2 Kinetics - Thermodynamics vs. kinetics THERMODYNAMICS...

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1 Kinetics Thermodynamics vs. kinetics THERMODYNAMICS •w h a t SHOULD happen • data generally available • data sometimes unreliable - often determined 40y ago • low temperature sensitivity (except reactions involving gas) • not used in design KINETICS h a t DOES happen data rarely available data determined for each individual system temperature sensitivity highly variable - E act vital for equipment design For example…. . 2Fe + 1.5O 2 = Fe 2 O 3 Δ G 298 = -742.3 kJ mol -1 since Δ G = -RT ln K ln K = 299.50 and K = 1.14*10 130 K = a Fe2O3 / (a Fe 2 .a O2 3/2 ) BUT, since a Fe2O3 = a Fe = 1 and a O2 = p O2 So, K = 1 / p O2 3/2 rearranging gives p O2 = (1/K) 2/3 =10 -86.7 However, the earths atmosphere has p O2 =0.2 so ALL iron on the earth should be present as Fe 2 O 3 Clearly, 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…. energy stability UNSTABLE: the ball can reach a lower energy position STABLE: the ball cannot attain a lower energy position METASTABLE: the ball can attain a lower energy position, but requires an energy input activation energy Steps in a gas-solid reaction e.g. O 2 -Cu 1. Gas diffuses to surface O 2 2. Solid diffusion to interface O 2 CuS 3. Adsorption, reaction and desorption 1.5O 2 +CuS SO 2 +CuO 4. Diffusion of products from the interface 5. Removal of products CuO Alternatively…. 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 flow 2. past the door you have to get through the crowd to reach the bar 3. you have to attract the bar staff’s eye for service 4. you then wend you way back through the crowd trying to get to the bar 5. back outside the bar you are carried away by the flow of pedestrians
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2 Rate determining step The slowest step in any reaction mechanism determines the overall rate of reaction rate limit gas flow rate explanation how to increase rate bulk fluid transport low reaction rate proportional to flow rate C A is low so Δ C A in Fick’s law is also smal increase gas flow increase C A diffusion medium gas flow and C A are high enough for the rate to be diffusion control ed rate of reaction is a function of the diffusion coefficient, D A increase gas flow rate increase stirring of fluid increase surface area chemical reaction high C A-initial >> C A-final due to high gas flow rate chemical processes are independent of gas flow increase temperature increase surface area increase active sites Schematically…… gas flow rate reaction rate, K gas flow diffusion chemical reaction Diffusion - Fick’s first law requires a concentration gradient to occur The flux (flow) of a particle (atom, molecule, ion etc) through a specific area perpendicular to the concentration gradient is given by: J A = -D A d C A / d x Where J A
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This note was uploaded on 05/13/2011 for the course METALLURGY 1133 at Murdoch.

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2 Kinetics - Thermodynamics vs. kinetics THERMODYNAMICS...

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