15 May 2016

Energy Content of Fuels--Which fuel is "Best?"

In-Class Activity

Submitted by Adam R. Johnson, Harvey Mudd College
Categories
Prerequisites: 
Corequisites: 
Course Level: 
Topics Covered: 
Subdiscipline: 
Description: 

There are many factors to consider when choosing a fuel. In this exercise, your group will work with a set of three different potential fuels and evaluate their performance in terms of price, energy density (per mole, per gram, and per volume) as well as in terms of CO2 emissions. You will then select which of your three fuels is the “best,” realizing that there are several possible considerations to select the “best” fuel. You will have to defend your choice, as well as your definition of “best!”

This is an activity that can stand alone, but I started with a lecture that outlined the major concepts of combustion, solar energy conversion, and catalysis. The slides I used are not mine, so I can not share them here, but I include an outline of the presentation as a guide.

I am indebted to VIPEr users Matt Whited and Karen Holman for discussions about developing and improving this activity. I am including them as co-authors on this activity for their help.

Learning Goals: 

1)    use Hess's Law to determine enthalpy changes associated with chemical reactions.
2)    use balanced chemical reactions, in combination with other relationships such as density, to derive other useful quantities related to a combustion reaction.
3)    identify trends in a set of related chemical reactions and note important characteristics of reactants and/or products that are related to or cause these trends.
4)    Account for for the many variables (chemical, societal, and political) associated with picking the "best" chemical fuels as well as for the ways in which their knowledge of thermochemistry and chemical reactions can be applied to understand these variables.
5)    Draw a reaction coordinate diagram showing the function of a catalyst in a chemical reaction, specifically showing:
a)    how one could limiting the combustion of methane to form methanol instead of CO2
b)    how one could maximize the storing solar energy in a fuel
 

Equipment needs: 

none, although demonstrating the combustion of isopropanol in a 5-L carboy is a good demo to do for the lecture. for example:

https://www.flinnsci.com/media/484580/95010-r.pdf

http://www.instructables.com/id/Woomf-Bottle-a.k.a-Fun-With-Fire/

Be mindful of safety doing this demo. Only bring a small amount of liquid fuel to do the demo (5-10 mL for a 4 L Erlenmeyer flask, and 20-30 mL for a 5 gallon carboy). The fire is reasonably self contained though it does jet upwards a few tens of inches to feet. It is a dramatic reminder/visual of thermodynamics and the quantity of energy that can be stored in chemical bonds.

Implementation Notes: 

This module was used during the Fall of 2015 in a general chemistry class. I started with a lecture outlining the major considerations of energy and the storage of energy in chemical bonds. An outline for this presentation and several slides are provided.

in a second class period, students worked in groups to determine the "best" fuel. They were allowed to determine which criteria to use, as there really is not a right answer here. I had the students upload their data to a google spreadsheet (template is provided). This allowed the students to compare a number of fuels even if they only calculated 3 of them.

the final column in the table is price ($) per kJ of heat, but these values are so low it might be better to ask the students to report cents per J.

I did my best to find accurate values for the prices of the fuels. If people have better numbers, I would appreciate comments when you use the LO!

Time Required: 
2 50 minute class periods (1 with no lecture)
Evaluation
Evaluation Methods: 

the group assignments were graded according to the attached answer key.

Evaluation Results: 

I allowed any criteria to be used as that for determining the "best" fuel. One particularly cheeky group decided that their goal was to raise global temperatures to bring back environmental conditions for dinosaurs, because they wanted dinosaurs to come back. For the purposes of the assignment, this was a "correct" answer, given that they justified their desire to maximize CO2 emissions. However, most groups did determine that minimizing CO2 emissions is probably a better goal. The discussions I had with groups was generally very good while in class.

Creative Commons License: 
Creative Commons Licence

Comments

This LO underwent a fairly significant update (including being lenthened to 3 days) for Fall 2016. It covers the same material in much the same way, but now it is much more of a guided inquiry exercise (pre class) with in-class group calculations. I am uploading new versions (LOv2016) for you to look at and evaluate. Reading comes from Atkins, Jones, & Laverman, 7th ed, 2016.

 

one interesting update (thanks to Prof. Lelia Hawkins at Harvey Mudd College for this) is the day 1 in-class activity where students write their own guided inquiry questions. I am providing a "key" for those as well.

The VIPEr community supports respectful and voluntary sharing. Click here for a description of our default Creative Commons license.