Question

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 stretching an exothermic or endothermic process?

c. Explain the above result in terms of intermolecular forces.

d. What is the sign of ΔS and ΔG for stretching the rubber band?

e. Give the molecular explanation for the sign of ΔS for stretching.

Short answer

(a)The temperature increases.

(b) This reaction is exothermic.

(c) Intermolecular bonds are broken down and the energy is released.

(d)ΔG>0, ΔH<0,ΔS<0

(e) Intermolecular structure has greater order

Step-by-step solution

Temperature of the rubber band on stretching

When the rubber band is near the mouth, our breath is warmer than the room temperature. Therefore, the temperature of the rubber band increases. The second process is stretching the rubber band. That process increases the temperature of the rubber band as well. Hence the temperature increases.

Stretching an exothermic or endothermic process

Stretching of the rubber band causes an increase in temperature of the rubber band. Because of the increase in temperature, this reaction will be exothermic. Hence this reaction is exothermic.

Terms of intermolecular forces

Intermolecular bonds between neighbouring polymer chains are broken down while stretching the rubber band. It results in releasing the energy to the environment in the form of heath (exothermic reaction). Hence Intermolecular bonds are broken down and the energy is released.

Sign of ΔS and ΔG for stretching the rubber band

Since energy is released in the form of heath while stretching the band, ΔH is lowerthan 0 (ΔH < 0). ΔG is greaterthan zero (ΔG > 0) because the reaction is not spontaneous. If we take a look at the equation below, we will see that the ΔS should be less than zero (ΔS < 0).

Hence ΔG>0, ΔH<0,ΔS<0

Molecular explanation for the sign of ΔS for stretching.

Rubber polymer chains have undefined structure (disorderly structure) when they are relaxed. When we start to stretch the rubber band, parallel polymer chains gain more orderly structure which decreases entropy. Hence Intermolecular structure has greater order.