Determining Energy Efficiency: A Lab Activity
In this introductory chemistry lab activity, students investigate the efficiency of heating water by different methods. They heat a known mass of water using a Bunsen burner, a hot plate, and a microwave oven. In order to determine efficiency, students must compare the energy absorbed by the water to the energy expended to heat it. Students use standard enthalpies of formation to calculate the amount of heat released in the combustion of methane while heating with a Bunsen burning. The power rating of the hot plate and microwave are used to calculate the energy expended while heating with these devices. Once students have calculated the efficiency of these different methods of heating, they are asked to determine both the economic and environmental cost of each method and to make a recommendation for the most energy efficient way to heat substances in their school lab.
- calculate the amount of energy absorbed by a substance during heating;
- use standard enthalpies of formation to calculate the amount of heat released in a chemical reaction;
- calculate efficiency of heating using different devices;
- assess the economic and environmental cost of different methods of heating.
Context for Use
The activity can be completed in a single two-hour laboratory period. Students may need additional time to do research into the method of electricity generation in their area.
Description and Teaching Materials
In this activity, students heat a known mass of water using a Bunsen burner, an electric hot plate, and a microwave oven. They record the temperature change of the water and then calculate the heat absorbed by the water using the specific heat capacity of water, the mass of the water, and its temperature change.
To calculate the efficiency of these different methods of heating, students must compare the amount of energy absorbed by the water to the amount of energy expended by the heating device. For the electric hot plate and microwave oven, the energy expended can be calculated from the power rating of the device and the duration of the heating.
In order to calculate the energy expended by the Bunsen burner, the energy released from the combustion of natural gas must be determined. If it is assumed that natural gas is mostly methane, the heat of combustion of methane can be calculated from standard enthalpies of formation. In order to determine the total energy released while heating with the Bunsen burner, the total number of moles of methane consumed during the heating must be determined. Using the flow rate through the burner and the time period of heating, the total volume of methane consumed during heating can be calculated. Using this volume and the ideal gas law, the total moles of methane consumed is determined. With this data and the heat of combustion of methane, the total heat released during heating with the Bunsen burner can be calculated. Students can now calculate the efficiency of heating by each method by comparing the energy absorbed by the water to the energy expended by the heating device.
Once the efficiency of each method known, students can use the cost of electricity and natural gas to calculate the economic cost of heating by each method. In order to determine the environmental cost of each heating method, students are asked to do research to determine the method of electricity generation in their region. Finally, students are asked to make a recommendation for the best method of heating substances in their laboratory by assessing both economic and environmental costs. Energy Efficiency Report Sheet (Microsoft Word 2007 (.docx) 21kB Apr15 16)
Assessing the environment cost of electricity generation is a huge task, and instructors may wish to narrow the focus of this question. Every method of electricity generation impacts the environment. These impacts include climate change, acid rain production, groundwater contamination, smog production, land degradation, water use, animal death...
Students can determine the fuel mix and air emissions rates of electricity generation in their region using the EPAs Power Profiler https://www.epa.gov/energy/power-profiler
A more in depth discussion of the environmental impacts of each method of energy production can be found at the U.S. Energy Information Administration's Energy and the Environment website http://www.eia.gov/energyexplained/index.cfm?page=environment_home
The National Academy of Sciences provides information on the environmental, security, and sustainability costs of energy production at http://needtoknow.nas.edu/energy/energy-costs/