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Mesoscopic Non-Equilibrium Thermodynamics Part 13

Tham khảo tài liệu 'mesoscopic non-equilibrium thermodynamics part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Insight Into Adsorption Thermodynamics 351 2.1 Activation energy Activation energy is an important parameter in a thermodynamic study as it determines the temperature dependence of the reaction rate. In chemistry activation energy is defined as the energy that must be overcome in order for a chemical reaction to occur. In adsorption separation it is defined as the energy that must be overcome by the adsorbate ion molecule to react interact with the functional groups on the surface of the adsorbent. It is the minimum energy needed for a specific adsorbate-adsorbent interaction to take place even though the process may already be thermodynamically possible. The activation energy of a reaction is usually denoted by Ea and given in units of kJ mol-1. The activation energy Ea for the adsorption of an adsorbate ion molecule onto an adsorbent surface in an adsorption process can be determined from experimental measurements of the adsorption rate constant at different temperatures according to the Arrhenius equation as follows . . E lnk lnA - 1 where k is the adsorption rate constant A is a constant called the frequency factor Ea is the activation energy kJ.mol-1 R is the gas constant 8.314 J.mol-1K-1 and T is the temperature K . By plotting ln k versus 1 T Figure 1 and from the slope and the intercept values of Ea and A can be obtained. The apparent activation energy of adsorption of heavy metal ions and synthetic dye molecules onto various low cost adsorbents is tabulated in Table 1. Fig. 1. A typical plot of ln k vs. 1 T Arrhenius plot 352 Thermodynamics The magnitude of activation energy may give an idea about the type of adsorption. Two main types of adsorption may occur physical and chemical. In physisorption the equilibrium is usually rapidly attained and easily reversible because the energy requirements are small. The activation energy for physisorption is usually no more than 4.2 kJ mol-1 since the forces involved in physisorption are weak. Chemisorption is .