Entropy of a rubber band

Place the rubber band flat on a table and mark two lines from where the rubber bands elasticity will be measured. Have the students touch the rubber bands to their lips. According to this law, a system or body will move from a state of order to another state of disorder naturally. Thermodynamics of rubber bands recommended for chapters. After reaching room temperature, the stretched band has much less entropy than the unstretched band. They contract when heated because the entropy is increased, which is the amount of disorder among the molecules. On stretching a rubberband does the entropy increase or. The cooling effect means that it is an endothermic process dh0 so that tds is positive. Contraction of the rubber band is entropy driven and enthalpyopposed. Thats present when you stretch the rubber band longer than its equilibrium length, that is, the length when its just sitting there. May 29, 2017 the answer involves the concept of entropy.

In this investigation, students work with a realworld item, a rubber band, to explore the concepts of gibbs free energy, enthalpy, and entropy and their relation to the spontaneity of a physical process. Using stirlings approximation, the entropy is s k nlnn n rlnn r n n r ln n n r 3 since this is a onedimensional system, the role of the pressure in a 3d system is taken here by the tension force f generated by stretching the rubber band. I understand that when you stretch a rubber band, it gives off heat an exothermic process but at the same time you are adding kinetic energy to the rubber band to make this happen, and its entropy decreases. Chemistry stack exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. If you stretch a rubber band, what happens to its entropy. When the bands are stretched, the energy is increased and the polymers untangle until they reach a local maximum of their length, and a local minimum of entropy.

Changes in rubber bands a then hold the stretched rubber band against your forehead to familiarize yourself with its temperature. Will this process be spontaneous at all temperatures. In effect, you are lowering the entropy of the molecules, but the universe prefers to move towards entropy. Individual 4carbon backbone units are forced into rotational conformations that have longer endtoend distances and this restricts the number of conformations states that are thermally accessible. And entropy can be defined as lack of order in a system. When i stretch a rubber band, the fibers straighten out causing lesser randomness and more order.

A higher temperature would increase the entropy, meaning that n r gets closer to its maximum entropy value of n. These long molecules behave in a special way because of a property in physics that we call entropy. Jan 04, 2018 the crosslinked polymers of a rubber band begin in a chaotic, low energy, tangled state. If all the chains are neatly lined up in rows, we say that the entropy is very low. Thats also why the rubber will actually contract when heating it, as shown below. So it is true that when we heat a rubber band it pulls, and this fact is definitely related to the fact that when we release the tension of it, it cools. If f 0 when the band is pulling inward, then work f dl is. Theoretical background the thermodynamic identity for a rubber band is du. When a rubber band is allowed to release and return to its highentropy state of tangles and knots, the elastic force that causes that contraction is considered an entropic force.

Push on this movable wall and you increase the pressure of this gas. Equation of state and molecular structure objective to determine the macroscopic thermodynamic equation of state of an elastomer, and relate it to its microscopic molecular properties. The thermal properties of rubber will be explored in this demonstration. You are treating the rubber band as an isolated system when in fact you are acting upon it. The ball is usually made from 100% rubber bands, but some instructions call for using a marble 20 or a pingpong ball 21 as a starting point. Clearly, the entropic force acting on the polymers wants to resist being stretched, so when we pull on a rubber band, we are performing work on the material. Rubber band size numbers a rubber band is given a standard number based on its dimensions. The rubber band changes temperature for a different reason 1 than the traditional heat pump, but the effect is easy to sense and the test equipment doesnt require a research grant. This allows students to experimentally check the predictions of the entropic spring model for elastomers and observe that the entropy does. To measure partial derivatives experimentally and test the validity of the cyclic rule of partial differentiation.

This means the entropy change of the rubber band upon being stretched must be 1. When released, the entropy rapidly increases until they tangle again. Introduction we are all familiar with the very useful properties of such objects as rubber bands, solid rubber balls. Because the rubber band is stretched when cooled with ice and is contracted at room temp, increasing temp causes the rubber band to contract. Generally, rubber bands are numbered from smallest to largest, width first. The cooling effect means that it is an endothermic process d h0 so that td s is positive. The amorphous phase in pe is also said to be rubbery it is above its tg but is. Therefore, the rubber band contracts more than it does at room temp.

This video is part of the flinn scientific best practices for teaching chemistry video series, a collection of over 125 hours. As the temp increases the change in entropy makes the contraction of the rubber band more and more spontaneous. The figure below shows the molecules of rubber in their natural state a and stretched state b. Thus, rubber bands numbered 819 are all 116 inch wide, with length going from 78 inch to 3 1. The criterion for spontaneity for an isolated system is. Feel the enthalpy and find the entropy when you stretch a rubber band. A rubber band ball is a sphere of rubber bands made by using a knotted single band as a starting point and then wrapping rubber bands around the center until the desired size is achieved. When a rubber band is stretched, some of the network chains are forced to become straight. A rubber band when stretched becomes warm from the work of stretching it plus the energy that can no longer be spread out in the more mobile molecular conformations of the unstretched rubber.

Does this mean a rubber band gets colder on stretching. Under tension, the molecules in a rubber band line up and the arrangement becomes much more ordered, lowering the entropy. Jan 23, 2020 when a rubber band is allowed to release and return to its high entropy state of tangles and knots, the elastic force that causes that contraction is considered an entropic force. Heat is produced in the rubber band as it is stretched. No, as t decreases, d g becomes less negative, and at some t, the d h term will dominate, and d g will be positive. As noted in part b of the above problem example, the gain in entropy when the rubber contracts drives. Thus, assuming that no other energy exchanges takes place, the tendency of the elastic band to shrink back to its original conformation is due to something called an entropic force, i. Rubber bands are unusual because they are made from looselypacked chains of atoms. A resource for teachers a script for classroom demonstration illustrating dg dh tds. If we stretch a rubber band entropy get decreased because macromolecules get uncoiled and arranged in a more ordered manner.

A rubber band also known as an elastic band, gum band or lacky band is a loop of rubber, usually ring shaped, and commonly used to hold multiple objects together. Although this relation was derived in the special case of constant energy u, it does seem to make sense. Jan 27, 2016 thermodynamics demonstration originally prepared for the coursera mooc. This should imply the entropy of the rubber band has decreased, but how does the entropy of the whole universe increase here. Entropy change of a rubber band chemistry stack exchange. The entropy increase occurs in your body as glucose is oxidize into water and carbon dioxide to produce the energy needed for your muscles to contract to stretch the rubber band. Thermodynamics demonstration originally prepared for the coursera mooc. Stretching forces those polymers to align themselves more closely to the axis of elongation, leading to greater ordering and decreased entropy thus, heating the rubber band should increase its restoring force. This means that a rubber band, held at constant tension in stretched state, will contract when it is.

Most rubber bands are manufactured out of natural rubber or, especially at larger sizes, elastomer, and are sold in a variety of sizes. By making this measurement at several di erent temperatures, we will be able to use a maxwell relation to determine the change in internal energy uand entropy sfor a rubber band that is stretched at xed temperature. We describe an upperdivision experiment in thermal physics where students measure the tension of a rubber band as a function of temperature and length and use a maxwell relation to find the change in internal energy and entropy for an isothermal stretch. Gale rhodes department of chemistry university of southern maine portland, maine 041049300. Next, observe the ml beaker and convert the distance the rubber band could stretch from ml to cm. The entropy change of the surrounding air is positive, since heat is given off. Rubber band numbers 3034 are for width of 18 inch, going again from. Suppose the system is the rubber band and the air in a large room.

The rubber band has lower entropy, is more orderd when stretched. Perform the following experiments with the supplied rubber band. So, when you stretch a rubber band, ill take my rubber band, ill stretch it a bit, there is a restoring force, lets call it f. But for now lets dive into the thermodynamics of a rubber band. Technical thrash metal early, groove metal later lyrical themes. Thus the entropy of the rubber band increases when it goes from the stretched state to the natural state. To determine how the entropy of a rubber band changes as the rubber band is stretched. November 4, 2003 rubber elasticity the entropic spring more disorder entropy a natural law that expresses the driving force towards disorder less disorder rubber bands are made from polymers, but the chains are crosslinked to provide a network. When you stretch a rubber band, you are aligning the polymers and eliminating all the chaotic tangles.

Energy and entropy of a stretched rubber band energy. Using the theory on entropy in connection with the elasticity of the rubber band, it can be said that the higher the temperature the higher the entropy level and lower the orderliness of the molecules that make up the rubber. Then measure the length of the rubber band within the marks just drawn. Entropy is a way of measuring the amount of disorder in a system. The rubber band was patented in england on march 17, 1845 by stephen perry. I have the students pick up a rubber band as they come into the room. Entropy change stretching and contracting at constant t. A rubber band is composed of a collection of tangled, crosslinked polymers. When the tension is removed, the stretched rubber band spontaneously snaps back to its original shape. Thermodynamics of a rubber band in this lab, we will measure the tension of a rubber as a function of length. Dec 18, 2012 feel the enthalpy and find the entropy when you stretch a rubber band. Entropy meaning, definition, formula, thermodynamic relation.

A rubber band is a single molecule as is a latex glove. The crosslinked polymers of a rubber band begin in a chaotic, low energy, tangled state. And, in fact, if we apply a gas flame to a rubber band holding a weight, we will see that the band contracts abruptly fig. To determine the number of polymeric chains per unit volume in a rubber band. The individual molecules that make up the rubber band are called polymers.