Thermodynamics And The Rubber Band

A Resource for Teachers

A Script For Classroom Demonstration Illustrating

DG = DH - TDS.

Gale Rhodes
Department of Chemistry
University of Southern Maine
Portland, Maine 04104-9300

Hand out heavy 3" rubber band to each student.

Consider this "reaction":

Q: Can you determine the sign of any thermodynamic parameters for this reaction?

A: Contraction is spontaneous, so DG < 0 for the forward (-->) reaction.

Q: Can you determine DH or DS?

A: You can determine DH by experiment

1. Stretch and hold the rubber band for twenty seconds.
2. Touch to lip (still stretched).
3. Release and quickly touch to lip.

Q: What is the sign of DH?

A: Band feels cooler, so it's taking heat from surroundings, so DH > 0.

Q: What is the sign of DS?

A: DG = DH - TDS, and signs of each state parameter are DG: (-), DH: (+)

So DS must be (+) in order to overcome DH and make DG negative.

 Q: Are the molecules in the band becoming more ordered or disordered during the forward reaction? Explain.

A: More disordered, in keeping with DS > 0.

Q: Explain.

A: They must be aligned when the band is stretched, and become more randomly oriented when it contracts.

Q: Are bonds breaking or forming during the forward reaction? Explain.

A: Breaking, in keeping with DH > 0.

Q: Explain.

A: All bond breakage is endothermic; all bond formation is exothermic. Alignment of molecules must bring them close enough to interact noncovalently.

Q: If the molecules of rubber bands are hydrocarbons, what type of bonds are formed when the band is stretched?

A: London dispersion forces (so-called van der Waals forces). It's startling to realize that you are feeling the heat effects of the weakest of the noncovalent forces.

CONCLUSION: Contraction of the rubber band is entropy-driven (and enthalpy-opposed).

Q: Will this process be spontaneous at all temperatures?

A: No, as T decreases, DG becomes less negative, and at some T, the DH term will dominate, and DG will be positive.

Q: This means that as temperature decreases, there is a decrease in the capacity of the contracting band to do work. How might this decreased capacity be manifest?

A: Two possibilities:

1. the band becomes longer at lower temperature, so it can do less work in contracting from a given stretched length; or
2. the tension of the band decreases at lower temperatures, so the band can do less work in contracting from a given stretched to its relaxed length because the contraction is less forceful.

Q: How could you decide between them?

A: Tests for the two possibilities:

1. Measure the length of the relaxed band at room temperature and at freezer temperature. If it's longer when cold, then lowering the temperature increases its equilibrium length.
2. Hang a weight from the band and measure its stretched length at room and freezer temperatures. If it's longer when cold, then lowering the temperature decreases its tension.

(Another oldie-but-goodie demo: Hang weight from band, heat band [heat gently -- easy to melt it or set it afire with a flame]. Possibility #2 says that the stretched band should contract when heated. Try it.)

Think About It

General-chemistry students: How does this example pertain to the gradual thinning and disappearance of icicles during freezing weather?

Biochemistry students: How does this example pertain to protein folding (or formation of lipid bilayers)?

NOTE FOR FUTURE REFERENCE TO THIS DEMONSTRATION

In discussion of spontaneous protein or nucleic-acid folding, or spontaneous assembly of bilayers, it's more important to distinguish the enthalpic and entropic contributions in these processes, rather than segregating the hydrophobic effect from other forces, which are entirely enthalpic. Reason: The hydrophobic effect has both enthalpic and entropic components (enthalpic: London dispersion forces; entropic: the ordering of the large molecules, and the disordering of water.)

Biochemistry Resources