To help students grasp thermochemistry better by doing a hot/cold pack experiment.
This lesson is the first of a three-part series on energy transformation. All three lessons have the general purpose of increasing students' understanding of energy transfer, its role in chemical change, and the factors that can influence this change. Then each lesson in addition has a specific purpose. The Transfer of Energy 1 or 2 can be done in any order; however, it is suggested that The Transfer of Energy 1 and 2 be done before The Transfer of Energy 3.
The Transfer of Energy 1: Thermochemistry is intended to increase students' understanding of heat and chemical reactions.
The Transfer of Energy 2: Electrochemistry is intended to increase students' understanding of electron transfer and its role in chemical changes.
The Transfer of Energy 3: Rust and Corrosion reinforces students' understanding of thermochemistry and electrochemistry by exposure to a process that they observe in life. This lesson allows students to understand how energy transfers in the chemical change of rust and corrosion and to understand the factors that influence this process.
By the end of elementary school, students should know several points about energy transformation. Students should know that when warmer objects are put with cooler ones (at a distance or next to each other), the warmer objects transfer internal energy (emitted as heat) to the cooler ones until they all reach the same temperature. They should understand things that give off heat can also give off other sorts of energy, including light. And, that heat is produced any time one thing rubs against something else and by mechanical and electrical machines. Students should also know that some materials transmit heat energy much better than others (materials that are poor conductors can reduce the transmission of heat from the object).
This prerequisite knowledge helps middle-school students learn the following four points about energy transformation:
- Energy cannot be created or destroyed, but only changed from one form into another.
- Most of what goes on in the universe—from exploding stars and biological growth to the operation of machines and the motion of people—involves some form of energy being transformed into another. Energy in the form of heat is almost always one of the products of an energy transformation.
- Heat can be transferred through materials by the collisions of atoms or across space by radiation. If the material is fluid, currents will be set up in it that aid the transfer of heat.
- Energy appears in different forms. Heat energy is in the disorderly motion of molecules; chemical energy is in the arrangement of atoms; mechanical energy is in moving bodies or in elastically distorted shapes; gravitational energy is in the separation of mutually attracting masses.
At this early stage, there may be some confusion in students' minds between energy and energy sources. Focusing on energy transformations may get around this somewhat. Food, gasoline, and batteries obviously get used up. But the energy they contain does not disappear; it is changed into other forms of energy through physical or chemical processes.
Heat itself is a surprisingly difficult idea for students, who thoroughly confound it with the idea of temperature. A great deal of work is required for students to make the distinction successfully, and the heat/temperature distinction may join mass/weight, speed/acceleration, and power/energy distinctions as topics that, for purposes of literacy, are not worth the extraordinary time required to learn them. Because dissipated heat energy is at a lower temperature, some students' confusion about heat and temperature leads them to infer that the amount of energy has been reduced. On the other hand, some students' ideas that dissipated heat energy has been "exhausted" or "expended" may be tolerably close to the truth.
Research most applicable to thermochemistry shows the following information on heat, temperature, and chemical changes:
Heat and temperature
After years of instruction, students do not distinguish well between heat and temperature when they explain the thermal phenomena. Their belief that temperature is the measure of heat is particularly resistant to change. Long-term teaching interventions are required for upper middle-school students to begin differentiating between heat and temperature. (Benchmarks for Science Literacy, p. 337.)
Some middle-school students have difficulty explaining the process of heating and cooling in terms of heat being transferred. Some think that "cold" is being transferred from a colder to a warmer object, others think that both "heat" and "cold" are transferred at the same time. Both middle- and high-school students have difficulty explaining heat-exchange phenomena as interactions. For example, some students think objects cool down or release heat spontaneously—that is, without being in contact with a cooler object. Even after instruction, students find it hard to give up their naive notion that some substances (for example, flour, sugar, or air) cannot heat up or that metals get hot quickly because "they attract heat," "suck heat in," or "hold heat well." Middle-school students even believe different materials in the same surroundings have different temperatures if they feel different (for example, metal feels colder than wood). As a result, students fail to recognize the universal tendency to temperature equalization. Few students (middle- and high-school) understand the molecular basis of heat transfer even after instruction. Although specially designed instruction as opposed to traditional instruction appears to give students a better understanding about heat transfer, some difficulties often remain. (Benchmarks for Science Literacy, pp. 337-338.)
The idea of energy conservation seems counter-intuitive to middle- and high-school students who hold on to the everyday use of the term energy. However, teaching heat-dissipation ideas at the same time as energy-conservation ideas may help alleviate this difficulty. Students in middle- and high-school tend to use their intuitive conceptualizations of energy to interpret energy conservation ideas. For example, some students interpret the idea that "energy is not created or destroyed" to mean that energy is stored up in the system and can even be released again in its original form. Although teaching approaches that accommodate students' difficulties about energy appear to be more successful than traditional science instruction, the main deficiencies outlined above remain despite these approaches. (Benchmarks for Science Literacy, pp. 338.)
To beging this lesson, students should use their What is Thermochemistry? student esheet to visit Thermochemistry, on the Chem4Kids website, to be introduced to the topic of thermochemistry.
Ask students the questions below. These questions address information in the website and common misconceptions that students have at this level. Give students time after each question so that they have time to write their ideas on the What is Thermochemistry? student sheet. Next, have students share what they wrote down.
Note: Questions 4-6 address some common misconceptions that students hold, thus you may wish to discuss the answers to these questions in more detail before moving to the next section.
- What is cold and when and why do we feel it?
("Cold" is the absence of heat. We feel a cold substance because the substance takes kinetic energy away from our skin—heat is transferred from our skin to the colder substance. We sense this change as "cold.")
- How do chemical reactions take away or absorb heat?
(Chemical reactions form and break bonds between atoms. Different bonds contain different quantities of energy—a net loss of energy is passed into the surroundings as "heat," and a net gain of energy is passed into the surroundings as "cold.")
- How does a thermometer read temperature?
(Energy is transferred from the solution to the thermometer bulb. A fluid within the bulb—usually alcohol or mercury, expands with the inflow of energy. The gradations on the thermometer measure the amount of expansion.)
- What is internal energy, what is heat, and what is temperature?
(Internal energy is the total kinetic energy of molecules comprising an object; heat is the mode of transmission of this energy; and temperature is the average kinetic energy of an object's component atoms.)
- Given a drop of water heated to 95 degrees Celsius and a gallon of water heated to 90 degrees Celsius, which has the greater internal energy? Which has the greater temperature?
(The gallon has the greater internal energy because there are more water molecules in a gallon of water. The drop has the greater temperature because a water molecule heated to 95 degrees Celsius has more average kinetic energy than a water molecule heated to 90 degrees Celsius.)
- When a pot of hot water is left out on a counter and cools to room temperature, where does the heat go?
(The heat is dissipated into the surroundings, including the air.)
Next, explain to students how this relates to the concept of energy conservation: The TOTAL energy in the room INITIALLY equals the energy in the air AND the energy in the pot of water; AFTER the pot has cooled, the energy in the air has increased and the energy in the pot has decreased but the TOTAL energy in the room is the same.
In this part of the lesson, you and your students will perform an experiment in which you will determine how the temperature of water changes when it is mixed with either calcium chloride or a ammonium nitrate.
Hand out the Hot and Cold Packs student sheet for students to follow along and complete while doing the experiment. You can refer to the Hot and Cold Packs teacher sheet for answers to the questions on the student sheet.
Hot and Cold Packs Experiment Procedure
- Take three 250 ml or larger beakers, fill them with water, and add a thermometer. Record the temperature.
- Then, add approximately 25 grams of calcium chloride to one beaker, and 25 grams of ammonium nitrate to the second, and leave the third without any chemical added.
- Stir continuously to ensure mixing (student volunteers should be able to help), and watch the changes in temperature over time.
- Record the temperature every five minutes for a total of 30 minutes.
- Have students volunteer answers to the questions listed on their student sheet. Help students with answers if necessary.
Ask these questions to review what students learned in the hot and cold packs experiment:
- In commercial hot and cold packs, how are the two chemicals kept separate until ready for use?
(They are separated into pouches via a fragile connection. A sharp blow to the pack will mix them and create the desired effect.)
- Would twice the chemical produce twice the temperature difference?
(No. While twice the energy will be released or absorbed, energy will be more readily exchanged with the external environment. This process is faster than the reaction of the chemical with water. For commercial packs, scientists determine the optimal ratio of additive to water to create the proper temperature.)
- What is thermochemistry?
(It is the division of chemistry that deals with temperature in chemical reactions.)
Follow this lesson with the next two lessons in The Transfer of Energy Series:
More engaging thermochemistry experiments can be found at Endothermic and Exothermic Reactions on the About.com Chemistry site. Check out the two reactions at the bottom of the page!