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2 Kinetics - Thermodynamics vs kinetics THERMODYNAMICS...

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1 Kinetics Thermodynamics vs. kinetics THERMODYNAMICS what SHOULD happen data generally available data sometimes unreliable - often determined 40y ago low temperature sensitivity (except reactions involving gas) not used in design KINETICS what 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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