Rubber band experiment

Simple household objects are involved in this practical, which shows off a simple principle in a clear and effective way.

This experiment should take 30 minutes.

Equipment

Apparatus

• Eye protection
• Rubber band, 0.5 cm wide (one for each participant)
• Hair dryer
• Weight >1 kg
• Ruler

Health, safety and technical notes

• Read our standard health and safety guidance.
• Always wear eye protection.
• Ensure rubber bands are sterile and clean.
• Ask participant to stand back so that broken rubber bands do not drop weights onto feet.
• Hairdryers should not be brought from home, ensure all electricals used have an up-to-date pat test.

Procedure

1. Take the rubber band. Quickly stretch it and press it against your lips. Note any temperature change compared with the unstretched band.
2. Now carry out the reverse process. First stretch the rubber band and hold it in this position for a few seconds. Then quickly release the tension and press the rubber band against your lips.
3. Compare this temperature change with the first situation.
4. Set up the apparatus as shown in the diagram. Make sure that if the rubber band breaks, the weight cannot drop on feet.
5. Predict what happens if this rubber band is heated with a hair dryer.
6. Write down your prediction.
7. Measure the length of the stretched rubber band.
8. Now heat the rubber band using the hair dryer and observe the result.
9. Measure the new length.

Notes

• The depth of treatment depends on the ability of the students.
• Students should recognise the difference between exothermic and endothermic reactions.
• A rubber band width of 1–1.5 cm and a 2 kg mass works well.
• A ruler standing beside the apparatus is effective as students can see the contraction as it occurs.
• Another alternative is to use a clampstand and adjust the height of the weight until it just touches the bench.

Theory

By placing the rubber band against their lips, students may detect the slight warming that occurs when the rubber band is stretched (exothermic process) and the slight cooling effect that occurs when the rubber band contracts (endothermic process).

The equation ΔG = ΔH - TΔS (where ΔG means change in Gibb’s free energy, ΔH is enthalpy change, ΔS is entropy change and T is the absolute temperature) can be rearranged to give TΔS = ΔH - ΔG. The stretching process (exothermic) means that ΔH is negative, and since stretching is nonspontaneous (that is, ΔG is positive and -ΔG is negative), TΔS must be negative.

Since T, the absolute temperature, is always positive, we conclude that ΔS due to stretching must be negative.

This tells us that rubber under 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, ΔG is negative and -ΔG is positive.

The cooling effect means that it is an endothermic process (ΔH > 0), so that TΔS is positive. Thus, the entropy of the rubber band increases when it goes from the stretched state to the natural state.

Questions

1. Based on your initial testing (by placing the rubber band against your lips) decide which process is exothermic (heat given out): stretching or contracting of the rubber band?
2. The chemist Le Chatelier made the statement, ‘… an increase in temperature tends to favour the endothermic process’. Explain in your own words how this statement and how your answer to question 1 can account for your observations when heating the rubber band.
3. Draw a number of lines to represent chains of rubber molecules, showing how they might be arranged in the unstretched and stretched forms.

Answers

1. Contraction.
2. They should observe that the rubber band contracts when heated, which may well be the opposite of what they have predicted. The most simplistic answer may be that since the endothermic process is favoured when heating occurs, this is a contraction in the case of the rubber polymer since this is the endothermic process.