucus, but this is a fluid with a lot of surface tension so that it is hard for oxygen to get through it. Anywhere there is a boundary between air and liquid there is a virtual "membrane" which we call surface tension. In the lungs, this is the place where the air you breathe in meets the alveoli, or lung tissue. In order to allow the oxygen to permeate the walls of the alveoli and pass into the blood stream, a substance called surfactant is produced by the lungs, significantly reducing the surface tension so that oxygen can easily pass through. Breathing works even though there is only about 1mmHg pressure difference between the air and where it will go and we can thank surfactant for that.
INTERMOLECULAR FORCES STATION 1 PRE-LAB QUESTIONS 1. What determines the strength of the attraction between molecules? 2. What kind of intermolecular forces have the weakest attraction? 3. What will be your evidence that a substance has strong intermolecular forces? 4. Where does air need to pass through in order to be absorbed by the body? 5. Does the surface tension need to be high or low in order to let oxygen pass through? MAKE A HYPOTHESIS: What do you expect will happen when you add the drops to the wax paper? Which substance will create the largest mound and which will make a small flat mound? MATERIALS Wax Paper Pipettes Water Ethanol Soapy water Oil PROCEDURE On a piece of wax paper, place 10 drops of each of the substances (in different piles), and record the shape of each liquid substance as viewed from the side. Your purpose was to find the liquid with the most surface tension and the least surface tension. The hypothesis may have been that water will have the highest surface tension because it has hydrogen bonds. Because ethanol has only one hydrogen bond it has weaker intermolecular forces. The soapy water has no hydrogen bonds because the soap breaks up the interactions between water, so it has weaker forces. Oil has only carbon- hydrogen bonds, so it is the weakest. This implies that it has dispersion forces. These weak forces mean that the molecules aren't attracted to each other and create a smaller mound when you add the droplets. QUESTIONS 1. List the substances by order of surface tension 2. Propane has only carbon- hydrogen bonds. Considering what you have learned, what kind of force would this substance have? What would you expect the droplets to look like if you had propane to experiment with?
INTERMOLECULAR FORCES LAB STATION 2 VOLATILITY LEARNING TARGET You should understand the effects of intermolecular forces on the volatility of a substance. BACKGROUND Have you ever noticed that some substances evaporate more quickly than others? For instance, when you wash your hands you need a paper towel, however when you use hand sanitizer the liquid just "disappears"... the reason for this is that those two chemicals have different volatility. Volatility is a measure of the ability of a substance to evaporate. This quality depends on the intermolecular forces and the vapor pressure of the substance. This can change as temperature changes, because a substance at a higher temperature has a higher vapor pressure. Evaporation occurs when the vapor moves away from the surface of the liquid. If a molecule gains enough kinetic energy to overcome the intermolecular forces it will vaporize. The lower the force, the quicker the substance will evaporate. Water has high intermolecular forces so it takes lots more time to evaporate than a non- polar substance such as acetone which has very high volatility. MEDICAL APPLICATION Putting you to sleep thanks to volatility Volatility is a key characteristic of many anesthetic agents. Due to the fact that many anesthesia medications are inhaled, these medications need to evaporate quickly so that they can be breathed in. They also need to be able to be released from the blood stream fairly easily so that the effects of the anesthesia wear off quickly once the procedure ends. If a drug is a liquid in a vial, yet it needs to be inhaled, there are two ways to administer the drug. Some drugs are made into a spray, however, in the case of anesthesia, this would result in a lot of liquid gathering in the lungs because it must be given over time. The administration of anesthesia is best done as a gas. If the medication came as a gas it would take up a huge amount of room, so having a medication which is liquid with high volatility is a much easier way to administer the drug. As air passes over the liquid it easily picks up the medication if it is highly volatile. The patient then breathes in the air and remains unconscious. The volatility is also important so that the anesthesia can leave the blood stream. Once the procedure is over the blood can easily release the medication into the lungs as you exhale so that the patient can just breathe away the medication. The more volatile the substance the more quickly it will leave the body. This means that anesthetics usually have weak intermolecular forces, such as dispersion forces.
INTERMOLECULAR FORCES STATION 2 PRE-LAB QUESTIONS 1. What is volatility? 2. What can allow a molecule to vaporize? 3. What kind of medicine depends on volatility? 4. How is it best to administer anesthesia? 5. What intermolecular force would be best in the case of anesthesia? MAKE A HYPOTHESIS: Which substance will evaporate the fastest and which will be the slowest? MATERIALS Acetone Water Ethanol Cotton Swabs Table top Timer PROCEDURE Look at the three different containers filled with water, ethanol, and acetone respectively. Put a different cotton swab into each liquid. At simultaneously pull out each swab and draw a straight line on the table top and record how long it takes for each liquid to evaporate completely. Today you determined which liquid evaporated fastest. The faster the evaporation, the lower the intermolecular forces. We hypothesized that acetone would be the most volatile because it has the weakest intermolecular forces. Water has two hydrogen bonds so it takes a lot of energy to vaporize so it is the last to evaporate. Ethanol lands somewhere in the middle, as a substance with just the one hydrogen bond, so it will evaporate more quickly than water, but longer than acetone. QUESTIONS 1. Which of the substances evaporated faster? What does this tell you about its intermolecular forces? 2. Which of the substances evaporated slowest? What does this tell you about its intermolecular forces? 3. Which of the substances seemed to smell the strongest? Use your knowledge from the experiment to explain why that substance made it to your nose most quickly.
INTERMOLECULAR FORCES LAB STATION 3 POLARITY LEARNING TARGET You should be able to identify that the more polar a molecule is the stronger its intermolecular forces. BACKGROUND Water There are many forces within a molecule that affect the way the molecule interacts with other molecules. These forces are generally a result of varying electronegativity between the atoms in a molecule. A highly electronegative atom attracts the electrons, drawing them away from other parts of the molecule. This leads the molecule to have partial charges. The more electronegative areas have a partial negative charge, while the less electronegative areas have a partial positive charge because the electrons have been drawn away. This polarity, or partial charge, causes the molecule to be attracted to anything else with a charge. In this case, by rubbing the balloon on your head it collects electrons, making it negatively charged. By passing the balloon near the liquid streaming from the burette you attract the polar molecules towards the balloon. A liquid made from polar molecules (like water) is highly attracted to the balloon, causing the stream to bend. MEDICAL APPLICATION Hi doc, can I get something polar? Medications need to bind to their receptors or target molecules in order to work, but how do they bind? It turns out that receptors have partially charged areas that match the polarity of the medication so that they are attracted to each other and fit just right like a puzzle piece. Ethanol In the diagram you can see that the oxygen atoms (which would be partially negative) are connected by a hydrogen bond to the positive hydrogen atoms nearby. These attractions allow the medication to bind. As the medication comes into contact with the enzyme in this case, it sticks to it so that the normal biological compound cannot bind. When we want to make a new drug, one of the important things to look at is the shape and partial charges in the target molecule or receptor. If we Here the sulfonamide drug binds just right with the enzyme called Carbonic Anhydrase. This acts in the kidney to increase urine output. This can reduce blood pressure in patients who have high blood pressure! can make a computer model of the biological target we can then test potential drugs digitally by having computers calculate the polarities and shapes. If the computer thinks it's a match then the drug can be made and tested. If the charges don't match then another molecule can be made.
STATION 3 PRE-LAB QUESTIONS 1. What creates the forces within a molecule? 2. What areas have a partial positive charge within a molecule? 3. What are polar molecules attracted to? 4. What allows a medication to bind to the receptor or target molecules in the body? MAKE A HYPOTHESIS: Which chemical will be more attracted to the negatively charged balloon? MATERIALS Burette filled with ethanol with a beaker below Burette filled with water with a beaker below Balloon PROCEDURE Pour water and ethanol into two different burettes; place a beaker under each burette. Charge a balloon by rubbing it on your head. Place the balloon a little bit to the side, and underneath a burette and turn it on. Observe the deflection angle. Repeat with other burette. You should have observed the water deflect significantly when the negatively charged balloon is brought near the stream. This supports our hypothesis that a polar substance will be attracted to the balloon. The ethanol was also deflected, but at a smaller angle. You can see that water is entirely polar, however ethanol is polar on one end but non- polar on the other so that it has less attraction to the negative balloon. QUESTIONS 1. Which substance is more polar, ethanol or water? 2. What is your evidence that one is more polar than the other? 3. What was on the balloon that caused it to become negative?
INTERMOLECULAR FORCES LAB STATION 4 SURFACTANTS LEARNING TARGET You should be able to investigate the effects of adding a surfactant to break the surface tension. BACKGROUND Intro: In some chemical reactions the result is something instant and unexpected. The reason for it is often unknown at first. In this case, the result is caused by the changes in intermolecular forces. Milk has several lactose molecules, which are rich in hydrogen bonds and are also very fatty. These molecules are normally attracted to each other and create surface tension. In the beginning the food coloring has no effect on the milk and will sit on top because it is less dense. Adding a drop of soap though causes everything to go crazy, why? The soap acts as a surfactant. It breaks the surface tension, so each molecule of milk it touches is no longer able to bind to the other molecules. When those molecules can no longer stay together they are drawn down by gravity and pushed away by the other parts of the milk that were not affected by the soap. MEDICAL APPLICATION Baby's breath When babies are born prematurely one of the biggest concerns is that they may not have begun to produce surfactant. Babies don't need surfactant until they breathe when they are born, because before that they get all of their oxygen straight from the mother's blood. This means it is one of the last developments before birth. A baby born without enough surfactant is in danger of having their lungs collapse because the alveoli cannot stay open when the baby exhales. There are two ways to treat this. First, if the doctor suspects the baby might be born early the mother can be given steroid shots to stimulate the baby to begin creating the surfactant. If this is not an option, doctors can give the baby synthetic surfactant until the baby begins to produce it themselves. When the surfactant hits the lungs, the oxygen that the baby breathes in is now able to cross through the membrane of the alveoli into the bloodstream. These surfactants have saved the life of many preemies, as it breaks up the mucus and lets the good air in!
STATION 3 PRE-LAB QUESTIONS 1. What causes the milk molecules to be attracted to each other? 2. What does the soap act as? 3. What breaks the surface tension? 4. Why don't babies need surfactant before birth? 5. What are the treatments for a premature baby lacking surfactant? MAKE A HYPOTHESIS: What will happen to the surface of the milk if a surfactant is added? MATERIALS Petri dish milk food coloring cotton swabs soap PROCEDURE Pour enough milk to cover the bottom of your Petri dish. Carefully add 4 drops of food coloring evenly spread throughout the milk. Dip your cotton swab into the soap. Tap the cotton swab on the surface of the milk and observe! The fatty milk which had even surface tension across the top held the food coloring in place. The added drop of non- polar liquid soap acted as a surfactant and began to break down the fatty milk molecules (lipids) and resulted in a lowered surface tension in that area. Because surface tension tries to make the smallest possible area it pulls towards the place of highest surface tension, in this case where the liquid soap didn t touch: the outsides of the petri dish. This imbalance in surface tension is what pulls the food coloring out towards the edges of the dish. The food coloring merely acted to show the movement of the surface of the milk and was not actually a requirement of the reaction QUESTIONS 1. What happened to the particles in the milk when the soap touched the surface 2. What was the effect of the soap on the molecular level 3. As you place the soap on the milk, where is surface tension the highest? 4. How is this similar to a baby's ability to breathe? 5. What was the purpose of the food coloring in this experiment?