Chemical Dominoes Activity 7 Reactions that Produce Heat GOALS In this activity you will: Explain the effect of particle size and use of a catalyst on reaction rate. Represent energy changes graphically. Explain the role of energy in chemical reactions. What Do You Think? You may already know that when American soldiers are in the field and don t have access to stoves, they eat pre-packaged meals called MREs or Meals, Ready-to-Eat. These meals include a packet of chemicals that allow soldiers to quickly heat their meals without having to build a fire. What is the source of the heat produced by the hot pack? How is the hot pack able to increase in temperature without an external (added) source of heat? Record your ideas about these questions in your log. Be prepared to discuss your responses with your small group and the class. Investigate In your Chemical Dominoes apparatus, you may want to use a chemical reaction that produces heat to cause something to happen. Perhaps you could melt an ice cube that is tied to a lever. To produce heat by a chemical reaction, you have to know a little about how the heat is produced, so you can have some control over it. Part A: Working With the MRE Heater In this part of the activity you will investigate the chemical method that military personnel use to heat their meals when they are in the field and do not have access to stoves. 318
Activity 7 Reactions that Produce Heat Tear here. Test tube MRE heater Thermometer Cut slit, and insert thermometer. Place tape over slit to seal. Fill lines MRE heater 1. Examine an MRE heater carefully. a) Record your observations in your log. 2. Open the MRE heater by tearing it open at the top. Look inside the bag. a) Record what you see. 3. Fill the MRE heater to the fill line with distilled water. a) Record your observations of the reaction as it proceeds for several minutes. From where do you think the heat comes? 4. After five minutes, hold a watch glass over the opening of the bag. a) Describe anything you notice forming on the watch glass. b) Place a thermometer in the MRE heater. Record the temperature in your log. 5. The chemical reaction that is occurring in the MRE heater produces a gas. Collect the gas escaping from the heater by carefully holding the opening of the bag around the mouth of a test tube. Caution: The bag and the gases are hot. Then, in an area designated by your teacher, test the gas. Bring a glowing wood splint to the mouth of the test tube. If the splint is extinguished, the gas might be carbon dioxide or some other non-flammable gas. If the splint glows brighter, the gas in the tube is oxygen. Finally, if there is a pop or barking sound, the gas is likely hydrogen. Water vapor is one of the gases produced by the reaction inside the MRE heater, but another gas is also produced. a) Based on the results of the woodsplint test, what other gas is produced? Explain how your results support your conclusion. 6. When the reaction is complete, add 20 ml of distilled water to the MRE heater. Then pour the solution from the MRE heater into a test tube. Add a few drops of phenolphthalein to the solution in the test tube. Phenolphthalein is an indicator that will turn the solution pink if a base is present. Is the solution basic? a) Record your results and explain how you arrived at your conclusion. 7. Dispose of the MRE heater according to your teacher s directions. 8. The MRE heater produces heat by reacting magnesium metal with water. The balanced chemical equation that represents the reaction that is occurring inside the MRE heater is: All unreacted magnesium metal from the MRE should be treated with dilute HCl before disposal. 319
Chemical Dominoes Test Tube A Test Tube B Test Tube C Test Tube D Mg(s) 2HOH(l) magnesium water Mg(OH) 2 (s) H 2 (g) energy magnesium hydrogen hydroxide gas a) Use the equation for the reaction in the MRE heater to account for your observations. Part B: How the MRE Heater Works Mg NaCl Fe Water 0.25 g granules 0.25 g granules 0.25 g granules 0.25 g strip Do not dispose unreacted magnesium metal down the drain or in the trash can. Put all magnesium metal in a waste beaker for proper disposal by your teacher. 1. Your teacher will provide you with four test tubes. These will be used to test several variables related to the function of the MRE heaters. MRE heaters contain magnesium granules, table salt and iron powder. Test tubes A and B contain only 0.25 g of magnesium. Test tubes C and D contain powdered iron as well as 0.25 g of magnesium but in D, the magnesium is in one large strip. a) Make the following data table in your log. none none 5.0 ml 1.0 g none 5.0 ml 1.0 g 1.0 g 5.0 ml T i T = f = 1.0 g 1.0 g 5.0 ml T i T = f = Temperature (ºC) Observations b) In comparing Test tubes A and B, what variable is being tested? In B and C, what variable is being tested? And in C and D, what variable is being tested? Record your answers in your log. 2. Measure out 5.0 ml of distilled water into each of two 10-mL graduated cylinders. While one group member starts a stopwatch, two other group members should simultaneously add the water to Test tubes A and B. a) Observe the two test tubes for five minutes. Write your observations and the final temperature in your log. Do either of the test tubes feel warmer? 3. Again, measure out 5.0 ml of distilled water into each of two 10-mL graduated cylinders. Measure the temperature of the water and record this as T i. Place thermometers into Test tubes C and D. a) While starting a stopwatch, simultaneously add the water to Test tubes C and D. b) Record observations for 10 minutes. Carefully touch the test tubes occasionally to feel for heat. Record the highest temperature reached in each test tube as T f. c) Which sample, C or D, showed the largest increase in temperature? 4. Write three short statements which summarize the results comparing A with B, B with C, and C with D. 5. According to the reaction equation in Part A, the reaction should produce a base, Mg(OH) 2. Test one of the solutions in Test tubes A, B, C, or D with red litmus paper or phenolphthalein. Does the test confirm that the solution is basic? 6. The iron in Test tubes C and D was partially ground into the magnesium with a mortar and pestle to ensure very close contact of the two metals. Design an experiment to determine if this step is necessary. With your teacher s approval, carry out your experiment. 7. Dispose of the magnesium, iron, and the solutions by placing them in a waste beaker. Do not put magnesium metal down the drain or in the trash because it can generate enough heat to ignite paper or melt plastic pipes. Clean your test tubes when you are done. 320
Activity 7 Reactions that Produce Heat THERMODYNAMICS Will it continue to occur? How fast will it occur? Those are two important questions that chemists and engineers need to be able to answer about any chemical reaction they plan to use. In this activity, you explored a chemical reaction that continues to occur on its own once it begins. Chemists call such a reaction spontaneous. You also explored some factors that affect how quickly a reaction will occur. In the case of a reaction that produces heat energy (like the reaction that occurs in the MRE heater), controlling the speed of the reaction will let you control how quickly heat is produced. For chemists and engineers to be able to control chemical reactions, they must be able to predict when reactions can occur spontaneously, and also what can be done to speed up or slow down reactions. One set of theories called thermodynamics is used to answer the question Can a reaction occur spontaneously? A second set of theories called kinetics is used to answer the question How fast can a reaction occur? Together, thermodynamics and kinetics help chemists and engineers to design reactions and processes that impact everyone s lives. Chem Words spontaneous: a change that, once begun, continues without an input of energy. thermodynamics: the study of how heat and other forms of energy are involved in chemical and physical reactions. kinetics: the study of reaction rates and how they can be affected by variables such as concentration, particle size, and temperature. Thermodynamics and Spontaneity There are two factors that determine if a change can occur spontaneously. You used these factors in Activity 1 as criteria with which to compare reactions that produced gas. The first factor that affects spontaneity is if the change gives off heat energy when it occurs or absorbs heat energy when it occurs. The second factor that affects spontaneity is if the change results in particles becoming more disordered (randomly arranged) or less disordered (more organized). Two commonsense rules apply to these two questions. First, lower energies are more stable than higher energies. So, in the same way that a ball tends to roll downhill, energy changes in chemical reactions tend to occur in ways that allow the substances to end up with lower energy. This means that changes that release energy tend to be favored. Second, everything tends to become more disorganized over time. So, changes in which particles become more disordered (spread out) are favored over those that make particles become more ordered. The two factors that affect spontaneity can work together. If a change both releases energy and results in an increase in disorder, the change is definitely spontaneous. If a change both absorbs energy and results in a decrease in disorder, the change is definitely not spontaneous. 321
Chemical Dominoes However, if one factor is favorable and the other is not, whichever is the stronger tendency controls whether the change is spontaneous. In this activity you explored a change that was spontaneous. Heat energy was given off as the system went from a higher energy to a lower one. This means at least one of the two factors affecting spontaneity is favorable for this reaction. This Chem Talk will focus on heat changes. Heat Energy Changes: Endothermic and Exothermic Reactions Consider the chemical reaction between magnesium and water that you observed in the activity. Mg(s) 2HOH(l) Mg(OH) 2 (s) H 2 (g) energy magnesium water magnesium hydrogen metal hydroxide gas The main points here are that: When chemical reactions happen, bonds the reactants (starting materials) break, and new bonds form to make products (ending materials). Breaking bonds requires energy input, so bond-breaking is an endothermic change. Forming bonds releases energy, so bond-forming is an exothermic change. The overall or net change can be endothermic or exothermic, depending on whether the total energy input (required to break bonds in reactants) or the total energy output (given off when bonds form in products) is greater. In the chemical reaction between magnesium and water, the total energy input required to break the bonds in the reactants is 572 kj. This number and similar numbers have been measured precisely and can be found in reference books. in 322
Activity 7 Reactions that Produce Heat (Endothermic changes are positive, because the system gains energy.) When the new bonds form in the products (making magnesium hydroxide and hydrogen gas), the total energy output is 925 kj. (Exothermic changes are negative, because the system loses energy.) The overall or net change is more exothermic than endothermic. More energy is produced than absorbed. The reaction in the MRE heater, like most changes (both chemical and physical) studied in chemistry, occurs at constant pressure. Chemists call an energy change that occurs at constant pressure change in enthalpy and represent it with the symbol ΔH. The Δ (the Greek letter delta ) is frequently used to mean change. Hess s Law states that, for a given reaction, enthalpy changes are additive. When the Standard Enthalpies of Formation (ΔH f, see Tables) of the reactants are subtracted from the products, the result is the net change in enthalpy for the reaction. A negative ΔH tells that the reaction is exothermic; a positive ΔH tells that the reaction is endothermic. The enthalpy change (ΔH) for the reaction in the MRE heater can be calculated as shown below. Chem Words enthalpy change: change in heat energy for a process that occurs at constant pressure; symbolized by ΔH. energy diagram: a graph showing how energy changes during the course of a reaction. ΔH = 925 kj (for Mg(OH) 2 2 ( 286 kj) (for 2H 2 O) = 353 kj ΔH is negative, which tells you that the net change is exothermic (the system loses energy to the surroundings). Therefore, the temperature of the MRE heater increased. You could also compare the energy absorbed and the energy released graphically like we did in Activity 1. The enthalpy change that occurs when a reaction occurs is sometimes included in the chemical equation as shown below. Mg(s) 2H 2 O(l) Mg(OH) 2 (s) H 2 (g) ΔH 353 kj Energy Diagrams Another way to represent an enthalpy change is to draw an energy diagram. An energy diagram shows the progress of the reaction from reactants to products along the horizontal axis. Along the vertical axis, the diagram shows the change in potential energy that occurs as substances progress from reactants to products. An energy diagram provides the following two important pieces of information: Endothermic or exothermic: The relative locations of the reactant (starting) energy and product (ending) energy indicate whether the overall enthalpy change (ΔH) is endothermic or exothermic. If the starting point (reactants) is higher than the ending point (products), then the system releases heat energy to the surroundings (an exothermic change), and the overall enthalpy change is negative (ΔH 0). If the energy of the reactants is lower than that of the products, then the system absorbs heat energy from the surroundings (an endothermic change), and the overall enthalpy change is positive (ΔH 0). 323
Chemical Dominoes Chem Words activation energy (E a ): the energy that must be gained by molecules so they can break existing bonds and undergo a reaction. activated complex: the intermediate state that is a combination of reactant and product atoms. Activation energy: Regardless of whether the change is endothermic or exothermic, bonds must always be broken in the reactants (requiring energy input) before new bonds can form in the products (releasing energy). Therefore, some initial amount of energy (always a positive value, since it is added to the system) must be supplied to the system in order for the reaction to begin. This is represented as an activation barrier, or an initial bump in the curve, to get from reactants to products. The height of this bump, measured from the reactant energy, is called the activation energy (E a ) and it is always positive (E a 0). The intermediate (in-between) state, between reactants and products, at the top of the barrier is called the activated complex. Energy Diagram for an Exothermic Reaction Energy E a activated complex Energy Diagram for an Endothermic Reaction activated complex Energy E a reactants H<0 H>0 products products reactants Reaction Coordinate Reaction Coordinate More about the Bump When you observed the reaction of magnesium with water and compared that reaction with the reaction of magnesium with NaCl and water, you could see that the reactions were very slow. Little heat was generated, although the salt solution reacted slightly faster than water alone. Tiny bubbles of hydrogen slowly formed. Clearly another substance was needed in the MRE heater in order to produce enough heat to warm a meal. When you compared the reaction of magnesium in a salt solution with the reaction of magnesium, salt water, and powdered iron, the difference was very dramatic. After a few minutes, the sample containing iron produced much more heat and bubbled vigorously. This combination could indeed be used to heat a meal in an outdoors setting. Powdered iron is an important component of the MRE heater. A comparison of the last two samples tested in the Investigate was simply a comparison of the same reaction and the same components, but the size of the magnesium particles was different. Test tube D contained a single large piece 324
Activity 7 Reactions that Produce Heat of magnesium strip while Test tube C used granulated magnesium. Granulated magnesium contains much more surface area than the single piece and the reaction proceeds much faster. This is because reactions can take place only when the reactants collide. When the reactant is a solid, these collisions can only occur on the surface. More surface area = faster reaction. The iron is a key component for speeding up the reaction and it must not only be present, but physically imbedded in the magnesium in order for it to function. According to the scientists and engineers who developed this type of MRE heater, the iron is a catalyst for the reaction: Mg(s) + 2H 2 O(l) Fe > Mg(OH) 2 (aq) + H 2 (g) + heat A catalyst is a substance that speeds up a chemical reaction without being used up itself. A catalyst works by providing a lower-energy alternative pathway for the reaction to take. In essence, it provides a lower activation energy (a lower energy barrier). As a result, more starting materials at a given temperature have enough energy to get over the energy barrier. This speeds up the reaction and generates heat quicker. As the temperature increases, the reaction speeds up even more. Energy Diagram for an Exothermic Reaction with a Catalyst Added reactants new activation energy activation energy The sodium chloride also plays a role in the reaction by being an electrolyte. When dissolved in water, the salt provides a pathway for the electrons to move from the magnesium, which is thus oxidized, to the hydrogen atoms, which are reduced. The oxygen in water remains in the same oxidation state as it becomes a part of the hydroxide ion. E pathway when a catalyst is added H products Chem Words catalyst: a substance that provides a lowerenergy pathway for a reaction which increases the speed of a reaction; it is not consumed (used up) during the reaction. Checking Up 1. What does it mean for a change to occur spontaneously? 2. What two questions must be answered to determine if a change can occur spontaneously? 3. What evidence do you look for to tell if a change is exothermic or endothermic? 4. From where does the energy in an exothermic reaction come? 5. What are two things that chemists or engineers designed into the MRE heater to make the reaction happen faster? 6. What does a catalyst do to speed up a reaction? 7. How does an energy profile graph look for a reaction with a catalyst vs. one without a catalyst? What s different? What s the same? What Do You Think Now? At the beginning of this activity you were asked: What is the source of the heat produced by the hot pack? How is the hot pack able to increase in temperature without an external (added) source of heat? Now that you have explored the MRE heater, how would you answer these questions? 325
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 observations that would lead you to conclude that a reaction is exothermic. Describe observations that would lead you to conclude that a reaction is endothermic. How do you know? Explain whether breaking a bond between atoms is an endothermic process or an exothermic process. Describe how a catalyst particle would interact with a molecule of a reactant to speed up the reaction. A catalyst lowers the activation energy and speeds up a reaction. What evidence do you have from this activity that this occurs? Why do you believe? The use of chemical reactions to release energy for human use is common. Describe some situations in which exothermic reactions are used to produce energy. Human use of endothermic reactions seems to be much less common than use of exothermic reactions. Why would this be the case? What are some situations where an endothermic change (either chemical or physical) might be useful? Why should you care? Draw an energy diagram for both an endothermic and exothermic process. Label the axes, ΔH, E a, reactants, and products for each diagram. Show how the diagrams are affected by adding a catalyst to the reaction. How might you use a reaction that produces heat as part of your Chemical Dominoes apparatus? What would set it off and what would it cause to happen next? Suppose the reaction you have chosen is not producing enough heat. What would be some ways to change the situation so more heat would be generated? 326
Activity 7 Reactions that Produce Heat Reflecting on the Activity and the Challenge In this activity, you learned how chemical reactions can be used to generate heat. Heat is produced in a reaction when the energy released by the formation of new bonds is greater than the energy used to break the old bonds. You also observed that increasing the surface area of the reactants and the addition of a catalyst will speed up a chemical reaction. You may want to use a chemical reaction that releases heat as a part of your Chemical Dominoes apparatus. You could use the heat produced to melt ice, heat a liquid or gas, or cause some other change you design. Knowing how to determine the amount of energy a change will produce is important so you will know how much of the reactants to use. Also, you will need to be able to control how rapidly the heat is produced so that the change happens when you want it to, rather than too soon or too late. 1. Identify each process as endothermic or exothermic. a) Burning wood b) Melting ice c) Evaporation of sweat d) A chemical hand warmer 2. Hydrogen peroxide decomposes to form oxygen gas and water. (Your teacher might have shown you this reaction as a demonstration.) 2H 2 O 2 (aq) O 2 (g) 2H 2 O(l) hydrogen peroxide oxygen water (dissolved in water) (gas) (liquid) a) Use the following information to determine the enthalpy change (ΔH) of the reaction. The energy required to break the bonds in 1 mol of H 2 O 2 is 188 kj. (Note: there are 2 mol of H 2 O 2 required in this reaction.) The total energy released in forming both 1 mol of oxygen gas and 2 mol of water is 572 kj. b) Is the overall change endothermic or exothermic? How do you know? c) Why is it necessary to use a balanced chemical reaction to calculate ΔH? d) How much heat would be released if 50.0 g of H 2 O 2 decomposed? Show your work. e) Sketch what the energy diagram. Label the following on your energy profile: reactants, products, activated complex, total energy change (indicate the sign by the direction of the arrow), activation energy (indicate the sign by the direction of the arrow), the energy axis, and the reaction coordinate axis. f) Sketch a dotted line on your energy diagram to show what it would look like if you added a catalyst. 327
Chemical Dominoes 3. Look back at the description of the reaction in the MRE heater. a) What kind of reaction occurs in the MRE heater? You ve seen these reactions before in a previous activity in this chapter. b) Why does the reaction occur spontaneously? c) How does the amount of energy required to break the bonds in the reactants compare to the amount of energy released when the products form? 4. Adding a catalyst to a chemical reaction results in: a) a decrease in activation energy and a decrease in the reaction rate b) a decrease in activation energy and an increase in the reaction rate c) an increase in activation energy and a decrease in the reaction rate d) an increase in activation energy and an increase in the reaction rate 5. Preparing for the Chapter Challenge One way you might use a heat-producing reaction in your Chemical Dominoes apparatus is to cause an ice cube to melt that would tip a lever (as described in the introductory words in the Investigate section). Brainstorm with your group to come up with an idea of how you could use a heat-producing chemical reaction to melt an ice cube that tips a lever that causes the LED to light. You may need to review your response to Preparing for the Chapter Challenge from Activity 4. Draw diagrams and make some notes so that you can return to these ideas when you build your apparatus. If your group has a different idea of how to use a heat-producing reaction, you may draw a diagram and explain that idea instead. Inquiring Further Calorimetry Calorimetry is the quantitative study of heat. Research coffee-cup calorimetry and build a simple calorimeter. Under the supervision of an adult, determine the heat capacity of your calorimeter. Calcium chloride is a salt used to melt ice on sidewalks. An interesting property of calcium chloride is that it produces a large amount of heat when it dissolves. Under the supervision of an adult, use your coffee-cup calorimeter to calculate how much heat is produced when different amounts of calcium chloride are dissolved in water. Other ionic salts such as ammonium chloride and sodium hydroxide may also be tested. 328