Entropy and Enthalpy: Changes in Rubber Bands

Transcription

1 Section 8 Entropy and Enthalpy: Changes in Rubber Bands What Do You See? Learning Outcomes In this section you will Determine if a change results in an increase or decrease in entropy. Determine if a change will be spontaneous by considering change in enthalpy and change in entropy. Describe the structure of polymers. What Do You Think? Rubber is an example of a polymer. Polymers are long strings of similar small molecules (monomers) joined together in many places by cross-links. Draw what you think the molecules inside a rubber band look like. Predict what will happen if a rubber band is heated. Explain why you think your prediction is correct. Record your ideas about these questions in your log. Be prepared to discuss your responses with your small group and the class. Investigate You could use the unusual properties of rubber to make a wheel that turns using heat energy from a lamp. If you are going to use rubber bands as a part of your apparatus, it will be important to understand why rubber has unusual properties. Part A: Adding or Removing Heat Energy from Rubber 1. In the first part of this investigation, you will explore what happens when a rubber band is heated and cooled. You will then investigate how a rubber band stretches and contracts. a) Make a data table similar to the one shown on the next page to record your data. 353

2 Chemical Dominoes Experiment heating rubber band cooling rubber band rubber band stretching rubber band contracting Observations Is heat added (ΔH 0) or released (ΔH 0)? Safety goggles and an apron must be worn at all times in the chemistry lab. Use caution when using electrical devices near water sources. 2. Set up the apparatus as shown in the diagram to the right. You will need a ringstand, a right-angle holder, an aluminum rod, a rubber band, and a 500-g mass. Adjust the height of the aluminum rod so that the 500-g mass just barely rests on a level surface. a) You will be heating the rubber band with a hair dryer for a few minutes. Do you predict that the rubber band will stretch, contract, or do nothing when heated? Explain the reasoning behind your prediction in your log. 3. Using a hair dryer, heat the length of the rubber band as evenly as possible. 4. Continuing to heat, note any changes in the height of the 500-g mass. Record your observations in the data table. a) When the rubber band heats up, does it gain heat energy (ΔH 0) or lose heat energy (ΔH 0)? 5. Remove the hair dryer and allow the rubber band to cool. Record your observations in the data table. a) When the rubber band cools, does the rubber band gain heat energy (ΔH 0) or lose heat energy (ΔH 0)? b) Was your prediction correct? If not, explain. Part B: Spontaneous Heat Changes 1. Obtain another large rubber band. Using both hands, hold the flat surface of the unstretched rubber band against your forehead. Keep the tension on the rubber band at a minimum. Do this several times until you become accustomed to the temperature of the rubber band at rest. 2. Move the rubber band away from your forehead and quickly stretch the rubber band as far as possible without breaking it. Immediately place it against your forehead while still stretched. Is it warmer, cooler, or is there no change in temperature? Try it several times to be sure of your results. a) Using the same data table, record your observations. As you determine the direction of ΔH for the stretching of a rubber band, consider if the rubber band is releasing energy to your skin, or absorbing energy from your skin. If the rubber band is releasing energy (ΔH 0), it will feel warmer. 3. Now, you will do the experiment the other way around to find out whether rubber gives off or takes in heat energy as it contracts after being stretched. Stretch the rubber band as before and hold it a few moments. 354

3 Section 8 Entropy and Enthalpy: Activity Changes # 38 RunningH Rubber Activity Bands a) Then hold the stretched rubber band against your forehead to familiarize yourself with its temperature. b) After that, move the stretched rubber band away from your forehead and quickly allow it to contract. Immediately hold it against your forehead again. Is it warmer, cooler, or the same temperature as your skin? Try this several times to be sure of your results. c) Record your observations in the data table. 4. There is a relationship between the results from Part A: Steps 4 and 5 and the results from Part B: Steps 2 and 3. Discuss this in your group and describe this relationship in your log. 5. Return to the model you drew of rubber molecules in the What Do You Think? a) Describe how your model would explain what happened when you heated the rubber band and when you cooled the rubber band. If your model does not explain the evidence you collected about heating and cooling the rubber band, revise your model and use the revised model in your explanation. Part C: Modeling the Behavior of Rubber Molecules 1. Obtain four chains of round beads and three pieces of string. The chains of beads represent polymers. Each bead represents a single part of a polymer, called a monomer. 2. Use three pieces of string to create a tangled mess of the chains of beads by tying each string around any two chains of beads, as shown. The drawing shows only two chains of beads as an example. You will link together four chains into one large group. Each string represents a cross-link that binds the polymer chains together to create a network of long-chain molecules. When you have all the strings tied, you should be able to lift the entire tangled mess by grasping any single location in the mess. In other words, no chains should remain unattached. 3. Set the tangled beads down randomly on a rectangular piece of flat rubber. Using duct tape, attach the beads to the rubber in about five different locations, so that the beads will move along with the rubber as you stretch it. a) Make a drawing of what the chains of beads look like, making sure to indicate the cross-links. Identify one of the chains of beads by shading the beads that you draw, as one of the chains is identified in the drawing above. 4. Now stretch the rubber gently, the long way, but try not to break any strings of beads. a) While one person holds the rubber stretched, everyone else should sketch what the polymer chains look like now. Try to identify the very same chain of beads as before, by shading the beads. b) Compare and contrast the two configurations of bead chains (not stretched and stretched). It might be helpful to look at the difference between the shaded chain in your first drawing and the shaded chain in your second drawing. How are the chains arranged differently in each configuration? Which configuration is more ordered (organized) and which is more disordered (disorganized)? 355

4 Chemical Dominoes Wash your hands thoroughly after the investigation. lower entropy 5. The amount of disorder is called entropy and has the symbol S. You can think of disorder as the opposite of order. Here are some other synonyms and their opposites to help you think about entropy. Less spread order neat organized low entropy More spread disorder messy disorganized high entropy a) Of the two configurations you drew, contracted, and stretched, which one has the higher entropy and which has the lower entropy? b) Determine whether a change of configuration that occurs has a positive or negative entropy change. One way to do this is to represent entropy as a number line (look back at the Chem Talk of Section 1 for a reminder and example). On a number line in your log, indicate the locations of the contracted rubber band and the stretched rubber band, similar to the following: higher entropy c) As you change from a contracted rubber band to a stretched rubber band, is the change of entropy in the positive or negative direction? d) The sign of the entropy change (ΔS) is the same as the direction of the change on the number line. Complete the table below to indicate if the entropy change (ΔS) for stretching and contracting the rubber band is positive or negative. Change Entropy change stretching rubber band ΔS is? rubber band contracting ΔS is? e) Now return to the drawings you made at the beginning of the investigation or your revision from Part A. Describe how your model would explain what happened when you heated the rubber band and when you cooled the rubber band. If your model does not explain the evidence you collected about heating and cooling the rubber band, revise your model and use the revised model in your explanation. 6. Clean up your workstation and save materials for future use as directed by your teacher. 356

5 Section 8 Entropy and Enthalpy: Changes in Rubber Bands Chem Talk SPONTANEITY When speaking of a change, chemists refer surroundings (everything else) to the system and the surroundings. Together, the system and surroundings make up the universe. You choose the system that system you want to investigate. You can imagine drawing a dashed surface around the system, to separate it from the surroundings. When considering change, you think about what the system is doing. Is the enthalpy of the system increasing or decreasing? Is the entropy of the system increasing or decreasing? For example, when stretching the rubber band, you asked, Is the rubber band producing heat energy or releasing heat energy? If the system is an open system, energy can pass between the system and the surroundings. For example, a teakettle heating on the stove is an open system. A closed system is a system that is isolated from the surroundings and heat can neither enter nor leave. This is very, very difficult to achieve. An attempt at a closed or isolated system is a thermos bottle. As you know, a thermos bottle slows the movement of heat, but eventually the hot soup or cold beverage in the thermos bottle will exchange heat with the surroundings. Chem Words system: the part of the universe under study. surroundings: all of the universe not under study. open system: a system whose boundaries allow the transfer of energy from the system to the surroundings. closed system: a system whose boundaries do not allow the transfer of energy. Energy is conserved within the system; also called an isolated system. law of conservation of energy: the law that states that energy cannot be created or destroyed; it is merely transferred. Enthalpy Changes When heat energy is released from the system, the surroundings gain that energy, while the system loses energy. If your hand is resting on the system (but you are part of the surroundings), you feel heat energy coming from the system. When a system loses heat energy, there is a loss in the amount of enthalpy that the system contains. The change of enthalpy is negative (ΔH 0). When heat energy is absorbed by a system, the surroundings must give that heat energy to the system. Energy must come from somewhere. The law of conservation of energy states that energy cannot be created or destroyed. It can only be transferred from one location to another. If your hand is resting on the system (but you are part of the surroundings), you feel heat energy leaving your hand and going to the system. In other words, your hand feels cold. Exothermic change, H 0 Endothermic change, H 0 heat heat 357

6 Chemical Dominoes When a system gains energy, there is an increase in the amount of energy the system possesses. The change of enthalpy is positive (ΔH 0). Let s review how to calculate enthalpy changes by considering a reaction that you used in Section 1 to generate CO 2 gas. In Method 3, you heated calcium carbonate (CaCO 3 ) to produce CaO and CO 2. This chemical reaction is symbolized as the following: CaCO 3 (s) energy CaO(s) CO 2 (g) The amount of energy absorbed and released by a reaction depends on the amount of starting materials, as you learned in Section 7. If you begin with a specific amount (1 mol) of CaCO 3 : It takes an input of 1207 kj (kilojoules) of energy to break apart the bonds in the CaCO 3. When the CaO and CO 2 form, the energy released is 1029 kj. For this change, you have to put more energy in than you get out. Looking at it from the perspective of the matter that is changing, the matter gains energy overall. Mathematically, the enthalpy change is determined as follows: ΔH ( 1207 kj) ( 1029 kj) 178 kj Entropy Changes Whenever a change occurs, particles inside the system are rearranged. The new arrangement is either more or less disorganized. When the new arrangement is more disorganized the entropy has increased. The change in entropy is positive, or ΔS 0. The final entropy is a larger value than the initial entropy. Conversely, when the new arrangement is less spread out or more organized, the entropy has decreased. The entropy change is negative, or ΔS 0. The final entropy is a lower value than the initial entropy. When materials change from gas to liquid or liquid to solid, the entropy decreases. Predicting Spontaneity in Nature Changes can occur either spontaneously or not. As you have learned, the two factors that affect spontaneity are changes in enthalpy (ΔH) and changes in entropy (ΔS). Exothermic changes (those with ΔH 0) drive a process toward spontaneity. This is because substances are produced that have lower energy than the reactants from which they formed. Lower-energy states are favorable. Changes that result in an increase in 358

7 Section 8 Entropy and Enthalpy: Changes in Rubber Bands entropy of the system also drive a process toward spontaneity. This is because nature tends to become more disorganized (ΔS > 0) over time. If both factors change in the direction that favors spontaneity, the reaction will definitely be spontaneous. If neither factor changes in the direction that favors spontaneity, the reaction will definitely not be spontaneous. If only one change occurs in a favorable direction, the more dominant change will determine if the reaction is spontaneous. This information is summarized below. Possible combinations of changes in enthalpy and entropy Enthalpy decreases (exothermic) Entropy increases (more disorder) Enthalpy increases (endothermic) Entropy decreases (less disorder) Enthalpy decreases Entropy decreases Enthalpy increases Entropy increases Signs of ΔH and ΔS ΔH 0 ΔS 0 ΔH 0 ΔS 0 ΔH 0 ΔS 0 ΔH 0 ΔS 0 Yes No Is the process spontaneous? Only if enthalpy change (ΔH) is dominant Only if entropy change (ΔS) is dominant A third factor determines if a process will be spontaneous. Consider the freezing of water to form ice. If the water is left on the counter, freezing is not spontaneous. However, if the water is placed in the freezer, the process becomes spontaneous. The temperature at which a process occurs can affect whether or not a process will be spontaneous. It would be convenient to combine the three quantities that affect spontaneity (ΔH, ΔS, and T) into one value. An American scientist Josiah Willard Gibbs, , developed a very useful mathematical relationship between ΔH, ΔS, and T. It is called Gibbs free energy (named in his honor). It is a measurement that can be used to tell whether a change will occur spontaneously. If the change in Gibbs free energy, ΔG, is negative, the change will occur spontaneously at the given temperature. In other words, the change is favorable. Chem Words Gibbs free energy: a combination of ΔH, ΔS, and T that is used to determine if a change is spontaneous at a given temperature. 359

8 Chemical Dominoes If ΔG is positive, the change cannot occur spontaneously at the given temperature; it is not favorable. The change in Gibbs free energy can be determined using the following equation: ΔG ΔH TΔS where the temperature (T) is measured in kelvins. If ΔH is negative and ΔS is positive, then ΔG will be negative no matter what the temperature. This is a situation where the change can occur spontaneously under any circumstances. If ΔH is positive and ΔS is negative, then ΔG will be positive no matter what the temperature. A spontaneous change cannot occur for such a system. When ΔH and ΔS both have the same sign, ΔG could be positive or negative, depending on whether ΔH or ΔS is more influential in determining the sign of ΔG. Notice that at the higher temperatures, ΔS is more influential on ΔG because ΔS is multiplied by the temperature. Multiplying ΔS by a large temperature would make it a large number. At low temperatures, the product of ΔS and T would be a small number. Therefore, at low temperatures, the ΔH is more influential on ΔG and spontaneity. Consider when CaCO 3 breaks down as the following: CaCO 3 CaO CO 2 The ΔH for the process is 178 kj. The value is positive, which indicates an endothermic reaction. This would drive the reaction to not be spontaneous. The ΔS is kj/k. This value is also positive, which indicates disorder increases during the change. This would drive the reaction to be spontaneous. So, which factor dominates and determines if the process is spontaneous? This is a place where the Gibbs free energy equation is useful. At room temperature (298 K), the ΔG would be calculated as shown below. ΔG ΔH TΔS 178 kj (298 K)(0.159 kj/k) 131 kj Since the value for ΔG is positive, the reaction is not spontaneous at room temperature. 360

9 Section 8 Entropy and Enthalpy: Changes in Rubber Bands What if the temperature were increased by heating CaCO 3, as in Section 1? Assume that the temperature was 373 K. ΔG ΔH TΔS 178 kj (373 K)(0.159 kj/k) 118 kj Notice that the ΔG is still positive indicating that the process is not spontaneous. In order to make the reaction continue to occur, energy must still be added to the substances. The reaction is not spontaneous at the temperature you used, so once you stopped supplying energy to the reaction, the reaction stopped. Polymers Have Unusual Properties Chem Words polymer: a very long-chain molecule composed of repeating monomers joined together. monomer: a small, repeating molecule that composes a polymer. Polymers are molecules made of long strings of monomers that are attached to each other. This structure causes materials that are made of polymers to exhibit some unusual behaviors. You probably predicted initially that when you heated a rubber band (with the hair dryer) it would get longer. In fact, it spontaneously contracted (got shorter). To understand this, you need to compare the enthalpy and entropy of the two states (stretched and contracted) of the rubber band. When the rubber band is in a stretched state, the entropy (disorder) is low because the molecules are pulled relatively straight and lined up. You saw this in your bead and string model. The enthalpy is also low when the rubber band is in a stretched state because the aligned molecules are able to experience attractions to each other. You learned in Section 1 that oppositely charged ions have less energy (and enthalpy) when they are near each other than when they are separated. Molecules exhibit a similar relationship between distance and energy. Molecules experience relatively weak attractions for each other when they are close together. This means when a molecule is attracted to another, it is at a lower energy (enthalpy) state. 361

10 Chemical Dominoes When the rubber band is in a contracted state, the entropy (disorder) is high because the molecules are tangled around each other. The enthalpy is also high because the molecules are farther apart from other molecules. (This may sound a little strange at first, but think about pulling the rubber with the bead strings attached. More molecules would be closer to one another when the rubber is stretched than when contracted.) When a molecule does not have other nearby molecules to be attracted to, the molecule is at a higher energy (enthalpy). A comparison of the two states of the rubber band is shown in the chart. State of rubber band Enthalpy (H) Entropy (S) stretched lower lower contracted higher higher Is a rubber band more likely to be stretched or contracted based on the two factors that affect spontaneity? Nature favors changes that result in lower energy (enthalpy). Therefore, the enthalpy factor would favor the rubber band becoming stretched. Nature favors changes that result in higher entropy. The entropy factor would favor the rubber band contracting. Which factor dominates and determines the state of the rubber band? At room temperature, nature favors the contracted state. A rubber band will not spontaneously stretch. Unless you apply force to the rubber band by holding it stretched, it will spontaneously return to its contracted state. So, at room temperature, the change in entropy dominates the change in enthalpy. Effect of Temperature on Rubber Band The rubber band stretched when you decreased its temperature by cooling it with ice. This means that at a lower temperature, the stretched state is favored. As you saw, the change in enthalpy drives the rubber band toward a stretched state. Therefore, at a lower temperature, the enthalpy factor becomes dominant over the entropy factor. Because the rubber band is stretched when cooled with ice and is contracted at room temperature, increasing temperature causes the rubber band to contract. You observed this when you heated the rubber band with the hair dryer. As the temperature is increased, the change in entropy makes the contraction of the rubber band more and more spontaneous. Therefore, the rubber band contracts more than it does at room temperature. 362

11 Section 8 Entropy and Enthalpy: Changes in Rubber Bands Biomedical Uses for Polymers Polymers probably are the most versatile substances that technology has produced. They can be very soft, like rubber bands, or very hard, like bowling balls. Recently, scientists have learned how to control the properties of polymers by varying the composition of the monomers, the chemical types of monomers, and the degree of cross-linking between polymer strands. One of the most exciting fields in which polymers are used is medicine. The development of mechanical heart valves, which have improved the quality of life for tens of thousands of people with damaged hearts, is one example of how polymers are used in medicine. A polymer called a thermoplastic polymer is used to stitch the working parts of the mechanical heart to the heart itself. Thermoplastics are usually polyesters or polymers in which the repeating monomer forms an ester linkage between an organic acid (an acid-containing carbon) and an alcohol. Polyethylene terephthalate (PET) is a copolymer of terephthalic acid and ethylene glycol connected by the ester functional group. PET is used to make clothing fibers as well as food and beverage containers. [-CH 2 CH 2 -O-CO-Ph-CO-O-] n The use of polymers in the biomedical field to provide skin grafts for the treatment of serious burns also is advancing quickly. First, a plastic mesh is made from the copolymer of lactic acid and glycolic acid. Next, a layer of healthy living cells from the burned victim s body, which is important for healing the wound, is placed in this mesh. Then, the thin mesh containing the cells is applied to the area where the skin was damaged. As the cells regenerate the skin over the burned area, the plastic mesh slowly dissolves, leaving only the newly formed layer of skin. This plastic is known as PLGA. Like PET, PLGA is a type of polyester; however, its properties, especially its ability to dissolve slowly, are very different from those of the polymer used in heart valves. [-CHCH 3 CO-O-CH 2 COO-] n There are many more polymer applications in the biomedical field, such as in prosthetic limb technology. Properties of polymers are such that they can be so sturdy that they can be used to construct cars and buildings, or so elastic that they can be used to make trampolines and playground balls. The fields of polymer chemistry and materials science are exciting and employ many chemists. Chem Words thermoplastic polymer: a polymer that can be softened with heat and then remolded; recyclable. polyester: a polymer in which the repeating monomer forms an ester linkage between an organic acid (an acidcontaining carbon) and an alcohol. terephthalic acid: an organic di-acid used as a copolymer to make PET, polyethylene terephthalate. Checking Up 1. What happens to a system during an endothermic change? 2. What happens to the entropy of your room when you go from a neat room to a messy room? 3. What two factors determine whether a change is spontaneous? 4. If a particular change has a negative enthalpy change (ΔH 0) and also a negative entropy change (ΔS 0), can the change be spontaneous? If so, what must be true for the change to be spontaneous? 5. Why does a rubber band contract when you heat it? 363

12 Chemical Dominoes What does it mean? Chemistry explains a macroscopic phenomenon (what you observe) with a description of what happens at the nanoscopic level (atoms and molecules) using symbolic structures as a way to communicate. Complete the chart below in your log. MACRO NANO SYMBOLIC Describe your observations about the behavior of a rubber band when heated and cooled. What Do You Think Now? At the beginning of this section you were asked the following: Draw what you think the molecules inside a rubber band look like. Predict what will happen if a rubber band is heated. Explain why you think your prediction is correct. Based on your study of rubber in this section, how would you answer the questions now? What makes a rubber band exist in the state that it does at room temperature? Chem Essential Questions Describe how the organization or particles within the rubber changes when the rubber band is stretched and contracted. Describe how the force of attraction between particles within the rubber changes when the rubber band is stretched and contracted. Draw a picture to represent a system with high entropy. Draw a picture to represent a system with low entropy. How do you know? Describe how you can tell if a process releases heat or absorbs heat. Why do you believe? The equation that represents the reaction that occurred in the MRE heater in Section 7 is Mg(s) 2H 2 O(l) Mg(OH) 2 (s) H 2 (g). Is the ΔH for this reaction positive or negative? Explain how you can tell based on your observations in Section 7. Is the ΔS for this reaction positive or negative? Explain how you can tell by looking at the phase subscripts provided in the equation. Is the reaction spontaneous at room temperature? Explain how you know. Why should you care? Would spontaneity be an important concept to add to your list of important criteria used to evaluate methods of producing CO 2 in Section 1? Why or why not? If you did decide to consider spontaneity, would you prefer a spontaneous or non-spontaneous reaction for your Chemical Dominoes apparatus? Explain your reasoning. 364

13 Section 8 Entropy and Enthalpy: Changes in Rubber Bands Reflecting on the Section and the Challenge For the Chapter Challenge, you must create a series of cascading physical and chemical changes that ultimately light an LED. Once the user starts the series, he or she is not involved again. You must be able to design the process so that energy transfers cause changes to occur. You have learned in this section how to determine if a chemical reaction or a physical change is spontaneous. If you choose to use a spontaneous reaction in your dominoes apparatus, you will likely have to find some way to supply the initial activation energy to the reactants to get the reaction started. If you choose to use a non-spontaneous reaction, you will have to find a way to supply energy to the reactants for however long you need the reaction to occur. The most common way to supply energy to a reaction is through the use of heat. Some reactions do not require the additional input of heat. The reactants are able to absorb the heat they need from their room temperature surroundings. An example of this would be the reaction that occurred in the MRE heater in Section 7. Once the water and magnesium come in contact, the reaction begins. Chem to Go 1. Make drawings depicting each change below. Then decide on what you believe the signs of ΔH and ΔS are for each change. Explain your reasoning. a) Ice melting b) A candle burning (the wax burns) 2. Here is a pictorial representation of how crystals of AgNO 3 dissolve in water. a) When AgNO 3 dissolves in water, the solution and beaker feel a little cold to the touch. Is dissolving AgNO 3 in water endothermic or exothermic? Explain. Which term describes this change: ΔG, ΔH or ΔS? Is the term positive or negative? b) Is the change creating more disorder or more order? (Look at the diagram.) Explain how you can tell. Which term describes changes in disorder: ΔG, ΔH or ΔS? Is the term positive or negative? c) Which of the two terms needs to be stronger in order for the change to be spontaneous? Support your explanation using the Gibbs free energy equation. 365

14 Chemical Dominoes 3. For the dissolving of silver nitrate, AgNO 3, shown in the following table, the initial (before) and final (after) information is: Enthalpy and Entropy Changes Energy required to break bonds between Ag and NO 3 ions, plus open up space between H 2 O molecules for those ions to fit between Energy given off when new attractive forces form between Ag and its surrounding H 2 O molecules, and NO 3 and its surrounding H 2 O molecules Entropy (amount of disorder) in the before (undissolved) state 124 kj per mole of AgNO 3 (positive because this bond breaking requires energy input) 102 kj per mole of AgNO 3 (negative because the forming of attractive forces releases energy) kj/k per mole of AgNO 3 Entropy in the after (dissolved) state kj/k per mole of AgNO 3 a) What is the system? What are the surroundings? b) Sketch a bar graph to illustrate the energy change. c) Based on the numerical information above, what is the change of enthalpy (ΔH) for the system? d) Does the change in enthalpy drive the process to be spontaneous or non-spontaneous? Explain. e) At the appropriate X on the line below, write in before and after. Then place an arrow connecting the two X s, showing the direction of the entropy change during the process of dissolving AgNO 3. lower entropy f) Based on the numerical information above, what is the entropy change (ΔS) for the system? g) Does the change in entropy drive the process to be spontaneous or non-spontaneous? Explain. h) At an absolute temperature, T, of 298 K (about room temperature), calculate the Gibbs free energy for this system, using the equation ΔG ΔH TΔS kj/k kj/k higer entropy Is ΔG positive or negative? Does this indicate that the change is spontaneous or non-spontaneous? i) What would be the sign of ΔG for the reverse process (separating the solution into solid AgNO 3 and liquid water) at room temperature? Explain your reasoning. 366

15 Section 8 Entropy and Enthalpy: Changes in Rubber Bands 4. At STP, a sample of which element has the highest entropy? a) Na(s) b) Hg(l) c) Br 2 (l) d) F 2 (g) 5. Systems in nature tend to undergo changes toward a) lower energy and lower entropy. b) lower energy and higher entropy. c) higher energy and lower entropy. d) higher energy and higher entropy. 6. Preparing for the Chapter Challenge It is time for you to compare all the different ideas you have generated throughout this chapter and select the ones that will make the best Chemical Dominoes apparatus. In your log, make a list of all the possible components you can build from different ideas you have start: component #1 How does component #1 start component #2? component #2 How does component #1 start component #2? explored in the sections. Remember that you must light an LED as the final step. List several different possible sequences of components that could be part of your apparatus. In between each component, explain how the previous step sets off the next step. You may use the example flowchart above as a method to organize your ideas. etc. end: light LED (what color?). Inquiring Further Build a rubber-band engine You can use what you have learned about enthalpy and entropy to make a rubber-band wheel or engine that runs on heat energy from a lamp. An infrared lamp works best. 1. Look at the photograph, and then design and build a rubber-band engine that spins when a lamp is turned on. Think about how rubber behaves when heated to figure out where to place the lamp. 2. The materials you can use include, but are not limited to, the following: rubber bands embroidery hoop wooden dowel with a large diameter metal eye-hook screws infrared lamp (or regular lamp) pliers, screwdriver, scissors 3. Once you have your rubber-band engine working, observe it carefully. What is actually happening to the rubber bands? Do your observations match your predictions that were behind the design of your wheel? 4. Think about how you could use the rubber-band engine in your Chemical Dominoes apparatus. Where would it be? Would you have something activate it, or would you turn it on at the beginning? What domino effect might the rubberband wheel cause? 367