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Effect of on the Length of : Thermodynamics

Objectives: To examine and explain the effect of temperature on elastomers (rubber bands)

Introduction:

Stretch a rubber band and allow it to contract while it is in contact with your upper lip. Is there any temperature change as the rubber band is stretched and allowed to contract? Record your observations below. Based on your observations, predict (and write down) the effect of temperature on the length of a rubber band while stressed.

Observations:

Hypothesis:

Materials:

Rubber band –thin and small

200-500 g

1000 mL graduated cylinder

Aluminum foil

Blow dryer or heat gun

Rod

Ruler

Procedure: See picture below for guidance in setting up the experiment.

1. Hook a small, thin rubber band around a 200 – 500 g weight. The rubber band should be moderately stretched (2 – 4 times its original length) with the weight attached to it. 2. Loop the rubber band/weight around a rod and place the rod on top of the graduated cylinder so that the rubber band with the weight is hanging inside the cylinder. 3. Place a ruler inside the graduated cylinder and record where a predetermined spot on the rubber band is on the ruler (we used the loop of the rubber band where it is connected to the weight). Leave the ruler in the graduated cylinder. 4. Place the aluminum foil over top of the graduated cylinder. 5. Using the heat gun or blow dryer, heat the rubber band through the glass of the graduated cylinder for 1 min. 6. Record where the predetermined spot on the rubber band is located on the ruler. 7. Allow the rubber band to cool to room temperature and again record the location of the predetermined spot on the rubber band using the ruler. 8. Repeat the procedure for another rubber band.

Observations:

Before Heating After Heating

Material Length of stressed Length of stressed Length of stressed rubber band before rubber band after rubber band after heating heating allowing to cool to room temperature Rubber band trial 1 9.0 in 9.6 in 9.0 in

Potential discussion uestions:

1. How does the rubber band () differ in behavior from most other materials?

2. How are the results obtained in the experiment consistent with the observations made of the temperature of the stretched and relaxed rubber band (from the introduction section)?

3. Explain your observations of the rubber band behavior in terms of .

Discussion

As students stretch the rubber bands across their upper lips, they should feel the rubber bands become warmer. As the rubber bands contract, the students should feel them become cooler. The rise in temperature of the rubber band as it is stretched is mostly due to (being converted to heat) acting on the rubber material to stretch. In terms of thermodynamics and the Gibb’s Free Energy equation, ΔG = ΔH – TΔS, ΔH is negative when the rubber band is stretched and positive when the rubber band is contracted. In order to feel the heat from the rubber band, the student needs to physically stretch the rubber band, meaning the reaction is not spontaneous and ΔG is positive. When the rubber band contracts, the reaction is spontaneous and ΔG is negative. In examining the Gibb’s Free Energy equation, ΔS must be negative when the rubber band is stretched if ΔG is positive and ΔH is negative. When the rubber band is contracted, ΔG is negative and ΔH is positive, causing ΔS to be positive. In a stretched state, the of the rubber band have fewer conformations that they can assume and are straightened out and aligned with each other, leading the to be ordered. This gives a negative entropy value (ΔS). When the rubber band is contracted the polymers are entangled, meaning they are disordered and have a positive ΔS value.

When the rubber band is stretched, heating the rubber molecules causes them to move around more. As a result, the polymers become less aligned and more naturally disordered. This causes the rubber band to shrink. In other words, the molecules become more tangled, leading the rubber band to contract.