Heat of Reaction Teacher Guide

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Unit

Heat of Reaction

Subject

Chemistry

Grade Level

HS

Activity Name(s)

Activation Energy

Baggie Chemistry

Explosion

Making Heat

Being Prepared

Two of the activities in this unit use Molecular Workbench models, therefore having one student per computer is ideal. If this is not possible, try to keep the number of students per computer as low as possible. With this in mind, if more than one student must use the computer, it will be necessary to ensure that all group members are participating in the activity and discussions.

Two of the activities in this unit require the use of temperature sensors. In these activities, having two students work together is ideal as one student can manipulate the sensor, chemicals, and equipment while the other is controlling the computer. If more than two students must use the computer, it will be necessary to ensure that all group members are participating in the activity and discussions. In addition, it is a good practice to have students switch roles as they proceed through the activity.

If you don't have computers in your classroom, remember to reserve the computer lab or mobile laptop cart as well as the temperature sensors if you share them with other instructors.

If using laptops that are not fully charged, arrange power cords in such a way that walkways remain clear.

Getting Started

The two activities in this unit that use Molecular Workbench models do not require sensors or any other equipment beyond the computer. Encourage students to pay close attention to the written instructions before attempting to interact with the model. Clear instructions are always provided in the text preceding the simulation. Point out to students that they may delete snapshots and reset a simulation if necessary.

Two of the activities use temperature sensors, chemicals, and other equipment. When using such materials, safety must be a high priority. While none of these chemicals are toxic, be careful handling them and clean up thoroughly after the experiment. Check state and local disposal requirements before releasing any waste down the drain or in the trash. Ensure that the match and wooden splint are properly extinguished after lighting by running water over them. Students should wear ANSI-approved chemical splash goggles at all times.

Carefully read instructions for your sensor set-up and use.

Suggested Timeline

Students should be able to complete the two Molecular Workbench activities (Activation Energy and Explosion) in approximately 20-30 minutes each. However, if they have no previous experience with using these activities, the first activity they complete may take longer.

Students should be able to complete the two activities using sensors (Baggie Chemistry and Making Heat) in approximately 45 minutes each. However, if they have no previous experience with completing activities using probes, the first activity they complete may take longer.

Thinking about the Discovery Questions

Overview

This unit investigates physical science concepts related to energy changes that occur during chemical reactions. The following overarching ideas pertain to the activities in this unit: (1) Energy is conserved during chemical reactions. (2) Exothermic reactions release energy to the environment. Endothermic reactions absorb energy from the environment. (3) Reactions produce substances with lower potential energy. (4) Activation energy may be required to initiate a reaction.

Misconceptions

When teaching a unit on energy changes that occur during chemical reactions, it is important to be aware of common student misconceptions: (1) If heat (or some other form of energy) is needed to start a reaction, the reaction must be endothermic. (Instead, heat provides the activation energy to initiate the reaction, but once activated the reaction can be exothermic.) (2) Energy is either used up or created by a reaction. (Instead, energy is conserved.)

Learning Objectives

  • NGSS
    • HS-PS1-4. Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy. [Clarification Statement: Emphasis is on the idea that a chemical reaction is a system that affects the energy change. Examples of models could include molecular-level drawings and diagrams of reactions, graphs showing the relative energies of reactants and products, and representations showing energy is conserved.] [Assessment Boundary: Assessment does not include calculating the total bond energy changes during a chemical reaction from the bond energies of reactants and products.]
    • HS-PS1-5. Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs. [Clarification Statement: Emphasis is on student reasoning that focuses on the number and energy of collisions between molecules.] [Assessment Boundary: Assessment is limited to simple reactions in which there are only two reactants; evidence from temperature, concentration, and rate data; and qualitative relationships between rate and temperature.]
    • Science and Engineering Practices
      • Developing and Using Models
        • Develop a model based on evidence to illustrate the relationships between systems or between components of a system. (HS-PS1-4)
      • Constructing Explanations and Designing Solutions
        • Apply scientific principles and evidence to provide an explanation of phenomena and solve design problems, taking into account possible unanticipated effects. (HS-PS1-5)
        • Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students' own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future. (HS-PS1-2)
        • Refine a solution to a complex real-world problem, based on scientific knowledge, student-generated sources of evidence, prioritized criteria, and tradeoff considerations. (HS-PS1-6)
    • Disciplinary Core Ideas
      • PS1.A: Structure and Properties of Matter
        • A stable molecule has less energy than the same set of atoms separated; one must provide at least this energy in order to take the molecule apart. (HS-PS1-4)
    • PS1.B: Chemical Reactions
      • * Chemical processes, their rates, and whether or not energy is stored or released can be understood in terms of the collisions of molecules and the rearrangements of atoms into new molecules, with consequent changes in the sum of all bond energies in the set of molecules that are matched by changes in kinetic energy. (HS-PS1-4),(HS-PS1-5)
  • Crosscutting Concepts
    • Patterns
      • * Different patterns may be observed at each of the scales at which a system is studied and can provide evidence for causality in explanations of phenomena. (HS-PS1-2),(HS-PS1-5)
    • Energy and Matter
      • The total amount of energy and matter in closed systems is conserved. (HS-PS1-7)
      • Changes of energy and matter in a system can be described in terms of energy and matter flows into, out of, and within that system. (HS-PS1-4)
    • Stability and Change
      • * Much of science deals with constructing explanations of how things change and how they remain stable. (HS-PS1-6)
  • NSES
    • Physical Science - Structure of Atoms
      • The nuclear forces that hold the nucleus of an atom together, at nuclear distances, are usually stronger than the electric forces that would make it fly apart. Nuclear reactions convert a fraction of the mass of interacting particles into energy, and they can release much greater amounts of energy than atomic interactions. Fission is the splitting of a large nucleus into smaller pieces. Fusion is the joining of two nuclei at extremely high temperature and pressure, and is the process responsible for the energy of the sun and other stars.
    • Physical Science - Chemical Reactions
      • Chemical reactions may release or consume energy. Some reactions such as the burning fossil fuels release large amounts of energy by losing heat and by emitting light. Light can initiate many chemical reactions such as photosynthesis and the evolution of urban smog.
    • Physical Science - Chemical Reactions
      • Chemical reactions occur all around us, for example in health care, cooking, cosmetics, and automobiles. Complex chemical reactions involving carbon-based molecules take place constantly in every cell in our bodies.
    • Physical Science - Structure and Properties of Matter
      • The physical properties of compounds reflect the nature of the interactions among its molecules. These interactions are determined by the structure of the molecule, including the constituent atoms and the distances and angles between them.
    • Physical Science - Chemical Reactions
      • Chemical reactions can take place in time periods ranging from the few femtoseconds (10 - 15 seconds) required for an atom to move a fraction of a chemical bond distance to geologic time scales of billions of years. Reaction rates depend on how often the reacting atoms and molecules encounter one another, on the temperature, and on the properties - including shape - of the reacting species.
    • Physical Science - Conservation of Energy and the Increase in Disorder
      • The total energy of the universe is constant. Energy can be transferred by collisions in chemical and nuclear reactions, by light waves and other radiations, and in many other ways. However, it can never be destroyed. As these transfers occur, the matter involved becomes steadily less organized.

    • Discussion: Setting the Stage

    It may be important to go over these terms before you have students start these activities: potential energy (stored energy), kinetic energy (energy of motion), initiate (to cause to begin), endothermic (energy is absorbed), exothermic (energy is released), concentration (the quantity of substance per defined amount; may be expressed in percent, ppm, etc.). Initial Discussion. There are many changes to materials in the world around us.

    • What kind of changes can occur?

      Chemical or physical.

    • What are the differences between these two kinds of changes?

      Physical changes don't change the composition of the substance. Chemical changes do change the composition; new substances are created.

    • What causes these changes to occur?

      These changes are caused by energy. For example, melting is a physical change caused by the addition of heat to a solid. Burning magnesium in air is a chemical change. In this case, heat provides the activation energy to initiate the reaction. Please note. You may or may not want to provide and/or discuss the answers to these questions. You may simply want to use them to stimulate and guide the students' thoughts prior to beginning the unit.

    Discussion: Formative Questions

    NA

    Discussion: Wrapping Up

    Energy is a huge part of the world around us. We use it daily in ways that we are acutely aware of and in ways that we rarely think about.

    • How do we use the energy created by chemical reactions?

      Batteries, candles, internal combustion engines, biological reactions, etc.

    Additional Background

    The energy of reactions is an important part of chemistry. If you need additional information, check the following website: http://apcentral.collegeboard.com/apc/members/courses/teachers_corner/49...

    Analysis

    Activity - Activation Energy

    • Describe the relationship between the potential and kinetic energy when new bonds were formed.

      When new bonds form, the kinetic energy decreases and the potential energy increases. However, the sum of the two remains the same.

    • The activation energy for a particular reaction (A) is low. The activation energy for a different reaction (B) is high. Which reaction is more likely to occur? Why?

      Reaction (A) with a low activation energy is more likely to occur because it is easier for a low amount of energy to be provided to initiate the reaction.

    • If a reaction happens very quickly, do you think it has a high activation energy or a low activation energy? Why?

      If a reaction happens quickly, it has a low activation energy. FINISH!

    • Notice that the sum of kinetic and potential energy is constant; if one goes up, the other goes down. Why is that?

      The sum of kinetic and potential energy is constant because energy is conserved. The energy can be transformed from one type to another, but it cannot be created or destroyed by the chemical reaction. Activity - Baggie Chemistry

    • In Collect Data I, did the reaction of the calcium chloride with water produce heat or consume heat?

      The reaction produced heat.

    • The reaction that occurs in Collect Data II is: CaCl2 (aq) + 2 NaHCO3 (aq) &rarrow; 2 NaCl (aq) + 2 CO2 (g) + Ca(OH)2 (aq). List the chemical changes that this reaction displayed.

      Gas was produced as well as heat was released.

    • Did the reaction in Collect Data II produce heat or consume heat?

      This reaction produced heat.

    • What gas was produced during this chemical reaction? Why do you think so?

      Carbon dioxide gas was produced. When a burning splint was exposed to this gas, it was extinguished. (Carbon dioxide is also used in fire extinguishers.)

    • From the Introduction, copy your initial list of observable changes that indicate a chemical reaction. What would you add to this list? What would you remove as changes that are not chemical in nature?

      Indicators that a reaction has occurred include the following: heat changes, light production, electricity production, precipitate production, gas production, and color changes of indicators. Check with students to see if they have additional items that don't belong (and may be physical changes) or are missing items.

    • If you were to measure the mass of the baggie and chemicals prior sealing the baggie, and again measured the mass after the chemical reaction was completed, would the mass of the baggie and its contents increase, decrease, or remain constant? Defend your answer.

      The masses prior to and after the experiment should be the same as mass is conserved during a chemical reaction. Please note that differences in mass may be due to gas escaping through a small leak, students neglecting to zero the scale, or using a scale that has had something spilled on it (and not cleaned up) between the two massings.

    Activity - Explosion

    • Explain, in your own words, what a homolytic fission is.

      A homolytic fission occurs when a chemical bond in a neutral molecule breaks creating two free fragments.

    • In the second model, was there enough energy to initiate homolytic fissions? What did you have to do to provide this energy?

      There was not enough energy to initiate a homolytic fission. In order to initiate a reaction, heat energy had to be provided.

    • Describe the critical point of an explosion.

      The critical point at which the first homolytic fission occurs. After this point is reached,the explosion will begin and additional energy is not required

    Activity - Making Heat

    • Recall that the concentration of the first solution was 3% hydrogen peroxide. What were the concentrations of the other two?

      The concentrations of the other two solutions were 1.5% and .75%.

    • Was the yeast - hydrogen peroxide reaction exothermic or endothermic?

      The yeast-hydrogen peroxide reaction was exothermic.

    • Compare the three temperature graphs with different concentrations of hydrogen peroxide. Were their shapes, their rate of temperature change and their maximum temperature change the same or different?

      The shapes of the graphs are similar because they are all exothermic reactions. The rate of changes are different. The rate of change is dependent on the concentration. The maximum temperature changes are also different. Once again, a

    • The chemical formula for hydrogen peroxide is HOOH. When it breaks down, it changes to water and oxygen: 2 HOOH &rarrow; 2 H2O + O2. In this reaction, yeast provides an enzyme, catalase, which acts as a catalyst. In the presence of oxygen, yeasts break sugars down into water and carbon dioxide, consuming oxygen and making some unintended hydrogen peroxide in the process, which must be broken down using catalase. So the yeast provides catalase that can break down the hydrogen peroxide you have added. Which molecules have more chemical energy stored in their bonds: 2HOOH, or 2 H2O + O2?

      Because it is an exothermic reaction, the hydrogen peroxide molecules have more energy. Energy is conserved during reactions: the sum of the energies in the water and oxygen plus the energy released to the environment must equal the energy in the hydrogen peroxide.

    • Explain why you think a change in concentration would change the heat generated by a chemical reaction.

      Higher concentrations means more molecules of reactant are available to react and release energy to the environment.

    Further Investigation

    Too often students only think of changes in heat energy when looking at chemical reactions. Another possible investigation would be to determine the activation energy of a light stick.