1KineticsThermodynamics vs. kineticsTHERMODYNAMICS•what SHOULDhappen•data generally available•data sometimes unreliable -often determined 40y ago•low temperature sensitivity (except reactions involving gas)•not used in designKINETICS•what DOEShappen•data rarely available•data determined for each individual system•temperature sensitivity highly variable - Eact•vital for equipment designFor example…..2Fe + 1.5O2= Fe2O3 ΔG298= -742.3 kJ mol-1since ΔG = -RT lnK lnK = 299.50 and K = 1.14*10130K = aFe2O3/ (aFe2.aO23/2)BUT, since aFe2O3= aFe= 1 and aO2= pO2So, K = 1 / pO23/2rearranging gives pO2= (1/K)2/3=10-86.7However, the earths atmosphere has pO2=0.2 so ALL iron on the earth should be present as Fe2O3Clearly, this is not the case and cars are only prevented from rusting by kinetic limitations. A phase which is kinetically hindered from changing is called metastable.Stability and metastability….energystabilityUNSTABLE:the ball can reach a lower energy positionSTABLE:the ball cannot attain a lower energy positionMETASTABLE:the ball can attain a lower energy position, but requires an energy inputactivation energySteps in a gas-solid reactione.g. O2- Cu1.Gas diffuses to surfaceO22.Solid diffusion to interfaceO2CuS3.Adsorption, reaction and desorption1.5O2+CuSSO2+CuO4.Diffusion of products from the interface5.Removal of productsCuOAlternatively….Gas-solid kinetics is similar to getting a drink….1.getting to the door of the bar along the street is easy, providing you go with the flow2.past the door you have to get through the crowd to reach the bar3.you have to attract the bar staff’s eye for service4.you then wend you way back through the crowd trying to get to the bar5.back outside the bar you are carried away by the flow of pedestrians
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