Energy in Chemical Changes

Abstract

In this exercise, students explore the heat energy in one hydrocarbon (paraffin) and extend their understandings to other fuels. They apply their understanding to an analysis of how heat energy is provided to their homes.

Unit of Study: Mass and Energy in Chemical and Physical Changes

TEKS

(6) Science concepts. The student knows the impact of energy transformations in everyday life. The student is expected to:

(D) Investigate and compare economic and environmental impacts of using various energy sources such as rechargeable or disposable batteries and solar cells

(The choice of fuel for A 380 economical and weight efficient reasons)

Goals

• Explain why mass is conserved in physical and chemical changes
• Describe molecular changes in terms of properties of reactants and products
• Describe energy transformations involved in physical and chemical changes and contrast their relative magnitudes
• Recognize and explain the limitations of measuring devices when developing questions or problems and conducting investigations that can be answered empirically

Key Concepts

Chemical change

Conservation of energy

Physical change

Resources

Balance

Calculator (per group)

Can (12 ounce soft drink can, empty with tabs still attached, per group)

Eye protection (per student)

Glass stirring rods (2 per group)

Graduated cylinder (100 ml)

Ice (to chill water, per group)

Index card (3 x 5, per group)

Large container to hold the chilled water (per group)

Matches (per group)

Paraffin candle (per group)

Ring stand w/ ring (per group)

Thermometer (per group)

Water

Teacher Resource

Blakeslee, Theron, and Heather Luoto. Unit 3 Lesson 8 Student Pages. Teacher-made material. Lansing, MI: Michigan Department of Treasury, 2002.

Blakeslee, Theron et al. Chemistry That Applies. Lansing, MI: Michigan Department of Education, 1995.

Texley, Juliana, and Heather Luoto. Grade 10 Unit 3 Teacher Background. Teacher-made material. Lansing, MI: Michigan Department of Treasury, 2002.

Safety Precautions

Eye protection and aprons are required.

Procedure

1. To generate a class discussion, ask students the following question: “What is a fuel?” Ask students to record their answers on their Student Pages. Then help them to understand: “Fuels provide energy when they are burned in the presence of oxygen. These combustion reactions are exothermic and can be used to heat our homes. Most fuels that we use for heating our homes are hydrocarbons. Hydrocarbons are compounds made of only two kinds of atoms, carbon and hydrogen. Methane, propane, and butane are all examples of hydrocarbons. Paraffin, a hydrocarbon that is used to make candles, also gives off heat when it is burned. In the next activity you will determine the quantity of thermal energy released when paraffin burns and compare it to other hydrocarbons.”

2. Ask the students to generate a list of fuels on the board. Ask them to copy the list onto their Student Pages.

3. Briefly explain to students what a hydrocarbon is and then circle the entire hydrocarbon fuels on the student-generated list. Ask students to do the same to the lists in their Student Pages. Note: A hydrocarbon is a compound formed of hydrogen and carbon. Many of these products are those we call “fossil fuels” because they are or were the products of photosynthesis. The carbon-carbon bonds in hydrocarbons store energy that is released when the substance is oxidized (burned). Sugars, fats, starches, and fuels ending in “-ane” are hydrocarbons.

4. Pair students into groups and ask them to perform the Heat of Combustion Activity in their Student Pages by following the directions given. Note: If your students have not covered the concept of a mole, this exercise can be done as “heat per gram.” Even if you have covered this concept in class, you may need to remind students how to determine the molar mass of a compound by using the masses listed on the periodic table. You may also need to explain to students that a mole is an amount of substance equal to 6.02 x 1023 units, where the units may be atoms, molecules or formula units depending on the type of substance you have.

5. Help students through the calculations. Question #13: Calculation…[We know that it takes 4.184 J to raise the temperature of 1 g of water 1°C. However, we have 20 times more water and 5 times greater rise in temperature as compared to this standard. Therefore we need 20 x 5, or 100 times more thermal energy to accomplish the task. We need 100 x 4.184 J or 418.4 J to heat 20 g of water 5.0 °C.]

6. When all students complete the activity and calculations, record their experimentally determined molar heats of combustion on the board. Compare and discuss the values as a class. Then ask students to use the class average for the molar heat of combustion when answering the analysis/conclusion questions.

7. Place the transparency of the Heats of Combustion table (in Teacher Background file) on the overhead while students answer the analysis/conclusion questions. When students complete the questions, discuss them as a class. Note: If your students are not comfortable with the concept of a mole, convert the Heat of Combustion of Butane to grams. Students should determine that the hydrocarbons with a higher molar heat of combustion would theoretically make better fuels. Students should realize that the energy used to heat the water was stored as chemical energy in the paraffin. As students examine the Heats of Combustion Table, they should notice that as the number of carbons and hydrogens in the hydrocarbon compounds increase, the molar heats of combustion increase.

8. Go over the calculations and the discussion questions carefully. Question #18: “Which is the better fuel, paraffin or butane?” [Butane.] Question #19: “Where was the energy stored that was used to heat the water?” [In the bonds of the paraffin molecule.] Question #20: “Explain the limitations of the measuring devices that you used in this activity.” [While the thermometer can only measure to a fraction of a degree, the main limitation was loss of heat to the surrounding air since not all heat rose to raise the temperature of the water.] Students should understand that the precision of the scale and thermometer they used influences their data and that some heat from the candle was lost to the surroundings.

9. Next, ask students to do the Energy Transformations and Going Further activity as homework. Ask students to describe their diagrams at the start of the next class period. They should realize that the greatest amount of energy is at the beginning of the sequence, and that energy is gradually dissipated.

10. Summarize by telling students: “In this lesson you determined the heat of combustion of paraffin and compared and contrasted it to the heats of combustion of other hydrocarbons. In the next lesson you will analyze properties of common household materials in terms of their risk/benefit balance.”

Assessment

Students describe the energy transformations that are involved in heating their home hot water supply and compare and contrast the relative magnitudes of energy released from different hydrocarbons.

Application outside the project

Fuels have economic and social implications for families. In some countries where coal is used to heat many homes (such as China) the cost in air pollution outweighs economic advantages.