Instructions - Lab 5 - Thermodynamics of Rubber Elasticity (PDF Instructions) PDF

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CHEM 115: EXPERIMENT 5 THERMODYNAMICS OF RUBBER ELASTICITY Introduction Polymers are an important part of our everyday world. We are all familiar in a general sense with different kinds of plastic and rubber, which are all polymers. Rubbers are also called elastomers; a term which simply refers to polymers that will recover their shape after being stretched or deformed. All of us are familiar with rubber bands, balloons, and bouncing balls, which are all made of elastomers. This experiment reveals some remarkable properties of these common objects. Rubber molecules are polymers (large molecules made from repeating units) with long hydrocarbon chains. Natural rubber, or latex, (from a rubber tree) is a polymer of isoprene. The structures of isoprene and its polymer are shown in Figure 1. The square brackets represent a repeating unit which may be many thousands of units long.

H

CH3

CH3 C

C

H

H C

*

H 2C

C

CH

CH2

C

H

*

n H

Polyisoprene (n large)

Isoprene Figure 1

Synthetic rubbers have similar structures. The physical properties of rubber are a consequence of the different degrees of order in the stretched and unstretched states. Simplified models of the hydrocarbon chains in these two states are shown in Figure 2. In which state do the molecules appear more/less ordered? Unstretched

Stretched

Figure 2 Schematic diagram of molecules in rubber stretched (left) and unstretched (right)

In thermodynamics, any process may be described by changes in the thermodynamic state functions that characterize the system under study. For example, the change in enthalpy ( H) for a process: H = Hfinal state – Hinitial state = qp is the heat transferred from the surroundings to the system when the process occurs at constant pressure. If H is positive, the process is endothermic; if H is negative, the process is exothermic. Similarly, the change in entropy ( S), which is a measure of molecular disorder, for a process is given by: S = Sfinal state – Sinitial state If S is positive, the entropy (disorder) of the system increases during the process, while if S is negative, the entropy of the system decreases during the process. We can also define a third thermodynamic function called the Gibbs free energy, G. The free energy change, G, for a process at constant temperature is given by:  G = H – T S

(1)

In this experiment, in order to understand the signs of H, S, and G for rubber stretching and relaxation, we must carefully define the initial and final states. a. For the stretching process, the initial state is the unstretched rubber and the final state is the stretched rubber. b. For the rubber relaxation process, the initial state is the stretched rubber and the final state is the relaxed rubber. For any spontaneous process, G must be < 0. From Equation (1) we can see that if H is negative (exothermic) and S is positive for a process, G must be negative and that process will be spontaneous. That is, processes that occur spontaneously are favored by both the release of heat to the environment and increased entropy or disorder. Conversely, if H is positive (endothermic) and S is negative, G will be positive and that process will be non-spontaneous. Note, however, that it is also possible to have enthalpy and entropy factors that work in opposition to each other, i.e. when H and S are both positive or both negative. In these cases, the spontaneity of the process will depend on the magnitudes of H and S and the temperature at which the reaction occurs. In today’s experiment you will qualitatively examine the enthalpy and entropy, and free energy changes involved in stretching and relaxing rubber and will re-examine Le Chatelier’s principle to predict changes when rubber is heated.

Experimental Procedures: Part A - Enthalpy Changes in Stretching and Relaxing Rubber This procedure and observations will be described in the video, however you may also try out this part at home (with a balloon, rubber band, etc.). 1. Touch an unstretched balloon against your forehead for a few seconds then remove it. Now, holding it close to your forehead, quickly stretch it once (until it is 3 times its original length), and immediately touch it to your forehead again. Do not stretch/relax the balloon repeatedly as you might if you were going to blow it up. The idea is to see what happens in one direction of the ‘reaction’. This process will be called the “unstretched  stretched” reaction. Repeat this step several times, if necessary, until you are certain of the results. Record/observe any (qualitative) temperature changes. 2. Stretch a balloon so that it is 3 times its original length and hold it in the air for about 20 seconds. Now let it contract (carefully but quickly) and promptly touch it to your forehead. This process will be called the “stretched  unstretched” reaction. Note any (qualitative) temperature changes. Repeat this step if necessary to confirm your observations. Worksheet Instructions: Copy the following Table A below and fill out the remainder of the table based on the videos (and/or your) observations. Insert a photo of this hand-written table (or screenshot photo of this table made in Microsoft Word) into the Worksheet Table A’s field. Table A:

Process unstretched  stretched

stretched  unstretched

Spontaneous or Non-spontaneous?

ΔG >0 or ΔG 0 or ΔH 0 or ΔS...