edited bv .-.GEORGEL. GILBERT Denison University Granville. Ohio 43023
Thermodynamic Changes, Kinetics, Equilibrium, and LeChatelier's Principle SUWMTED BY Robert C. Hansen University of Wisconsin-Platleville Planeville. WI 53818 CHECKED BY Paul F. Krause University of Central Arkansas Conway. Arkansas 72032 T h e concents of tbermodvnamic changes. kinetics a n d reaction mech&ms, equilibrium, a n d ~ e e h k e l i e r ' sprinciple a s related t o chemical systems are often vague a n d difficult for beginning chemistry students t o visualize a n d comprehend. T h e development of a n understanding of these concepts c a n b e aided b y correlation with a physical model. W a t e r in beakers a n d t h e flow of water between beakers can b e used as a model t o represent t h e components of a n exothermic chemical reaction and t h e flow and ananti& of thermal e n e r m involved in chemical changes. Materials a n d Equipment T h r e e 1-1beakers, one 100-ml beaker, assortment of 0.5-m lengths of rubber tubing of different internal bores (3116 in., 318 in., etc.) a l-%meter length of %-in. rubber tubing, assortment of wood blocks 10-15 c m square of varying thickness, a n d a sponge. Demonstration Procedure Experiments t h a t c a n b e done with this equipment a r e outlined helow, a n d t h e concepts illustrated are presented a s discussion questions. Function of State, Standard State, andEnthalpy. Place a heaker of water a t a given point on the demonstration desk. Move the beaker around and return it to the same point on the desk. Has there been a change in the net condition of the beaker of water (function of state)? What is the energy content (enthalpy) of the beaker ofwater? Select a reference point (standard state) for some physical changes (similes of chemical changes) that are to he observed. Entropy Changes and the Second Low of Thermodynamics. Place a small beaker, half full of water, on the desk. Ignoring the restraint by the wall of the heaker, can the water flow out across the desk? Tip the heaker over. Willlcan the reverse process of the water returning to the heaker occur? Consider the orderliness (entroov) of the water work on the system). Enthalpy Change for a Chemical Reaction. Place a 1-1 beaker approximately 415 full of water (the water may be colored to enhance its visihilitv) on a c o u ~ l eof wood blocks of such a height that when water is syphoned fro" it into a beaker on the desk ;depth of 2-4 cm of water will remain in the upper heaker when the water level in the two beakers equalizes. With an appropriate pathway, what is the energy change (enthalpy change) for the water in the upper beaker all flowing to the bottom of the second beaker? How does thisenergy change depend upon our initially selected reference point (standard statej? AetiuotionEnergy. Hang an empty0.5-mlength of rubber tubing between the two beakers. A oathwav (reaction mechanism) for the
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Journal of Chemical Education
water, that is, do work on the system, (provide activation energy) to . ~ energy available (heat of start the syphon (chemical r e a c t i ~ n )The reaction, enthalpy change) from the change in water level between the two beakers (reactants going to form products) provides the necessarv enerw (activation enerw) to raise more water over the edge t,f the (upper braker {ralscrnergyof rcactnntv ruthc trancniunslarel, i r . , tmrn acrivatrd complex and runtlnur rhe Rarer flow ichemlral reaction). Kinetics, Reaction Mechanism, and Equilibrium. As the water syphons from one heaker to the other (chemical reaction proceeds) note the rate of change in the water levels in the two heakers (concentrations of reactants and products) as the levels equalize (equilibrium is approached). (The water levels in the two beakers may be measured as a function of time and a plot of the data made.) Repeat the syphoning experiment using the same initial starting conditions, hut different sizes, lengths, and twists of the rubber tubing (different catalyst and reactions mechanism^).^ What determines the final water level (equilibrium position)? LeChatelierS Principle. After the water level in the two beakers has come to the same level (eauilibrium is reached) the effects of stress on the s\+wm can be ulmrved by adding ur rrmminr: water treartnnts or products) trom [he beakrrs ireaction sysrpm,. The kyphon~ng. ~chem~cal reaction) can be furred to complet~onby the remuvol uf watrr trrmoval ot a rracrlon product) trom the h e r beaker. 'l'he Gtbh's Frrp I,'n.llew C h n n p and rhe Entrrrp) ( ' h a n p /or a Chcrnirol R ~ a r l l v n Consider the water in the benkerias b~ing.the thermal energy involved in a chemical change. Have the student imagine that a small water wheel is fastened to the end of the syphon and that it can be harnessed t o d a useful work. Note the vigor of the flow of water during the syphoningand the changein the distance of the water drop (energy drop), as the water levels equalize (system approaches thermo equilibrium, "heat death". Is all of the enthalpy change as defined above for the system available for doing useful work (Gibb's free energy change)? During the syphoning process, what is the change in the orderliness of the system and the amount of ent h a .l ~,vchanee not available for useful work (entronv ., chanee). What can he i l r m 10 incrra>t rhe (;ith'\ tree m r q y ch;ulgcQ E//.YI. of T,rnpemtun,~mtlztI'wriwt olEqut1,bnt.m. After the water level has equalized between the two beakers (equilibrium attained), insert blocks of different thickness under both beakers (an increase in temperature, i.e., enthalpy). The difference in the increase of the heights of the two beakers is analogous to the difference in ca.oacitv. of the reactants and oroducts of a chemical reaction to absorb thrrmal energy. ~ h f t r r m c ein apeciiic heats,. How doc5 arhangc in tPmpPrature affect the poi:tion of a ihemirnl cquilil,rium?
Presented at Chemical Demonstration Program. Western Illinois University. Macomb. Illinois, sponsored by Department of Chemistry, Western Illinois University and Quincy-Keokuk Section, American Chemical Society, May. I. 1981. For demonstrations involving specific chemical reactions that will illustrate each of tne aoove concepts see: "Tested Demonshat#onsin Chem slry." (Edilors: Alyea, Huoerl N.. and D~rton.Freder ck 6.1 J. CMEM. EOLC..6th ed. 1965. Snaknashiri, Bassam 2,"Cnemcal Demonstrat ons, A Handbook for Teachers of Chem.stry" Volume 1. The Unwerslty of Wisconsin Press.
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1983.
NOTE: All rubber tubing should be filled with water from the third 1-1 beaker s o account does not have to be taken of the volume of water entrained in the tubing. For a model of dynamic equilibrium see Sorum, C. H., J. CHEM. EDUC..25, 489 (1948) and Marlin, D. F., J. CHEM.EDUC., 53, 634 (1976).
