T590: Entropy – Stretching Rubber Bands
Introduction
Students stretch pre-cut rubber bands to their lips, stretch the rubber bands, and then allow them to contract. The rubber bands grow warmer as they expand and cool as they contract.
To Conduct Demonstration
- Pass out rubber bands; one to each student.
- Instruct students to place rubber bands against their lips and stretch the bands, observing any temperature change.
- Instruct students to allow rubber bands to contract, observing any temperature change.
Explanation & Equations:
Obtain a rubber band at least 0.5 cm wide. Quickly stretch the rubber band and then press it against your lips. You will feel a slight warming effect. You can also carry out the reverse process. First, stretch a rubber band and hold it in position for a few seconds. Then quickly release the tension and press the rubber band against your lips. This time you will feel a slight cooling effect. A thermodynamic analysis of these two experiments can tell us something about the molecular structure of rubber.
Rearrangement of the equation: D G = D H - TD S gives: TD S = D H - D G
The warming effect (an exothermic process) due to stretching means that D H < 0, and since stretching is non-spontaneous (that is, D G>0 and -D G<0), TD S must be negative. Since T, the absolute temperature, is always positive, we conclude that D S due to stretching must be negative. This observation tells us that rubber in its natural state is more disordered than when it is under tension.
When the tension is removed, the stretched rubber band spontaneously snaps back to its original shape; that is D G is negative and -D G is positive. The cooling effect means that it is an endothermic process (D H>0) so that TD S is positive. Thus the entropy of the rubber band increases when it goes from the stretched state to the natural state.
The figure below shows the molecules of rubber in their natural state (a) and stretched state (b). An overhead transparency of this illustration is available.