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Chapter 21: Problem 155

Stretch a rubber band while holding it gently to your lips. Then slowly let it relax while still in contact with your lips. a. What happens to the temperature of the rubber band on stretching? b. Is the stretching an exothermic or endothermic process? c. Explain the above result in terms of intermolecular forces. d. What is the sign of \(\Delta S\) and \(\Delta G\) for stretching the rubber band? e. Give the molecular explanation for the sign of \(\Delta S\) for stretching.

Short Answer

Expert verified
When the rubber band is stretched, its temperature decreases, making the process endothermic. This is due to elastomer chains requiring energy to be pulled apart and intermolecular forces becoming weaker. During stretching, the entropy (ΔS) decreases, and Gibbs free energy (ΔG) increases, resulting in a more ordered molecular structure. The decrease in entropy is due to chains being aligned with fewer molecular arrangements available when stretched compared to the relaxed state.

Step by step solution

01

Understand the Experiment

First, hold a rubber band gently against your lips while you stretch it and then slowly let it relax. Energize your senses and try to figure out what changes you feel in the temperature of the rubber band in both stretched and relaxed stages. This exercise aims to understand the thermodynamics of rubber bands and their intermolecular forces.
02

a. Temperature on Stretching

When the rubber band is stretched, the temperature of the rubber band decreases. You can feel this as a cooling sensation when the stretched rubber band is in contact with your lips.
03

b. Exothermic or Endothermic Process

The stretching of the rubber band is an endothermic process. Endothermic processes absorb heat from their surroundings, which is evidenced by the decrease in temperature felt when the rubber band is stretched.
04

c. Intermolecular Forces Explanation

Rubber bands are made of long chains of molecules called elastomers. When the rubber band is stretched, these chains are pulled apart and require energy to do so. This energy comes in the form of heat absorbed from the surroundings (including your lips). As the elastomer chains are extended, the intermolecular forces between them become weaker, which is why it's an endothermic process.
05

d. Sign of ΔS and ΔG for Stretching

When stretching the rubber band, the entropy (ΔS) decreases, and Gibbs free energy (ΔG) increases. This conclusion can be reached by observing that stretching a rubber band leads to a more ordered molecular structure due to extended chains that are more aligned.
06

e. Molecular Explanation for the Sign of ΔS for Stretching

With the rubber band in a relaxed state, the elastomer chains and other molecules have more freedom to move and adopt different configurations. However, when the rubber band is stretched, the chains are aligned and have fewer molecular arrangements available. This leads to a decrease in the number of available microstates and therefore a decrease in entropy (ΔS).

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