Alternative Reaction Pathways

Transcription

1 Section 1 Energy and Entropy: Alternative Reaction Pathways What Do You See? Learning Outcomes In this section you will Apply the engineering-design process to scientific and everyday situations. Generate evaluation criteria and use those criteria to compare and evaluate various methods to achieving a goal. Determine how energy and disorder change during physical and chemical processes. What Do You Think? Imagine that you are making chocolate-chip cookies. At the same time, your friend across town is also making chocolate-chip cookies. Suppose some of your ingredients are not the same as those of your friend. Can you and your friend produce identical chocolate-chip cookies? How? Suppose you and your friend both have identical sets of ingredients. Could you produce chocolate-chip cookies that are completely different from those your friend produces? How? The What Do You Think? questions are meant to get you thinking about what you already know or think you know. Don t worry about being right or wrong. Discussing what you think you know is an important step in learning. Record your ideas about these questions in your log. Be prepared to discuss your responses with your small group and the class. 279

2 Chemical Dominoes Safety goggles and apron must be worn at all times in a chemistry lab. Report any broken, cracked, or chipped glassware to your teacher. Investigate Often, there are many ways to achieve a goal. In this investigation, you will compare different ways of arriving at a product and learn how you can make decisions about which way is best. Also, you will consider if a change in circumstances might cause you to choose a different method. 1. Carbon dioxide (CO 2 ) is a gas at room temperature. You will examine four different methods of generating CO 2 gas. The gas you generate will then have to blow up a balloon that will tip a lever. 2. Your teacher will assign each lab group one method of CO 2 production. Your group has 15 minutes to set up the apparatus and practice making the lever tip by a distance of 2 cm. At the end of 15 minutes, each group will demonstrate their method for the class. 3. As you practice making CO 2 gas using your group s method, make observations about the changes that happen using the CO 2 production method. You will want to record the following: a) the time it takes to generate the gas b) the volume of gas generated c) the mass of the starting materials You will also make other observations during the experiment. d) You will need to decide the amount of each substance to use. Make sure you record quantitative data. (These are measurements that involve numbers, such as the size of the balloon and the mass of starting materials necessary.) Record qualitative data as well. (These are observations of what happens.) 280

3 Section 1 Energy and Entropy: Alternative Reaction Pathways Method 1 Starting materials: Sodium bicarbonate and acetic acid Sodium bicarbonate is also known as baking soda. Sodium bicarbonate is represented by the chemical formula NaHCO 3. A chemical formula shows how many atoms or ions of an element are needed to combine with the other elements in the compound. The formula for sodium bicarbonate shows that one sodium ion, one hydrogen atom, one carbon atom, and three oxygen atoms bond together to make sodium bicarbonate, NaHCO 3. Acetic acid, HC 2 H 3 O 2, is the ingredient that gives vinegar its characteristic odor. Acetic acid is present in vinegar in low concentration, about 5%. The majority of vinegar is water. The reaction that occurs between sodium bicarbonate and acetic acid is NaHCO 3 (s) HC 2 H 3 O 2 (aq) sodium acetic bicarbonate acid NaC 2 H 3 O 2 (aq) H 2 O(l) CO 2 (g) sodium water carbon acetate dioxide The symbol (g) indicates that the substance is a gas, (s) indicates a solid, and (l) indicates a liquid. The symbol (aq) shows that the substance is dissolved in water. Note that the equation is balanced. The number of each element (Na, H, C, O) is identical before and after the reaction. Method 2 Starting materials: Calcium carbonate and hydrochloric acid Calcium carbonate, CaCO 3, is the principal component of limestone and chalk. Hydrochloric acid, HCl(aq), is one of the acids in your stomach. Like vinegar, the majority of a hydrochloric acid solution is water. The reaction that occurs between calcium carbonate and hydrochloric acid is CaCO 3 (s) 2 HCl(aq) calcium hydrochloric carbonate acid CaCl 2 (aq) H 2 O(l) CO 2 (g) calcium water carbon chloride dioxide Note that the equation is balanced. The number of each element (Ca, Cl, H, C, O) is identical before and after the reaction. Avoid getting chemicals on skin. Clean up spills immediately. 281

4 Chemical Dominoes Be careful with the heat source. Have your teacher check your procedure before beginning. Do not eat or drink anything while in the lab. Wash your hands and arms thoroughly after the investigation. Method 3 Starting material: Calcium carbonate When heat energy is added to calcium carbonate, bonds between atoms break and then re-form in a different way. This allows carbon dioxide gas to escape while leaving calcium oxide as a solid CaCO 3 (s) energy calcium carbonate CaO(s) CO 2 (g) calcium carbon oxide dioxide Method 4 Starting material: Carbonated beverage Note: This method of CO 2 production will be demonstrated by your teacher. Some beverages, such as naturally occurring mineral water, contain natural carbonation. Carbon dioxide also results from fermentation processes, causing some alcoholic beverages to become carbonated. Other beverages, such as cola and ginger ale, are artificially carbonated. Carbon dioxide is less soluble in water at higher temperatures because the molecules have more kinetic energy (move faster) at higher temperatures. This enables the gas molecules to overcome intermolecular forces of attraction with water and return to the gas phase. For example, soda loses its fizz more quickly at room temperature than when refrigerated. You can cause soda to lose carbon dioxide rapidly by placing a hot object in the soda to warm it. CO 2 (aq) energy dissolved carbonation H 2 O(l) CO 2 (g) water gaseous carbon dioxide Notice that the equation above shows a physical change rather than a chemical one. Both the starting materials and ending materials are water and carbon dioxide. Adding the hot object to the soda doesn t change the carbon dioxide and water into other materials. The carbon dioxide and water are merely separated from each other. 4. Present your group s method to the class. 5. Dispose of the materials as directed by your teacher. Clean up your workstation. 6. Decide which CO 2 production method is best for blowing up a balloon to tip a lever. There may not be one correct answer. a) Once you decide on the best method, record it in your log and describe why you decided that was the best method. b) Explain why each of the methods you did not choose was not as good as the one you chose. 282

5 Section 1 Energy and Entropy: Alternative Reaction Pathways Chem Talk ENERGY AND ENTROPY CHANGES IN CHEMICAL REACTIONS In this investigation, you generated CO 2 gas and used that gas to both blow up a balloon and move a lever. Each production method used different materials and generated a different amount of gas in a different amount of time. There are other ways in which chemists look at the generation of the gas. You will now look at the reactions in terms of energy and entropy. Does the Generation of the Gas Require Energy Input or Does It Release Energy? When matter undergoes a change, there often is a change in energy involved, meaning energy will move from being stored in one form or place to another form or place. However, the total amount of energy in the entire universe stays the same. If the matter involved in a change loses energy, then the energy has to go somewhere. It can t just disappear. Likewise, if the matter involved in a change gains energy, the energy must come from somewhere. It doesn t just appear. One indication of whether energy is entering or exiting matter is heat energy. Is heat energy gained to create a change or is heat energy lost when the change occurs? If heat energy goes in, then the chemicals gain energy. This is shown in the diagram as A. energy is added end (matter has higher energy) start (matter has higher energy) energy is released start (matter has lower energy) A B end (matter has lower energy) Conversely, if heat energy is released and the chemicals lose energy, the change can be represented as B. Many reactions can be understood in terms of energy change by reflecting on your experience with water. If you place ice cubes on a heated stove, the ice will melt (as it gains more energy), the water will increase in temperature (as it gains energy), and the water will boil and become vapor (as it gains even more energy). On the other hand, changing vapor to water, cooling water, or making ice requires the removal of energy. 283

6 Chemical Dominoes Where Is the Energy? When you add heat energy to something, the particles gain the energy. Therefore, the particles have more energy than before the heat energy was added. The additional energy has to go somewhere. One place it can go is into making the particles (atoms or molecules) move faster. The extra energy could also change the bonds that exist between atoms or molecules. It is important at this point to make a distinction between bonds holding a molecule together and attractive forces holding molecules together. Bonds exist within molecules. They hold the atoms in an individual molecule together. For example, it takes heat energy input to break bonds between ions in materials (such as the CaCO 3 crystals you heated in Method 3). Attractive forces exist between molecules in solids and liquids. These attractive forces hold the molecules of solids and liquids together and prevent them from separating to form a gas. For example, it takes heat energy input to break apart the forces that hold water molecules together in ice to turn them into liquid water. It takes the input of energy to overcome weak intermolecular forces or chemical bonds. Of course, it takes less energy to break the attractive forces holding molecules near each other in the liquid and solid phase. These forces are involved in the phase changes of a substance. On the other hand, chemical bonds are much stronger than the intermolecular forces of attraction so it takes a lot more energy to break bonds. In any chemical reaction, bonds are broken so that new and different molecules can be formed. Whether breaking chemical bonds or intermolecular forces of attraction, it takes an input of energy. Conversely, when attractions between molecules cause a phase change, energy is given up and heat energy is released to the surroundings. Most processes you will encounter involve both breaking and forming bonds or forces between molecules. In those cases, knowing whether the overall process will absorb or release heat energy requires an answer to the following question: Does it take more heat energy input to break the original bonds or forces, or is more heat energy released when new bonds or forces form? A Matter of Perspective Whether an energy change is positive or negative depends on the perspective from which it is viewed. In chemistry, the perspective is always from that of the reactants. In the previous example of heating 284

7 Section 1 Energy and Entropy: Alternative Reaction Pathways the CaCO 3 to form CaO and CO 2, you have to add energy. However, if you consider the chemicals, they gain energy during the change. The energy change from start to end in the chemicals is a positive one. This is called an endothermic change, because heat energy goes into the chemicals. If you consider a match that is burning, the matter (wood) is releasing (losing) energy. The ash that remains after burning is complete and has less energy than the unburned match. The change in energy for the match is negative. This kind of energy change is called exothermic. Chem Words endothermic: a process that absorbs heat energy. exothermic: a process that releases heat energy. In the four reactions in the investigation used to produce CO 2, the two in which heat was added were endothermic reactions. The two reactions which occurred spontaneously were exothermic reactions. What Happens to the Organization of Particles That Make up the Matter? Most changes also involve a change in how the particles that make up the matter are organized. Chemists choose to measure, or speak in terms of, disorganization. If the arrangement of particles in the matter gets more disorganized (less organized), the change can be shown as in the diagram. The way that chemists consider disorder is to think about how many arrangements of the particles are possible. You can think of this as the space available for individual particles to move. A better term for this might be spread out. For example, consider the two situations 1 and 2 in the diagram. In situation 1, the particles are arranged in a regular repeating structure. The particles are tightly packed, so it is not possible for any particle to move to a different location. A particle can only wiggle around within its little space. In situation 2, the same number of particles is shown in an arrangement in which each particle has the ability to move to other locations throughout the entire space. In situation 2, the matter is 285

8 Chemical Dominoes Chem Words entropy (S): the disorder of particles in a substance. Checking Up 1. What is the difference between bonds holding together atoms and forces between molecules? 2. What two things compete to determine whether a particular change releases heat energy overall or absorbs heat energy overall? 3. What is the amount of disorder called? 4. Which has more disorder, a gas or a solid? Why? more spread out. If particles move from situation 1 to situation 2, chemists say that the disorder increases. A number line can be used to represent an increase in disorder as shown in the diagram on the previous page. On the other hand, if particles move from situation 2 to situation 1, chemists would say that the disorder decreases. The arrangement of particles in the matter is less spread out. This kind of change represents an increase in order. Gases have their molecules very spread out. Reactions that create gases from solids and liquids have an increase in disorder. All of the reactions that were used to generate CO 2 gas increased in disorder. In general, any substance going from solid to liquid to gas is increasing in disorder. Chemists quantify spread or disorder under the name entropy, which is given the symbol S. The entropy that a material has is a measure of its amount of disorder. For example, one can look up in standard reference tables the entropies of the three substances in the decomposition of calcium carbonate (CaCO 3 ) into calcium oxide (CaO) and carbon dioxide (CO 2 ): CaCO 3 (s) energy CaO(s) CO 2 (g). One mole of calcium carbonate will produce one mole of calcium oxide and one mole of carbon dioxide when it goes to completion. With the entropy values, it can be seen mathematically that the total entropy increases. What Do You Think Now? At the beginning of the section, you were asked the following: Imagine that you are making chocolate-chip cookies. At the same time, your friend across town is also making chocolate-chip cookies. Suppose some of your ingredients are not the same as those of your friend. Can you and your friend produce identical chocolate-chip cookies? How? Suppose you and your friend both have identical sets of ingredients. Could you produce chocolate-chip cookies that are completely different from those your friend produces? How? Imagine that this time, you want to make the cookies unique. After looking through the cabinets, you find nuts, cinnamon, chocolate syrup, and marshmallows. Describe criteria you may use to decide on the best recipe. How do the energy and entropy of the matter change when you make cookies with these other ingredients? Support your answer with an explanation. 286

9 Section 1 Energy and Entropy: Alternative Reaction Pathways 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 the observations you might make in each of the following situations: Disorder of matter increases Disorder of matter decreases Energy of matter increases Energy of matter decreases How do you know? Describe the four methods used to produce carbon dioxide gas and how you could know what type of change(s) occurred in energy and entropy in each case. Why do you believe? Changes in energy and entropy are common occurrences in everyday life. Describe the energy and entropy changes that occur in a situation that you have experienced or are aware of. You might choose cooking over an open fire, burning gasoline in a car, or another situation. Why should you care? Chem Essential Questions Explain how the arrangement of particles changes as disorder changes. Explain how the bonding of atoms may be affected by changes in energy. You will be developing a series of physical and/or chemical changes that result in the lighting of a small bulb called an LED. Assume that tipping a lever will be the first step in your apparatus. Brainstorm a list of things that tipping the lever could cause to happen next. Show how you would represent a decrease in disorder using a drawing and a number line. Show two different methods you would use to illustrate how a change releases energy overall. Reflecting on the Section and the Challenge There are lots of alternative pathways for arriving at a destination, but one is usually better than all the rest. In this section, you practiced some of the steps in the Engineering Design Cycle. In order to design an apparatus for the Chapter Challenge, you will have to complete the entire process, inventing your own pathways and deciding which is best. It will be helpful to establish criteria for judging different possible solutions so that you can evaluate them. You have also learned several methods for producing carbon dioxide gas. You may want to use one of those methods in your toy. One use for producing a gas could be to blow up a balloon. There could be other uses for the gas as well. 287

10 Chemical Dominoes Chem to Go 1. Identify each of the following changes as endothermic, exothermic, or not possible to tell. The matter to keep in perspective is in italics in each case. Explain your decision in each case. a) When effervescent antacid tablets are dropped into water, the tablets slowly disappear and the water becomes fizzy. b) On a hot day, water vapor condenses out of the air and turns into liquid droplets on the outside of a cold glass of lemonade. c) The copper on the Statue of Liberty oxidizes and turns blue. 2. Identify each of the changes listed in Question 1 as disorder (spreadedness) increases, disorder (spreadedness) decreases, or not possible to tell. Explain your decision in each case. 3. Use the following chemical equations to answer Parts a) and b). 4Fe(s) 3O 2 (g) 2Fe 2 O 3 (s) and 2KClO 3 (s) 2KCl(s) 3O 2 (g) a) Identify the reactants and products in each chemical equation. b) Decide if each change represents an increase in disorder or decrease in disorder. Explain your choice in each case. 4. What occurs when an atom of chlorine and an atom of hydrogen become a molecule of hydrogen chloride? a) A chemical bond is broken and energy is released. b) A chemical bond is broken and energy is absorbed. c) A chemical bond is formed and energy is released. d) A chemical bond is formed and energy is absorbed. 5. Given the balanced equation: KNO 3 (s) J K (aq) NO 3 (aq) H Which statement best describes this process? 2 O a) It is endothermic and entropy increases. b) It is endothermic and entropy decreases. c) It is exothermic and entropy increases. d) It is exothermic and entropy decreases. 6 Which of these changes produces the greatest increase in entropy? a) CaCO 3 (s) CaO(s) CO 2 (g) b) 2 Mg(s) O 2 (g) 2 MgO(s) c) H 2 O(g) H 2 O(l) d) CO 2 (g) CO 2 (s) 7. Preparing for the Chapter Challenge a) You have just investigated several methods for blowing up a balloon that can tip a lever. Review your Why should you care? response. Which method for blowing up the balloon would be most appropriate in each case you listed? Provide reasons for your selections. b) Provide at least one situation in which you might want to inflate the balloon slowly. Which method would be best in that case? 288