Rubber band thermodynamics– Office supplies are used to explore enthalpy, entrophy and Gibbs free energy
Purpose: To allow students to figure out the thermodynamics of a common office supply.
Procedure: Give all students a rubber band and write the “reaction” stretched going to unstretched on the board. Ask students about spontaneity of this reaction. Stretch a rubber band and point it in general direction of professor. Say that if have a stretched rubber band it will “spontaneously” fly at someone. Get the sign of ΔG. Then move on to enthalpy. Have students hold rubber band against face while stretched out for a little while. Then have them allow it to relax and check the temperature change. Talk about heat flow for this process and record sign of ΔH. Then talk about entropy. Have them deduce sign. Ask if makes sense. Have students hold rubber band in stretched out position and try to wiggle it. Shouldn’t move much. Then have them hold a relaxed rubber band and repeat. Also, can use larger model of linked rubber bands and ruler to show how system becomes rigid. Record sign for entropy.
Rubber Band Thermodynamics
Perform the following experiment
- Stretch and hold the rubber band for twenty seconds.
- Touch stretched rubber band to the skin directly above your upper lip or forehead. (Not where your fingers are holding the rubber band.)
- Quickly relax rubber band while still against you lip or forehead. (DO NOT LET GO of RUBBER BAND! Try not to thwack yourself or your neighbor!)
Q1: What did you notice?
Q2: Is this a spontaneous or non-spontaneous reaction?
Q3: What is the sign of ΔG?
Q5: Are the molecules in the band becoming more ordered or disordered during the forward reaction?
Q4: What is the sign of ΔH for the rubber band going from stretched to contracted?
Q6: What is the sign of ΔS?
Q7: Are the intermolecular forces breaking or forming during the forward reaction?
Q8: Will this process be spontaneous at all temperatures?