Question

Find a wide (³ 5 mm) rubber band (I have some if you need one). Touch the rubber band to your lip and quickly stretch the rubber band; it should get a bit warm. After holding the rubber band in a stretched position for a few seconds, touch the rubber band to your lip and quickly release the tension; it should cool.

1) Provide a thermodynamic analysis (explain the signs of DG, DH, and DS) of these phenomena (be sure to explain how you know).

**Are the DG, DH, and DS positive or negative? explain why.

Answer 1

When we stretch the rubber band the changes occured in the rubberband thermodynamically

We know the thermodynamic equation:
D G = D H - TD S
On rearranging it 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.