TEKS 8.10 C

Hot Stuff

TAKS Objective 3 – The student will demonstrate an understanding of the structures and properties of matter.

Learned Science Concepts:

  • Matter is composed of atoms.
  • Substances have chemical and physical properties.
  • Complex interactions occur between matter and energy.

TEKS Science Concepts 8.10

The student knows that complex interactions occur between matter and energy. The student is expected to:

(C)identify and demonstrate that loss or gain of heat energy occurs during exothermic and endothermic chemical reactions.

Overview

The students will understand that breaking chemical bonds always involves energy. When bonds are forming energy is released. An endothermic or exothermic reaction is classified by comparing the amount of energy need to break the chemical bonds of reactants to the amount of energy released when the bonds of the products are formed. If less energy is needed to break the bonds of the reactants than to make the products, an exothermic reaction occurs. The opposite is true when an endothermic reaction occurs. Though this TEKS focuses on chemical changes, physical properties and physical changes have been included in the activities. It is important for students to understand that endothermic and exothermic reactions are also involved in physical changes. Several activities and assessments compare and differentiate chemical and physical changes.

Instructional Strategies

The activities in this TEKS focus on the concepts of chemical change, exothermic, and endothermic reactions. Students will make use of their senses to observe a red to yellow color change as chemicals are combined, cold and hot mixtures, and the production of gas as evidenced by fizz and bag expansion. Hot/cold packs, making ice cream and various activities will be used to examine endothermic and exothermic reactions through feel.

Learning Objectives

  1. The student will identify endothermic and exothermic reactions associated with chemical changes.
  1. The student will be able to explain how a substance losses or gains heat during a chemical reaction.
  2. The student will identify physical and chemical changes associated with endothermic and exothermic reactions.

For Teacher’s Eyes Only

This section concentrates on energy changes in chemical and physical processes. Chemical reactions usually involve the absorption or release of thermal energy. The energy available from a chemical reaction is chemical potential energy. If a reaction releases energy, it is called an exothermic reaction; one that takes in heat is called endothermic.

System and Surroundings

When energy is transferred, it is important to differentiate between what is losing the energy and what is gaining the energy. By definition, the system is enclosed within the defined boundaries where we are focusing our attention. Usually, this includes reactants and products in a chemical reaction, or a single substance in the case of a physical change like melting or boiling. The surroundings is everything else in the universe which is not included in the system. When we are looking at a physical change or a chemical reaction, the surroundings mean the container which holds the reactants and products, any solvent which is not considered a reactant or product, and the rest of the universe. For example, if aqueous solutions of hydrochloric acid and sodium hydroxide are mixed, thermal energy is released by the system – in this case, the reactants. The surroundings – the water, the container holding the solutions, the air in the room, the rest of the country, etc. absorb the energy. Any heat lost by the system must be gained by the surroundings, and vice-versa.

Internal Energy

The internal energy of a system is defined by all possible forms of energy in the system. This includes all the vibrational and rotational motion of molecules and atoms as well as all the kinetic and potential energy each particle possesses. Since it is impossible to measure all of this energy, it is impossible to know the total internal energy of a system. As it turns out, it isn't the absolute internal energy of a system which is important. The important aspect of a system’s energy is the change in internal energy when a system changes from one state to another. And this can be determined. There are usually two types of transformations which dictate a change from state 1 to state 2:

A change in physical state (liquid to gas, solid to liquid, etc.). Energy is absorbed or released by these processes.

A chemical reaction, in which the reactants represent the initial state and the products represent the final state. Chemical reactions are accompanied by a gain or loss of energy.

It is important to recognize that internal energy is a state function: its value depends only on the state of the system, not on how the system arrived at that state. There are two ways to change the internal energy of a system: add or remove heat, or perform or extract work from the system.

Hot and Cold Packs

Hot and cold packs demonstrate endothermic and exothermic processes. A coldpack can be stored at room temperature, but within seconds can become cold enough toice an injury. Hot packs meanwhile are often used as hand-warmers in the winter. Youcan carry them in your pocket until you are ready to activate them, and they will then staywarm for 15 minutes to half an hour.

A typical cold pack consists of two chambers, one containing water and the othercontaining a chemical like ammonium nitrate. When you break the bag, the twochambers combine and the ammonium nitrate dissolves in the water. This generates achemical reaction. This particular reaction is endothermic, which means that heat isabsorbed. The absorption of heat causes the pack to get cold.A hot pack works in the same way, except it uses a chemical that reacts withwater exothermically, which means that heat is given off. Calcium chloride andmagnesium sulfate are typical choices for this chemical. Both hot packs and cold packswill last for about twenty minutes on average.

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Lesson Layout


Student Misconceptions

Misconception

Chemical reactions always generate heat.

Science Concept

Chemical reactions can generate heat or absorb heat.

Rebuild Concept

Use hot and cold pack chemical reactions and let students feel how the temperature of the substance changes due to the chemical reaction.

Misconception

Heat can be lost, destroyed, or just disappear.

Science Concept

Heat energy is transferred from one object to another or transformed into another form of energy, but it never just disappears.

Rebuild Concept

With each chemical reaction discuss where the energy comes from and where it goes. Discuss the kinds of energy that are involved. Make students aware of the conservation of energy.

Misconception

Color change is always a physical property

Science Concept

Color is a physical property of a substance. Color change, however, is usually the result of a chemical change.

Rebuild Concept

Describe and/or show chemical formulas of what is occurring when a substance changes color.

Student Prior Knowledge

TEKS 6.8 (B) analyze energy changes that accompany chemical reactions such as those occurring in heat packs, cold packs, and glow sticks to classify them as endothermic and exothermic reactions.

(TEKS 8.9) The students know that substances have chemical and physical properties should be introduced prior to the study of exothermic and endothermic chemical reactions.

5 E’s

Engage

Engage 1

Let students squeeze warm-packs and cold-packs from athletic stores. This is a good kinesthetic activity that allows students to feel heat being generated or absorbed.

Engage 2

Provide each student with a rubber band. Ask students to hold the rubber band to their upper lips. Does the rubber band feel warm or cool? Ask the students to stretch the rubber band out and touch it to their upper lips. Does the rubber band feel warmer or cooler than before?

Engage 3

Provide each lab group with a light stick. Examine stick before and after breaking it. List as many observations as you can. Group and Label them as physical and chemical changes/properties.

Explore

Activity: Hot or Not…Adapted from © Project LIFE 1998.

Materials:

Phenol red solution

Sodium bicarbonate powder (baking soda)

Calcium chloride powder

Quart-size freezer bags

Plastic spoons

Goggles

Gloves

Medicine measuring cups

Toothpicks

Procedure

  1. Add one spoon full of sodium bicarbonate and 2 spoons full of calcium chloride to a plastic Ziploc freezer bag.
  2. Measure 10 ml phenol red solution in a medicine measuring cup and place the cup inside the Ziploc freezer bag. Be careful not to spill the phenol red solution. Squeeze most of the air out of the bag, and then seal the bag.
  3. While the bag is still sealed, spill the contents of the phenol red solution inside the Ziploc freezer bag. Record observations.
  4. The Chemical Reaction. The chemical reaction that is occurring is:

CaCl2 / + / 2NaHCO3 / → / 2NaCl / + / CaCO3 / + / H2CO3
Calcium Chloride / Sodium Bicarbonate / Sodium Chloride / Calcium Carbonate / Carbonic Acid

5. The carbonic acid is unstable and breaks down according to the reaction

H2CO3 / → / H2O / + / CO2
Carbonic Acid / Water / Carbon Dioxide

Questions

  1. Name physical properties of a substance such as:its state (solid, liquid, gas), color, clarity, density, boiling point, solubility, crystal structure, and odor.
  2. Record 3 physical properties for each of the following chemicals.

Substance / Property
Calcium chloride / White, solid brittle, chalky, has no odor, large particles
Baking soda / White, powdery, smooth, has an odor, dry and won’t clump, tiny particles
Phenol red solution / Red, liquid, odor, clear, flows easily
  1. Describe what happens when the baking soda and calcium chloride are mixed together. Nothing happens.
  2. Describe what happens when the phenol red solution comes in contact with the baking soda and calcium chloride. Powders dissolve, turns into a yellow solution, fizzes, gets hot, the bags gets puffy, and there is a slight sulfur smell.
  3. What are evidences of chemical change? Change in color, heat or light produced, bubbles of gas produced, solid formed in liquid, smell change, texture change.
  4. What is an exothermic reaction? An exothermic reaction is one in which energy is released. In this experiment heat is released.
  5. Give 3 examples of everyday exothermic reactions.A burning candle, combustion when driving a car, compost decay, or cellular respiration.

Explain

Debrief the results of the lab, “Hot or Not.” Allow students to discuss physical change, chemical reactions, endothermic and exothermic reactions.

Elaborate

Elaboration 1

Activity: Ice Ice Baby

Materials

Sodium bicarbonate

Vinegar

Ziploc freezer bag

Plastic spoon

Medicine measuring cup

Procedure

  1. Add one spoon full of sodium bicarbonate to a plastic freezer bag. Record observations.
  1. Measure 10 ml of vinegar in a medicine measuring cup and place the cup inside the Ziploc freezer bag. Be careful not to spill the vinegar. Squeeze most of the air out of the bag, and then seal the bag.
  2. While the bag is still sealed, spill the vinegar in the medicine measuring cup inside the Ziploc freezer bag. Record observations.

Questions

  1. What is an endothermic reaction? An endothermic reaction is one in which energy is absorbed. In this experiment heat is absorbed and the bag becomes cold to touch.
  1. Give 3 examples of everyday endothermic reactions.Photosynthesis, photography, cold packs, setting of plaster of Paris.

The bag will feel cold to touch when sodium bicarbonate is mixed with vinegar. This chemical reaction is called an endothermic reaction because heat is taken in as it is absorbed into the reactant (sodium bicarbonate). In this experiment, the temperature of bag drops.

The students may also design their own experiment to determine which combination of chemicals produces an endothermic reaction. Provide students with the following substances:sodium bicarbonate, water, phenol red, and calcium chloride. When sodium bicarbonate is mixed with a liquid, an endothermic process occurs.

Elaboration #2

Activity: Full of energy

Materials

50 mL beaker

Thermometer

Goggles

Gloves

glass stirring rod

plastic spoon

copper (II) chloride dehydrate

15 cm square of aluminum foil

 Always use caution when working with chemicals. Wear goggles and check how to discard the resulting solutions.

Procedure

Record observations in the table as your perform the experiment.

  1. Place 25 ml of tap water in a beaker. Take the temperature of the water.
  1. Add 5 milliliters of copper (II) chloride dehydrate crystals to the beaker. Do NOT stir the solution. Take the temperature of the solution.
  2. Stir the mixture with the glass rod until the crystals dissolve. Take the temperature of the solution.
  3. Crumple the 15 cm square of aluminum foil into a ball and carefully place it in the solution. DO NOT LOOK DOWN INTO THE BEAKER but rather make observations from the side of the beaker.
  4. Observe the mixture for 10 minutes and record any change in temperature.
  5. Pour liquids down the sink with water. Discard solids in a chemical waste container provided by the teacher. Wash your hands thoroughly.

Procedure / Observations
Examine dry Copper (II) chloride dehydrate
Add 5 ml of Copper (II) chloride dehydrate crystals to 25 ml of tap water
Stir mixture until crystals dissolve
Carefully place aluminum foil ball into solution
Monitor temperature for 10 minutes
  1. Question:Is this an endothermic or exothermic reaction?Exothermic Why? Heat is given off.

Elaboration #3

Activity: I scream, you scream, we all scream, “Ice Cream”

Materials

½ cup milk or cream

1 teaspoon vanilla

1 Tablespoon sugar

4 cups crushed ice

4 Tablespoons salt

Thermometer

Quart size freezer bag (2)

Gallon size freezer bag

Use the following materials and procedures to continue investigating endothermic and exothermic reactions and processes. In this laboratory activity, you will explore energy changes that occur during the process of making ice cream.

Procedure

  1. Mix milk, vanilla, and sugar together in a quart sized Ziploc freezer bag. Remove air and zip closed.
  2. Place the bag in a second quart sized Ziploc freezer bag to provide extra protection for the ice cream in case the seal accidentally breaks.
  3. Place the double-bagged ice cream mixture into a gallon size Ziploc freezer bag. Fill the gallon baggie ½ full with ice and layer in 6 tablespoons of salt. Remove extra air from the gallon bag and seal it closed.
  4. Use your hands to carefully shake or massage the gallon size Ziploc freezer bag for about 2 minutes. It is possible to wrap the freezer bag in a towel or use gloves if the mixture is too cold for the students to handle.
  5. Crack open the seal of the gallon bag, slide the thermometer in and take the temperature of the outside of the ice cream baggie. Record the temperature in the table. Remove the thermometer and reseal the gallon bag.
  6. Continue shaking and massaging the gallon size Ziploc freezer bag. Repeat step #5 every two minutes for approximately 15 minutes.

Minutes / Temperature of the OUTSIDE of ice cream baggie
0
2
4
6
8

Complete the following questions or activities.

  1. Based on you observations, Is the ice cream mixture an endothermic or exothermic process? What about the salt & ice mixture? Exothermic for ice cream, endothermic for salt-ice-water.
  1. Create a graph of the results.
  1. Explain how heat is gained or lost in this experiment. Heat is transferred OUT of the ice cream (exothermic) so the left over ice cream gets colder as it heats up the surroundings. The ice-salt-water mixture is gaining heat from the ice cream, causing the ice to melt and ice cream to cool off and freeze. Since the salt-ice-water mixture is causing the heat to decrease in the surroundings, it is endothermic.

Evaluation

Use the following table to (1) classify the following as examples of exothermic or endothermic reactions and (2) identify physical and chemical changes.

Type Change?
Physical/Chemical / Description / Endothermic or Exothermic?
Physical / Melt ice cubes / Endothermic
Chemical / Rusting iron / Exothermic
Physical / Condensation of rain / Exothermic
Physical / Evaporation of water / Endothermic
Chemical / Combining H2 and O / Endothermic
Physical / Freeze ice cubes / Exothermic
Chemical / Candle flame / Exothermic
Chemical / Cold packs / Endothermic
Chemical / Combustion / Exothermic
Chemical / Setting of plaster of Paris / Exothermic
Chemical / Photosynthesis / Endothermic

Varying amounts of Compounds A and B were added to separate containers and then dissolved in 500 ml of water. As each quantity of the compound was added to each of the containers, a temperature was recorded. Use the graph to answering the following questions.

  1. What is the independent (manipulated) variable? Amount of the compound
  2. What is the dependent (responding) variable? Temperature of the solution
  3. According to the graph, is the chemical reaction for Compound A endothermic or exothermic? Endothermic
  4. According to the graph, is the chemical reaction for Compound B endothermic or exothermic? Exothermic
  5. Predict what would happen if 30 milliliters of Compound A were dissolved in 500 ml of water. The temperature would remain at 20° Celsius
  6. Predict what would happen if 50 milliliters of Compound B were dissolved in 500 ml of water. The temperature would remain at 45° Celsius
  1. Assuming there is no chemical interaction between Compound A and Compound B, predict the final temperature of a mixture containing 15 milliliters of Compound A and 5 milliliters of Compound B. The temperature would be 24° Celsius. One teaspoon of Compound B does not change the temperature.
  2. List at least two variables that should be controlled in this experiment. Answers will vary.


Hot or Not