Using enthalpy to assess fuel sources

Enthalpy, balancing equations, critical thinking about scientific data in the media

This activity is appropriate for a general chemistry class after they have been exposed to the concept of enthalpy of reaction and how to use energy transfer to drive endothermic processes.

The only material required for this activity is a "grab bag" of different fuels with their enthaply of formation. I cut these pages into small slips that are passed around the room in a bucket for students to randomly draw.

While the bucket is being passed around the classroom, I typically review combustion reactions with the generic equation:
"hydrocarbon fuel" + __O₂(g) --> __CO₂(g) + __H₂O(g)
The students are told to:

1. Write a correctly balanced chemical equation for their reaction
2. Calculate the enthalpy of the reaction from enthalpy of formation values
3. Calculate the heat liberated by burning 100.0g of their fuel

The students begin working on this problem individually and after ~5 minutes are told to find other people in the class with the same fuel and compare their answers. During this time, a table is generated on the board and each "fuel group" records their enthalpy of reaction and heat liberated values. After all data is listed, a discussion of how "good" each fuel is can be initiated. The "goodness" of the fuel can be evaluated on a number of different criteria, with the "best" fuel depending upon the criterion used.

By compiling all groups' data during the class period and discussing it relevance, errors in calculation and conceptual understanding can be addressed directly during the class period.