Biochemistry - Sticky Molecules SE

Student Exploration: Sticky Molecules Directions: Follow the instructions to go through the simulation. Respond to the questions and prompts in the orange boxes. Vocabulary: adhesion, capillary action, capillary tube, cohesion, hydrogen bond, intermolecular force, molecule, newton, nonpolar, partial negative charge, partial positive charge, polar, surface tension, tensiometer Prior Knowledge Questions (Do these BEFORE using the Gizmo.) 1. James adds some magnetic marbles to a glass jar full of ordinary marbles, and then shakes up the jar. What do you think will happen to the magnetic marbles? The magnetic marbles are going to attract each other and form almost a chain of magnetic marbles attached to each other, but the ordinary marbles will remain still and won’t attract any of the marbles. 2. James then dumps the marbles on a steel cookie sheet and tilts it. Which type of marble will roll off more easily? The ordinary marbles will roll off more easily because they have no magnetic force that is holding them from rolling off the steel cookie sheet. Also, the magnetic force of the magnetic marbles causes more friction and a greater pulling force compared to the ordinary marbles, so ordinary marbles will roll off more easily. Gizmo Warm-up Just as some marbles are attracted to one another while others are not, certain molecules stick together more than others. In the Sticky Molecules Gizmo, you will discover what causes this “stickiness.” You will investigate a variety of phenomena that result from the attraction of molecules to one another. To begin, drag a dropper bottle of Water and a Petri dish (labeled Polarity) to the simulation area. Drag the dropper over the dish to add water. Examine the molecules. 1. What do you notice about the water molecules? Water molecules are dispersed in this case, and hydrogen atoms are attracted to oxygen atoms as they float towards one another and also away from one another. Different areas of the water molecules are electrically charged. The red portions of the molecules are negatively charged, while the blue regions are positive. Purple is neutral. 2. Note the yellow lines which show attractions between the molecules. Why do you think these attractions occur? There is a magnetic charge that connects the molecules, and this is what causes their attraction. Hydrogen atoms have a positive charge and oxygen atoms have a negative charge, so they are attracted to each other. Activity A: Polarity Get the Gizmo ready: ● Check that the Petri dish contains water. Introduction: All molecules are neutral overall. However, the charges within molecules are not always arranged in the same way. In a polar molecule the positive and negative charges are separated into distinct regions. In a nonpolar molecule the charges are evenly dispersed. Question: Are polar or nonpolar molecules more “sticky?” 1. Classify: Based on what you observe, are water molecules polar or nonpolar? Based on what I observed, water molecules are polar. Indicate your choice in the data table using the drop-down menu. The presence of charged regions indicates a polar molecule. However, these charged regions do not represent full charges as found in compounds such as sodium chloride. Instead they are partial positive (δ+) and partial negative (δ–) charges, which are much weaker than full charges. 2. Hypothesize: Note the yellow lines that form between molecules, which represent hydrogen bonds. A hydrogen bond (H-bond) is an example of an intermolecular force (IMF). IMFs occur between all molecules, but hydrogen bonds are the strongest. A. What causes these H-bonds to form? (Hint: Look at the charged regions.) Electron attraction causes these H-bonds to form. B. Why do you think the H-bonds only last a short time before breaking and reforming? I think the H-bonds only last a short time before breaking and reforming is because the H-bonds are possibly weak. 3. Explore: Replace water with hexane (an ingredient in paint thinner). Examine the molecules. A. Do you see any charged regions? No Do any H-bonds form? B, Is hexane polar or nonpolar? nonpolar Indicate this in the data table. C. Observe and classify the other two liquids. Which one is polar? Glycerin No Which one is nonpolar? Mineral Oil Indicate this in the data table. 4. Summarize: Molecules that are attracted to one another can be called “sticky.” Which types of molecules, polar or nonpolar, are the “stickiest”? polar What causes this stickiness? H-bonds (Cohesion) Activity B: Cohesion and adhesion Get the Gizmo ready: ● Drag the Drop diameter tool (a piece of wax paper) to the simulation area. Introduction: 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. Question: How can the strength of cohesive and adhesive forces be determined? 1. Predict: All drops do not look the same—some are rounded while others are flatter. A. Do you expect polar or nonpolar liquids to form the roundest drops? I expect polar liquids to form the roundest drops. B. Explain your reasoning. I expect polar liquids to form the roundest drops because of the electric charge attraction that helps keep the molecules together, so the shape of the polar liquid will be held. With a nonpolar liquid, however, there is no electric charge that can attract the molecules together, so it won't hold a shape and will be flatter. 2. Measure: Drag the dropper bottle of water to the simulation area. Add a drop of water to the wax paper. The drop holds together and assumes its shape due to cohesive forces. A. Describe the shape of the water drop. Oval, bubble-like, thick B. Use the ruler to measure the diameter of the drop. What is its diameter? 6 mm Enter this measurement in the data table. 3. Analyze: Measure the drop diameter for each of the other substances. A. Enter these values in the data table and then record below: Hexane B. 19 mm Glycerin 5 mm Was the prediction you made in 1A confirmed? Mineral oil 8.5 mm Yes, the prediction I made in 1A was confirmed. C. How do the drops of polar liquids differ from those of nonpolar liquids? The drops of polar liquids were round, measured about a diameter of 5 mm, and they have pretty small diameters. Nonpolar liquids are flatter, and measure a larger diameter. D. How do intermolecular forces affect the shape of a drop? Because of intermolecular forces between molecules, the droplet takes on a sphere-like shape. 4. Predict: Drag the Tilt angle tool to the simulation area. Adhesive forces can be measured using this instrument, which can be tilted to form an inclined plane. The greater the adhesion, the greater the angle at which it can be tilted before the drop begins to slide. Which substances do you think will experience the greatest adhesive force? Why? Water will exhibit the strongest adhesive forces since they have strong H-bonds that can adhere to other substances, keeping their shape and creating a strong bond with other substances. Creating a strong bond with other substances will take a higher angle to drag polar molecules down. 5. Investigate: Drag the dropper bottle of water to the simulation area. Add a drop to the tilt angle instrument. Using the slider, gradually increase the incline until the drop starts moving. A. At what angle does the drop begin to slide? 18° Enter this value in the table. B. Repeat the above procedure with the other substances. Enter the angle at which each droP begins to slide in the data table, and then record below: Hexane C. 9° Glycerin 12° Mineral oil As the angle increases, do polar or nonpolar molecules tend to slide first? 6. Explain: To understand adhesion, consider the charges on the surface of the inclined plane. Since it is nonpolar, the positive and negative charges are evenly distributed, as shown to the right. The negative charges (electrons) can freely move, while positive charges (protons) tend to be fixed in place. 8° nonpolar molecules A. If the positive end of a water molecule were to encounter the surface shown above, what might happen to the surface’s negative charges? Explain your reasoning. If the positive end of a water molecule were to encounter the surface shown above, the surface’s negative charges(electrons) will be attracted to the water molecule and the water droplet will be attached to the surface. Water molecules are polar in nature, so they can use H-bonds to attract opposite charges to one another and repel the same charges. B. On the image at bottom right, draw in the charges after a water molecule contacts the surface. C. How does this explain why polar molecules have better adhesion than nonpolar molecules? Nonpolar molecules have more difficulty adhering to surfaces because they need an inert state to remain on the surface, which makes it difficult for them to adhere to the surface and have an adequate surface tension. Polar molecules, however, have a more stable form, allowing them to have a good surface tension. Activity C: Surface tension and capillary action Get the Gizmo ready: ● Drag the Surface tension tool (a tensiometer) to the simulation area. Introduction: Cohesion and adhesion give rise to a variety of phenomena. Surface tension represents the force exerted by the surface of a liquid. Capillary action occurs when a liquid creeps up the sides of a thin tube, in apparent defiance of gravity. Question: How do intermolecular forces create surface tension and capillary action? 1. Predict: A tensiometer contains a ring immersed in a liquid. A sensor connected to the lever registers the force needed to lift the ring out of the fluid. The greater the surface tension of the liquid, the greater the force required to lift the ring out of the fluid. In which substances would you expect to see greater surface tension? Why? I would expect greater surface tension in water because of the strong H-bonds that the water molecules share with each other that holds its form. 2. Experiment: Drag the dropper bottle of water to the simulation area. Add some water to the dish. Pull down on the lever until the ring breaks free from the surface of the water. Note that the force is measured in units of millinewtons (mN). 1 mN = 1/1000 of a newton (N). A. How much force was required? 33 mN Enter the measurement in the data table. B. Repeat this procedure with the other liquids. Enter the force required to break the surface tension in the data table, and then record below: Hexane 13 mN Glycerin 35 mN C. Do polar or nonpolar liquids have greater surface tension? Mineral oil 17 mN polar liquids D. Explain how polarity affects surface tension. Polarity affects surface tension by attracting molecules all around each other which creates a higher surface tension if the attraction is stronger like water molecules that attract to each other when molecules are everywhere and hold in place which keeps them on the surface where the molecule is still attached to. 3. Compare: Examine the measurements for drop diameter in the data table. What is the relationship between surface tension and drop diameter? The relationship between surface tension and drop diameter is the representation of how all the molecules attract each other and hold molecules together. Surface tension is responsible for a drop’s rounded shape, with cohesive forces holding its molecules together. The interior molecules are pulled equally in all directions, but those on the surface are only pulled inward. As a result, the drop shrinks to the shape with the smallest possible surface area, which is a sphere. 4. Investigate: Drag the Capillary rise tool (a capillary tube) to the simulation area. Fluids have a tendency to rise up into these thin glass tubes due to capillary action. A. Which liquids do you think will rise the highest? Polar B. Drag the dropper bottle of water to the simulation area and add water to the dish. Make sure the tube diameter is set to 1 mm. Measure the height of the water. How high did it rise? 22 mm Enter this measurement in the data table. C. Repeat this procedure with the other liquids. Enter the capillary rise in the data table, and then record below: Hexane 11 mm Glycerin 18 mm Mineral oil 14 mm D. What patterns do you see? The pattern that I see is that the polar liquids rise higher than the nonpolar liquids Capillary action depends on the balance between cohesion and adhesion. In order for a liquid to rise up a tube, the force of adhesion between the liquid molecules and the sides of the tube must be stronger than the cohesive forces within the liquid. 5. Explore: Select Water. Use the Tube diameter slider to adjust the width of the tube. A. How do you think the width of the tube might affect how high the fluid rises? It's harder for molecules to stay in place when there is a larger opening because there is less surface area and more molecules. B. Adjust the width of the tube. Why do you think width affects capillary rise? I think the width affects capillary use because there is less surface for the molecules. 6. Summarize: Use the arrows beneath the data table to adjust the columns so the polar substances are grouped together and the nonpolar substances are grouped together. A. What do the polar substances have in common? The common thing between polar substances is that they tend to have higher intermolecular forces so that they stick to surfaces more easily. B. What do the nonpolar substances have in common? There is a common characteristic of nonpolar substances in that their diameters are large, but their intermolecular forces are very low so they do not stick to surfaces easily.