Thermodynamics (of a rubber band). Perform the following experiments with the su

Question

Thermodynamics (of a rubber band). Perform the following experiments with the supplied rubber band. (5a) Quickly stretch the rubber band and then press it against the skin between your lips and nose. Note an observations. (5b) Based on your observations, determine the signs of DeltaH and DeltaG first, and then DeltaS for the process. Explain how the observations lead to the determination of the signs for the three thermodynamic functions. (Did you write an equation?) (5c) Stretch the rubber band and hold it for a few seconds, quickly release the tension and then press it against the skin between your nose and lips. Note any observations. (5d) Based on your observations, determine the signs of DeltaH and DeltaG first, and then DeltaS. Explain how the observations lead to the determination of the signs for the three thermodynamic functions. (5e) Based on the entropy observations, make a prediction about the molecular structure of the rubber band in its normal state and in its excited state (sketch each!).

Explanation / Answer

H : when the rubber band is stretched, it should give out heat, and conversely, when contracted, it should absorb heat

S : The rubber band has lower entropy, (is more orderd) when stretched

and has higher entropy when released.. ie. it is more disordered.

The disorder in the rubber band is described in scientific terms by its “entropy:” By

definition, the entropy of the band is high when disorder is present. Upon stretching, the

molecules straighten out and align along the axis of the stre

tch. This stretching causes

the coils of molecules to uncoil and thus the structure becomes more ordered; this means

that the entropy of the band decreases when stretched. When the entropy of a system

goes down, the system gives out heat. Thus, when the

rubber band is stretched, it should

give out heat, and conversely, when contracted, it should absorb heat. This can be felt by

holding a rubber band on a sensitive place on the body (try lips or forehead) and then

alternately stretching and allowing the

rubber band to contract. Using the same effect, a

stretched rubber band will contract on its own when heated.

By heating the rubber band,

we increase the entropy. The molecules become more disordered and tangled, and thus

the rubber band

shrink

s

. This is

just the opposite of what happens when we stretch the

band.

When the stretching force is removed from a stretched rubber band, the molecules also

have trouble returning to their original length. This is because the structure of the band

has been altered

. Stretching uncoils the kinks and tangles in the molecules, which do not

return to their original positions when unstretched. This is called “hysteresis.” Further, if

the band is stretched past its elastic limit, the cross links can also be stretched,

causing a

permanent lengthening of the rubber band.

The important point here is that these properties of the rubber band are related directly to

the molecular structure of the rubber band. Without the unique distribution of the

molecules in the rubber band, these properties would not exist.

I would suggest you to go through the following links where this experiment is beautifully explained.

http://depts.washington.edu/chem/facilserv/lecturedemo/EntropyofRubber-UWDept.ofChemistry.html

http://www.materialseducation.org/educators/matedu-modules/docs/The_Odd_Behavior_of_Rubber_Bands.pdf

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