In thermodynamics, any process may be described by changes in the thermodynamic
ID: 1033340 • Letter: I
Question
In thermodynamics, any process may be described by changes in the thermodynamic state fiunctions that characterize the system under study. For example, the change in enthalpy (AH) for a process, is the heat transferred from the surroundings to the system when the process occurs at constant pressure. If AH is positive, the process is endothermic; if AH is negative, the process is exothermic. Similarly, the change in entropy (A S), which is a measure of molecular disorder, for a process is given by If AS is positive, the entropy (disorder) of the system increases during the process, while if AS 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 (AG) for a process at constant temperature is given by In this experiment, in order to understand the signs of AFI, AS, and AG 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 ruber relaation proces, the initial state is the stretched rubber and the final state is the relaxed rubber. For any spontaneous process, AG must be 0. From equation (1) we can see that if AH is negative (exo AG must be negative and that process will be spontaneous. That s,processes that occur spontancously are favoured by both the releas of beat to the environment and increased entro yor disorder. Conversely, if ?? is positive (endothermic) and asisnegative. ?@will be positive and at process will be nonspontaneous. Note, however, that it is ako posible to hove enthalpy and entropy factrs that work in opposition to each otber ic, when AH and AS are both positive or both negative. In these cases, the spontaneity of the process will depend on the magnitudes of AH 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 in- volved in stretching and relaxing rubber and will reexamine Le Chatelier's principle to predict changes when rubber is heated. In addition, because this experimental procedure is so short, you will also be doing a short problem session on thermodynamics. The questions will be given to you in the lab. 8.2 PRE-LAB ASSIGNMENT 1. Read the experiment and understand its objectives. 2. Read the appropriate sections in your textbook, including Chapter 7 of the CHEM 115 workbook.Explanation / Answer
1. For a spontaneous process ?G will be negative but it still depends on temperature
At low temperature : ?Hspontaneous = - ; ?Sspontaneous = - / + ; ?G = -
At high temperature : ?Hspontaneous = +/- ; ?Sspontaneous = + ; ?Gspontaneous = -
For a non-spontaneous process ?Gnon-spontaneous = +
At low temperature : ?Hnon-spontaneous = + ; ?Snon-spontaneous = +/- ; ?Gnon-spontaneous = +
At high temperature : ?Hnon-spontaneous = +/- ; ?Snon-spontaneous = - ; ?Gnon-spontaneous = +
2. For stretching rubber : ?H = - ; ?G = - ; ?S = +
when stretched the system goes from less disorder to more disorder hence entropy is positive and
heat willl be released.
Relaxation of rubber : ?H = + ; ?G = + ; ?S = -
3. As a way to stabilize when the balloon is stretched it weighs down on the opposite side which is very much
similar to Le Chatlier's principle that states that equilibrium moves towards the opposite direction from where
stress is applied.