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9 Cards in this Set
- Front
- Back
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What are the assumptions of the Langmuir Isotherm ? |
1.Adsorption is limited to a monolayer. 2.The surface is completely uniform sothat there is the same probability ofadsorption on all sites. 3.The ability of a molecule to adsorb isindependent of whether or not anadjacent site is occupied. ΔHads isindependent of surface coverage. |
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Define the term fractional coverage ? |
number of adsorption sites occupied / number of adsorption sites available |
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What is an adsorption isotherm ? |
An adsorption isotherm is the equilibrium amount of gas adsorbed on a given adsorbent as afunction of the gas pressure at constant temperature |
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What is the Langmuir isotherm equation for non-dissociative adsorption ? |
A + * -> A*
ka and kd – the rate constants for adsorption and desorption, respectively; Ka = Ka/Kd is the adsorptionequilibrium constant fractional coverage = Kp / 1+Kp |
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What is the Langmuir isotherm equation for dissociative adsorption ? |
A2 + 2* -> 2A* fractional coverage = (squr) Kp / 1 + (squr) Kp |
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What is the Langmuir isotherm equation for competitive adsorption ? |
A + * -> A* B + * -> B* fc = KaPa / 1+KaPa+ KbPb fc = KbPb / 1+KaPa+ KbPb |
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Describe physical adsoprtion ? |
Physical adsorption is an exothermic process. Therefore, when the temperature is increased at agiven pressure, the equilibrium shifts towards the reverse reaction (desorption) and K decreases,resulting in a decreased amount of adsorbed material. |
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What is the equation for isosteric enthalpy of adsorption ata fixed surface coverage ? |
(sigma ln p/ sigma(1/T) ) fc = deltaH*/R |
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Describe the BET isotherm ? |
The BET isotherm is based on the same assumptions as the ones for the Langmuir isotherm exceptfor the monolayer restriction – it assumes that adsorption can extend beyond a monolayer(multilayer adsorption). Multilayer adsorption occurs on the majority of solids and therefore theBET model is universally used for determination of the surface area of catalysts by physicaladsorption. |
