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Active Chemistry Cool Chemistry Show Activity 5 Chemical Energy 7HAT$O9OU3EE GOALS What Do You Think? In this activity you will: Injury seems to be part of professional sports. Athletic trainers have large kits with first-aid items to treat athletes during a game. One item they use is a cold pack. • Make hot packs and cold packs. • Observe energy changes when matter undergoes a change. • Determine whether energy changes are endothermic or exothermic from a particular point of reference. • Do the instant ice packs found in first-aid kits require refrigeration? • How do the cold packs work? Record your ideas about these questions in your Active Chemistry log. Be prepared to discuss your responses with your small group and the class. Investigate 1. To make a cold pack, place 10 g of ammonium nitrate in a quart-size, resealable plastic bag. Add 20 mL of water to the bag and seal. a) Record your observations. 468 SE_CH_C06_CH6.indd 468 5/23/07 12:43:43 PM Activity 5 Chemical Energy In an endothermic chemical reaction, energy in the form of heat is absorbed in the process. In an exothermic chemical reaction, energy in the form of heat is given off in the process. b) Was the cold pack an example of an exothermic or endothermic chemical reaction? 2. Make a hot pack by placing 20 g of sodium carbonate (or calcium chloride) in a quart-size, resealable plastic bag. Add 20 mL of water to the bag and seal. a) Record your observations. b) Was the reaction exothermic (heat generating) or endothermic (heat absorbing)? 3. Your teacher will perform the following demonstration. To a flask containing 16 g of ammonium thiocyanate, 32 g of barium hydroxide is added. A rubber stopper is placed in the mouth of the flask. The flask is shaken vigorously. The stoppered flask is placed on a wood board that has been wet down so that there are puddles of water. Safety goggles and a lab apron must be worn at all times in a chemistry lab. a) Record your observations. Be careful when working with the sodium hydroxide pellets. Wear rubber gloves and eye protection. If you should accidentally drop a pellet, do not try to pick it up with your bare hands as it may burn them. Use gloved hands to retrieve the pellets. b) Was the reaction exothermic (heat generating) or endothermic (heat absorbing)? Wash your hands and arms thoroughly after the activity. a) Record your observations. Cool chemistry! b) Was the reaction exothermic (heat generating) or endothermic (heat absorbing)? 4. Using a chemical scoop, transfer a few pellets of sodium hydroxide to a test tube half-full of water. Carefully feel the side of the test tube. 5. Dispose of the materials as directed by your teacher. Clean up your workstation. 469 Active Chemistry SE_CH_C06_CH6.indd 469 5/23/07 12:43:46 PM Active Chemistry Cool Chemistry Show Chem Words endothermic change (reaction): a change in which energy in the form of heat is absorbed from the surrounding environment resulting in an increase in the internal energy of the system. exothermic change (reaction): a change in which energy in the form of heat is released from a system resulting in a decrease in the internal energy of the system. activation energy: the minimum energy required for a chemical reaction. kinetic energy: the energy of motion. KE = _1 mv2 2 ENDOTHERMIC AND EXOTHERMIC PROCESSES Reactions That Absorb and Release Heat Energy A process is described as endothermic when heat energy is absorbed, increasing the internal energy of the system. The cold pack you made with ammonium nitrate is an example of an endothermic reaction. Another example is the decomposition of potassium chlorate. In this reaction, energy must be added to the system in order to cause the decomposition of the potassium chlorate to form the products of oxygen gas and potassium chloride. If you touch a container that holds a spontaneous endothermic process, it will feel cool to the touch. An exothermic process results when heat energy is released, decreasing the internal energy of the system. If you touch a container that holds an exothermic process, it will feel warm or hot to the touch. The hot pack you made with sodium carbonate is an example of an exothermic reaction. Another example is the combining of sodium hydroxide solution with hydrochloric acid. This reaction produces sodium chloride and water and releases energy to the environment. The terms endothermic and exothermic can be used when describing both physical and chemical changes. Importance of Energy Why is energy so important? In order for a chemical reaction to take place, the particles (reactants) involved in the reaction must interact. Not all collisions result in a chemical reaction. The particles involved must have enough energy to enable them to react with each other. The colliding particles must have enough kinetic energy to break the existing bonds in order for new bonds to be formed. (Physics reminder: 1 Kinetic energy is the energy of motion. KE __2 mv2) The minimum energy required for a chemical reaction is called the activation energy for the reaction. Bond breaking is an endothermic process and requires an addition of energy. Bond formation is an exothermic process and is accompanied by a release of energy. As products form, energy is released as new bonds are created. If this released energy is greater than the energy needed to break the bonds of the reactants (bond-forming bond-breaking), the reaction is 470 Active Chemistry SE_CH_C06_CH6.indd 470 5/23/07 12:43:47 PM Activity 5 Chemical Energy exothermic, as shown in the graph. Exothermic Reaction The prefix exo (activation energy) means “leaving” and Ea C6H12O6 + 6O2 thermo means heat energy and so, heat P.E. (heat of reaction) H energy is leaving. 6 CO2 + 6H2O The products have less energy than the reactants. For Reaction Coordinate example, the cells in your body use glucose to get energy for cellular functions. Glucose plus oxygen (from breathing) provides you with energy. C6H12O6 6O2 → 6CO2 Reactants 6H2O energy Products This energy is used by the body. When the released energy from bond formation is less than the energy needed to break the bonds of the reactants (bond-forming bond-breaking), the reaction is called endothermic, as shown in the second graph. The prefix endo means “taking in” and so the products have more energy than the reactants. The reverse reaction, which is performed by plants during photosynthesis, requires energy and is endothermic. 12H2O 6CO2 energy (sunlight) → C6H12O6 6O2 6H2O Plants produce glucose for their own growth and release oxygen for living things (including the plants) to breathe. In the process of photosynthesis, plants use energy from light to break the bonds of carbon dioxide and water in order to form the products, glucose and oxygen. Endothermic Reaction C6H12O6 + 6O2 + 6H2O Ea (activation energy) H 6CO2 + 12H2O (heat of reaction) P.E. Reaction Coordinate 471 Active Chemistry SE_CH_C06_CH6.indd 471 5/23/07 12:43:51 PM Active Chemistry Cool Chemistry Show Chem Words Law of Conservation of Energy: states that the energy absorbed from the surroundings or released to the soundings is equal to the change in the energy of the system. gravitational potential energy: is a stored energy determined by an object’s position in a gravitational field. Conservation of Energy Energy transfer is an important feature in all chemical changes. Energy is transferred whether the chemical reaction takes place in the human body, like the metabolism of carbohydrates, or in a car, like the combustion of gasoline. In both of these cases energy is released into the surroundings. The photosynthesis that occurs in living plants and the decomposition of water into hydrogen and oxygen both require energy from the surroundings. Energy is the great organizing principle of all science. The conservation of energy allows you to better understand the world around you. Energy exists as light energy, heat energy, sound energy, nuclear energy, kinetic energy, and chemical energy, as well as other forms. According to the Law of Conservation of Energy, the energy absorbed from the surroundings or released to the surroundings is equal to the change in the energy of the system. The total energy in any isolated (perfectly insulated), closed system remains the same. In the simplest physical systems, it is quite easy to describe the energy changes. When a bowling ball falls, its original gravitational potential energy becomes kinetic energy as the ball increases its speed. After the ball hits the ground, this kinetic energy is converted to sound energy (you hear the crash) and heat energy (you can measure a temperature rise of the ball and ground) and the compression and vibration of the ground. Each of these can be measured and it is always found that the total energy before an event is equal to the total energy after the event. When a human being is involved, you notice other energy interactions. A person is able to eat food and digest the food. The energy released from this slow-burn of the food (metabolism) is able to keep the body at about 472 Active Chemistry SE_CH_C06_CH6.indd 472 5/23/07 12:43:52 PM Activity 5 Chemical Energy Chem Words 37ºC. As all non-living things in a room cool down to room temperature, humans are able to stay warm in the 22ºC room environment. People also use the food energy that they ingest for moving muscles, keeping the heart pumping, and operating all human functions. Living organisms are superb energy conversion systems. heat: a form of energy that results from the motion of atoms and molecules. temperature: a measure proportional to the average kinetic energy of all the particles of a material. Heat and Temperature Both heat and temperature have been mentioned in this activity. It is important to note that heat and temperature are not the same, although they are related. Heat is one form of energy. When two materials of different temperatures interact, they exchange heat energy until they arrive at the same temperature. Temperature is a number associated with how hot or cold something is. On a molecular level, temperature is related to the average kinetic energy of the atoms in the material. All particles in a material are in a constant state of motion. The temperature of the material is a measurement of this molecular motion. If the kinetic energy of the particles increases, the temperature increases. A thermometer is an instrument that measures temperature. Heat is the transfer of energy, which often results in a change in the kinetic energy of particles. That is, it results in a change in the temperature of the system. Checking Up 1. Describe an endothermic reaction. 2. Describe an exothermic reaction. 3. Explain whether each of the following is endothermic or exothermic: a) bond breaking b) bond forming 4. Distinguish between heat and temperature. What Do You Think Now? At the beginning of the activity you were asked: • Do the instant cold packs found in first-aid kits require refrigeration? • How do the cold packs work? How would you answer these questions now that you have completed this activity? 473 Active Chemistry SE_CH_C06_CH6.indd 473 5/23/07 12:43:55 PM Active Chemistry Cool Chemistry Show What does it mean? Chemistry explains the macroscopic phenomenon (what you observe) with an explanation of what happens at the nanoscopic level (atoms and molecules) using symbolic structures (formulas and measurements) as a way to communicate. Complete the chart below in your Active Chemistry log: MACRO What do you observe when heat energy is absorbed during a chemical reaction? What do you observe when heat energy is released during a chemical reaction? NANO What happens to atoms and molecules as they become heated? SYMBOLIC How can you graphically depict an exothermic reaction and the activation energy required for the reaction to take place? How do you know? In the activity, what chemical reaction did you observe that was an exothermic reaction? What evidence did you have that the process was exothermic? Why do you believe? Consider the chemical reactions that you encounter in daily life. Which are more common, endothermic reactions or exothermic reactions? Hand and feet warmers are used in the winter. Why do you think that hand and feet coolers are not used in the summer? Why should you care? Experiences that involve more than one sense (as in sight, sound, etc.) add interest to a presentation. Consider and explain how the concept of chemical energy can safely incorporate the sense of touch in your chemistry show. Reflecting on the Activity and the Challenge Energy is involved in any process that requires the breaking or making of bonds. The process may be a chemical or physical change. Sometimes energy changes are not noticed or measured, but other times an energy change is significant enough to be detected. In this activity you explored both endothermic and exothermic processes. As you select activities to include in the Cool Chemistry Show, you must be aware of any heat energy released or absorbed. The audience may be interested to learn about the energy changes that accompany chemical processes. In addition, your awareness will ensure that the presentations are safe for both the presenters and the audience. 474 Active Chemistry SE_CH_C06_CH6.indd 474 5/23/07 12:43:57 PM Activity 5 Chemical Energy 1. Identify the following changes as endothermic or exothermic. (Ask yourself whether the reaction requires the addition of heat energy to occur or does it release energy in the form of heat.) a) Melting ice b) Lighting a match c) Dry ice subliming into carbon dioxide gas d) Frying an egg e) Burning gasoline f) Explosion of hydrogen gas when it reacts with oxygen gas 2. The water in a teapot is heated on a stovetop. The temperature of the water increases. Is this an endothermic or exothermic process? 3. If a red-hot piece of iron is dropped into a bucket of water, what type of heat change takes place in reference to the water? What type of heat change takes place in reference to the iron? Use the conservation of energy to describe the event. 4. Explain in terms of energy flow how a cold pack works on a sprained ankle. 5. If a sample of ice is at –20ºC and you set a beaker of the ice on a warm hot plate, explain why the ice does not appear to start melting immediately. 6. Given the reaction: S(s) O2(g) → SO2(g) energy Which diagram best represents the potential energy changes for this reaction? 2EACTION#OORDINATE 0OTENTIAL%NERGY 2EACTION#OORDINATE D 0OTENTIAL%NERGY 2EACTION#OORDINATE C B 0OTENTIAL%NERGY 0OTENTIAL%NERGY A 2EACTION#OORDINATE 7. Rumor has it that a leading cola producer is preparing a new can of cola that does not need refrigeration! That’s right, you simply take the can right out of the cupboard, open the can, and after waiting for five minutes you’re able to enjoy an ice-cold can of your favorite soda! a) Do you think this “instant cold” is possible? Give an explanation for your answer. b) How much would you be willing to pay for such a feature (in addition to the cost of a regular can of soda)? 8. Preparing for the Chapter Challenge Review the chemical reactions that you have so far planned to feature in your Cool Chemistry Show. Have you included both endothermic and exothermic reactions? In a paragraph describe the difference between the two types of reactions. Be sure to mention why energy transfers are important. 475 Active Chemistry SE_CH_C06_CH6.indd 475 5/23/07 12:43:57 PM Active Chemistry Cool Chemistry Show Inquiring Further 1. Commercial cold and hot packs Research several cold and hot packs. What materials are used in the packs, and how is the chemical reaction activated? You should be aware that there are a variety of commercially available hot and cold packs. Some are reusable and some are not. Those that are not reusable are typically chemical reactions. Those that are reusable are typically physical changes. Design a process to make a cold pack, using your research. Have your teacher approve your design before you actually try it out. 2. One colligative property of a solvent A salt solution will depress the freezing point of water. This is commonly known as the colligative property of a solution. When you add anti-freeze (ethylene glycol) to water, you find that the freezing point of the solution is lowered, which prevents the water in the car radiator from freezing at 0ºC. It also elevates the boiling point of the solution, which will prevent the water solution from boiling at 100ºC. Design an experiment to demonstrate the colligative property of a solvent. With the approval of your teacher carry out your experiment. 476 Active Chemistry SE_CH_C06_CH6.indd 476 5/23/07 12:43:59 PM