Lesson Heat Transfer:
From Hot to Not

Learning Objectives

After this lesson, students should be able to:

• Describe that specific heat capacity is the amount of energy an object can absorb before changing in temperature by one unit temperature.
• Explain how heat capacity, heat of reaction and heat transfer can be applied in engineering to understand and control chemical processes and physical systems.
• Identify exothermic reactions as heat generating, and endothermic reactions as heat consuming.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

Common Core State Standards - Math

International Technology and Engineering Educators Association - Technology

State Standards

Worksheets and Attachments

Pre-Req Knowledge

Algebra: Students need to be aware of basic algebraic manipulation of equations and substitution techniques.

Chemistry: Students should be aware that chemicals interact in reactions that change the chemical and/or physical properties of a system.

Physical Science: Students should be familiar with the concept of energy, that it can be exchanged, and that it comes in different forms.

Introduction/Motivation

Some of the most interesting demonstrations in science and engineering involve energy and heat. Think of a balloon full of hydrogen being ignited with a match, a cold-weather hand warmer releasing warmth, or salt melting ice. Heat exchanges such as these are only a small sample of the broad applications in engineering and science of heat transfer. The explosive ignition of the fuel in a rocket that provides energy for lift-off, the tough material on the surface of the space shuttle, the lining inside of a high-temperature reactor, or even the chemical processes that go into making ice-cream are all drawing from our knowledge of heat transfer.

Engineers use the concept of heat of reaction to know how much energy we can expect from burning rocket fuel, or how much energy will be absorbed by reactions that make everything from plastic to cookies. Knowledge of heat capacity allows engineers to predict how much energy a material can hold before reaching a certain temperature, and then can design a product accordingly.

What is heat? It may seem hard to describe exactly what heat is. Heat could be described as hotness by some people, but that is already described by temperature. For scientists and engineers, heat is simply a term referring to thermal energy transferred between two bodies. Heat can also be energy released in a reaction. There are many applications for knowledge of heat and heat transfer. As we will see, engineers use knowledge of heat of reaction to predict how much energy will be produced in a chemical system, which is important for keeping the reactor safe and efficient. Beyond chemical reactions, heat is exchanged for physical reactions, too. Some examples include dissolving one chemical in another, or phase changes between solid, liquid and gas.

Heat of reaction is the amount of heat energy generated or absorbed for a given physical or chemical reaction. Reactions can either give off heat or they can absorb heat. When something gives off heat, it is called exothermic. Examples of exothermic reactions are easy to name. They include burning wood, lighting a hydrogen balloon, and ice freezing. On the other hand, endothermic reactions are reactions that absorb heat. Baking a cake, boiling water, and dissolving certain salts in water are examples of endothermic reactions. Heat of reaction is also often called the enthalpy of reaction. Heat/enthalpy of solution is another important concept. This is the same idea, but instead of chemical reactions, it refers to dissolving one chemical in another (as a simple definition). Other terms to keep in mind are heat of vaporization and heat of fusion. These describe the energy inputs and outputs in boiling and freezing.

Another important concept is heat capacity. A chemical engineer needs to know how much a given amount of energy will raise the temperature of the reaction components and the reactor itself. An aerospace engineer needs to understand the tolerances of a spaceship's building materials so that it can successfully survive the extreme temperatures of space. Even in cooking it is important to understand heat capacity in order to determine how long and how hot to cook a turkey, for instance. Heat capacity can be thought of as how much energy must be put into something before it will get one degree hotter. This is different for different substances. The heat capacity for different substances depends on complex atomic and molecular interactions, such as the way in which atoms are connected to one another, the atomic bond strength, and how quickly the atoms transfer energy among themselves. Refer to the associated activity Hot Potato, Cool Foil to have students investigate different material properties and the basic principles of heat transfer using calorimeters to determine the specific heat of several substances. 

Finally, how heat transfers between two systems is an important part of engineering. The knowledge of how quickly and how much heat will be conducted is important for controlling reactions as well as keeping important materials and components in a device within operating conditions. For example, a chemical engineer might need to know how much heat will be transferred from the outside of a reactor to the surrounding air, or a computer engineer would require a certain type and size of cooling fins and fans to keep the processor from overheating.

Lesson Background and Concepts for Teachers

Heat of Reaction and More!

What causes an energy change in a reaction, dissolution or phase change? It has to do with the rearrangement of atoms and molecules. For a chemical reaction, energy is either absorbed or released to form chemical bonds between atoms. If the new bond arrangement is more stable than the original arrangement, then is less energetic than before and releases the extra energy it does not need. Thus, it is an exothermic reaction.

If the bond is less stable and requires more energy to exist, it absorbs energy from its surroundings until it has stabilized. This is an endothermic reaction. The same principle applies for dissolution. When you dissolve table salt NaCl in water, you break it into sodium and chloride ions surrounding molecules. Since this requires an input of energy, it is labeled endothermic. Some salts are exothermic and release energy in the same situation.

Finally, boiling water is an endothermic reaction because the water molecules need to absorb energy to break their inter-molecular interaction with each other and become a gas. That is why we boil water on a hot stove!

The amount of heat transferred in a given reaction can be predicted if we know certain things about what is going on. First, we have to identify if it is a chemical or a physical process. Is there a chemical change, such as burning wood to get ash and CO2? Or is it physical, such as melting ice to get water? After we have identified the type of reaction, we can look up the standard heat of reaction. Negative heats of reaction are exothermic, whereas endothermic reactions are positive. These standards are on a per mole basis. Therefore, the heat of reaction is the amount of heat produced for the number of moles of product in the base chemical equation.

Heat Transfer

Heat transfer between two systems is governed by a relatively simple equation that relates energy exchanged to the heat capacity and quantity of the substance. The equation is as follows: Q=mC_P ∆T

In this equation, Q refers to the amount of energy transferred, m is the mass of the object in question, C_P is its heat capacity, and ∆ T is the change in temperature of the substance between the starting temperature before heat is transferred and the temperature after heat transferred. Students can illustrate this idea with the Counting Calories associated activity by constructing constant pressure calorimeters to determine the heat of solution of potassium chloride in water.

We can set the heat that exits the system equal to the heat entering the surroundings, which can be the air surrounding an object, or something the object is touching. The idea of energy conservation is known as the first law of thermodynamics. Another way of saying this is that energy can neither be created nor destroyed, just changed into different forms. So if energy is leaving the system, it must be entering another system because it cannot disappear.

In the case of two substances/objects originally at different temperatures which are brought into contact (e.g., two fluids mixed together, hot ball bearings quenched in cool water), the energy equation can be used to set up a mathematical model to determine the energy change in one substance/object when the energy change in the other is known. For instance, if a hot cube of aluminum is submerged in cool water, the energy equation can be utilized to show that the energy lost be the aluminum is gained by the water.

Q_{lost by Al} = Q_{gained by water}

m_Al C_p,Al(T_f,Al - T_i,Al) = m_w C_p,w (T_f,w - T_i,w)

If the aluminum cube is left in the water for a long enough period, the water and aluminum will reach the same temperature. Assuming that we know the masses, specific heats, and initial temperatures of the water and aluminum, the energy balance above can be rearranged to solve for the final temperature. This final temperature can then be used to solve for the energy change of the water and aluminum.

T_f,Al = T_f,w

T_f = (m_Al C_p,AlT_i,Al - m_w C_p,wT_i,w)/(m_AlC_p,Al - m_wC_p,w)

Associated Activities

• Counting Calories - Students discover the basics of heat transfer by constructing constant pressure calorimeters to determine the heat of solution of potassium chloride in water.
• Hot Potato, Cool Foil - Students explore material properties and the basic principles of heat transfer using calorimeters to determine the specific heat of several substances.

Lesson Closure

Heat transfer is an extremely important aspect of nearly all fields of engineering. Whether it be cooling fins on a computer component or the cooling system in a car's engine, engineers apply their knowledge of heat transfer in many situations. Heat capacity describes how much heat a substance can hold when increased by one degree of temperature. This is important in applications such as industrial-scale cooking, chemical reactors and material tolerances for machines, such as cars and spacecraft heat shielding. Heats of reaction, solution and phase changes describe the energy absorbed or released in the rearrangement of atoms and molecules in their interactions. Applications of this include everything from formulating powerful rocket fuels to creating an effective road de-icer. Finally, understanding that reactions such as burning rocket fuel are considered exothermic because they release energy, and baking a cake or boiling water are endothermic because they require energy input to be considered "complete."

Vocabulary/Definitions

endothermic : A process or reaction which absorbs energy.

enthalpy: The enthalpy change is the amount of heat released or absorbed when a chemical reaction occurs at constant pressure.

exothermic: A process or reaction which releases energy.

heat: Energy transferred between two systems as a result of a temperature difference.

heat capacity: The amount of energy transfer required to raise or lower a given amount of a substance by one unit temperature at a constant pressure

heat of reaction: The amount of energy released or absorbed for a given amount of reacting chemicals.

Assessment

Pre-Lesson Assessment

Discussion: Gather and discuss student ideas.

• What is energy? (Answer: The capacity of a system for doing work; kinetic energy, potential energy, electrical energy, etc.)
• What is heat? (Answer: Energy transferred between two systems as a result of a temperature difference.)
• What is the difference between heat and energy? (Answer: Heat is a form of energy; it has the same units but is specifically the transferable energy across a temperature gradient.)

Post-Introduction Assessment

Everyday Examples: As a class, think of examples of heat transfer and heat of reaction and heat of solution in everyday life and list them on the board. (Answers may include: ovens, road salt for ice melt, furnaces, etc.)

Lesson Summary Assessment

Applying Concepts to Problem Solving: Have students work in small groups of 2-3 to complete the Heat Transfer Problem Sheet. Go through the solutions as a class, allowing groups to show their solutions on the board if desired. The problems ask students to use the concepts they learn in this lesson to develop computational models to calculate change in energy in the form of heat. Solutions are available for the teacher's use in the Heat Transfer Problem Sheet Answer Key.

Brainstorming: Put students in small groups to brainstorm ideas about how knowledge of heat of reaction and heat transfer might be useful. This should include applications for everyday life, industry, science/engineering and at least one other category (cooking, fixing cars, etc).

• Have students present their ideas to the class.
• Have students think of ways they would explain the causes behind heat of reaction, dissolution and phase changes to someone else. They should come up with good analogies, or even act out the roles of the atoms for these situations.

References

Copyright

Contributors

Supporting Program

Acknowledgements

The contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation GK-12 grant no. 0338326. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.