Determining the Energy of Combustion

Using Common Fuels

Objective:

As previously discussed energy is involved in chemical reactions when chemical bonds are broken and then reformed. Reactions can either be endothermic or exothermic based upon the amount of heat energy absorbed and released during the chemical reaction. We use a variety of fuels to provide heat energy for many industrial and household processes. Many of you use natural gas or propane to heat your homes, cook food, heat water, or dry clothes. Some of you may have done an experiment in an earlier science class where you burned a food (like nuts or chips) to determine the amount of energy in the food. This experiment does something very similar. To calculate the amount of heat energy absorbed by the water the following equation can be used ΔH = m · c · ΔT.

ΔH – heat absorbed by the water (common units of energy are joules)

m – mass of the water in grams

c – specific heat (specific heat of water = 4.18 J/g oC)

ΔT – change in temperature of water

Students should be able to determine the amount of energy released per gram and per mole of fuel. As a further extension looking at the grams of carbon dioxide produced compared to energy released.

Concepts Learned:

  • Energy released vs. energy absorbed
  • Calculating energy released by various fuels
  • Determining costs of various fuels for heat
  • Amount of carbon dioxide released per joule or kilojoule of heat released

Fuels for Use:

  • Butane
  • Methane
  • Ethanol (ethyl alcohol)
  • Isopropol alcohol (2-propanol)

Procedures:

For this experiment you will be trying to determine the amount of heat energy released by burning various fuels. In order to do this it will be necessary to determine how much energy water absorbs when heated by the fuels. To do this set up the ring stand and iron ring with a clay triangle on the iron ring. Measure the mass of the water to be heated, between 100 and 150 ml. Record the initial temperature of the water. You will need to determine the mass of fuel used in this experiment, by measuring the initial and final mass of fuel. Heat the water for 2 minutes and then determine the water’s temperature after heating. Repeat the above steps with each of the fuels provided.

Results: Show all of your work.

  1. Calculate the amount of heat absorbed by the water.
  2. How many grams of fuel were used?
  3. Calculate the amount of heat absorbed by the water per gram of fuel used.
  4. Write the chemical formula for each fuel used.
  5. Determine the gram formula mass of each fuel used.
  6. Calculate the number of moles of fuel used.
  7. Calculate the amount of heat absorbed by the water per mole of fuel used.
  8. Construct a graph of heat absorbed per mole versus the gram formula mass.

Conclusions:

  1. What is the relationship between the fuel’s gram formula mass and the heat absorbed per mole?
  2. If propane was used as a fuel how many joules of heat per mole of propane might be released? Hint: knowing the formula and gram formula mass of propane will help.
  3. How does the amount of heat absorbed by the water compare to the heat released by the fuel? Explain.
  4. Write a balanced equation for the combustion of each fuel. Assume the only products are carbon dioxide gas and water vapor.
  5. Describe any relationship that exists between the number of carbon atoms and the amount of energy released by the fuels.
  6. Besides heat energy going to the water from the fuel where else might some of the fuel’s heat energy have been transferred? Describe how this might have affected the amount of heat you calculated for the fuels. Would the calculated value have been greater than, less than what you calculated? Explain.
  7. Looking at your parent’s natural gas bill or on the internet what is the cost of natural gas. Include not only the dollar amount but also the units for the amount of natural gas used. If your family uses propane instead of natural gas, indicate that. If you use neither natural gas or propane at home check out my blog.

Further Investigation:

Description of what other question(s) were raised during the experiment that could be investigated with new experiments. Also a list of how the current experiment could be modified to eliminate or minimize the sources of error that occurred during the experiment. Be specific.

Something Extra:

One concern over the use of carbon based fuels such as fossil fuels is the amount of carbon dioxide released when these fuels are burned. For each of the fuels used in this experiment calculate the grams of carbon dioxide produced. Assume the there is an excess amount of oxygen gas used to burn the fuels. Sections 9.2 and 9.3 in your textbook will be helpful. This part of the lab report is extra credit.