7 Mar 2013

Energy Content and Properties of Fuels

In-Class Activity

Submitted by Matt Whited, Carleton College
Categories
Prerequisites: 
Corequisites: 
Course Level: 
Topics Covered: 
Subdiscipline: 
Description: 

This is a group activity I developed for my "Introduction to Chemistry" class, which is set up primarily to cover the topics we consider to be prerequisites for the first course in our chemistry sequence at Carleton.  However, it covers aspects of thermodynamics (e.g., particularly Hess's Law) that are core topics for most intro courses.

The class is split into groups of ~4 students and each group is given a selection of 3 fuels (or potential fuels) from the list.  They are asked to consider the combustion reactions (balancing the equations) and use those to determine a number of important properties that relate enthalpy of combustion to energy content.  Then they use the balanced equations to determine relative carbon impact (as measurd by mmol CO2 per kJ of heat released).

Following these calculations, students pool their answers to identify trends, which I think are very instructive and interesting ways of applying general chemistry principles to real-world problems!

The final question is an interesting way of relating this exercise to the current literature, and I viewed this as sort of a "challenge problem" that could possibly be assigned outside class, though we were not able to get to it during out meeting.

Note that I intentionally simplified to include the most common categories, while still assuming, for instance, that coal is entirely carbon and that all coal has the same density (this is an average over several types).

Learning Goals: 
  • A student should be able to use Hess's Law to determine enthalpy changes associated with chemical reactions.
  • A student should be able to use balanced chemical reactions, in combination with other relationships such as density, to derive other useful quantities related to a combustion reaction.
  • A student should be able to 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.
  • Students will gain appreciation for the many variables 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.
Equipment needs: 
  • Calculator
  • Computer (w/ projector) or chalkboard for compiling answers
Implementation Notes: 

I began the class by talking a little about energy in general, the need for renewable/sustainable energy sources, and the reasons why chemical fuels are so important (and cannot be replaced by batteries or mechanical energy storage anytime in the foreseeable future).  I used a PowerPoint presentation for this, and I am happy to share it but would prefer not to post on VIPEr since I received some figures in it from others.

I set up a GoogleDoc at the front of the classroom and had groups enter their results as they got them, then we circled everyone up and picked out trends related to composition of fuels and other characteristics (density, for instance).  I probably needed to allot more time for this exercise, and would love to hear ideas about how parts of it could be done outside of class (either before or after the meeting).

Some of the groups worked much more slowly than others, which kept us from being able to have all of the numbers available for our discussion.  One way around this would be to have each group start out with one fuel and then have them pick up more after they finish their first.

Time Required: 
50 minutes
Evaluation
Evaluation Methods: 

I assessed students' performance by correct answers to the mathematical parts of the questions as well as how quickly they were able to integrate the information.

I also used their ability to pick out trends from the data as an indication of how well they understood what their numbers meant.

Evaluation Results: 

By the first criterion, variation was pretty wide, i.e., the best students found the exercise to be relatively simple and the weaker students sometimes struggled to get a balanced equation for the combustion reaction.  However, every group was able to make some good progress on their questions by the end.

Since this was a problem for an "intro to intro chem" course, I found that many students still struggled with the idea of "moles of reaction" when relating values for ΔH to heat released per mole of fuel.  However, this was part of the reason I designed the exercise in the way I did, so I found it to be a useful application of the molar relationships conveyed by balanced chemical equations.

The students did an excellent job of picking out trends and identifying strengths and weaknesses of different fuels.

Creative Commons License: 
Creative Commons Licence

Comments

Only content-related comment is for the states of the compounds involved in combustion on page 1 to be included in the table. Once you make that change, the LO will be approved.

This is a well-constructed LO with lots to offer students. An instructor could definitely pick and choose different components to adjust for time constraints. You have very complete implementation notes and evaluation results. Thanks for sharing!

Planning to use this in the spring.  It will be a nice expansion of some problems I have done with students before.  Thanks!

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