Distinguishing between physical and chemical

Distinguishing between physical and chemical adsorption using the value of adsorption heat cannot lead to unambiguous results, too. An arbitrary classification of physical adsorption as having small heats Q - 0.01 + 0.2 eV typical for and attributing - 1 eV to diemisorption is often violated. A typical example can be provided by a dissipative chemisorption diaracterized by small total heat effect. 

One can distinguish between physical and chemical methods of immobilization (Fig. 42.2). The former makes use of weak interactions between the metal... 

In order to accurately determine the chemisorbed amount from the overall adsorption isotherm, the sample can be further outgassed at the same temperature to remove the physically adsorbed amount, after which a new adsorption procedure is carried out to obtain isotherm II. The difference between the first and second isotherm gives the extent of irreversible adsorption ( ) at a given temperature (Figure 13.5b), and can be considered as a measurement of the amount of strong sites in the catalyst. However, in the first approximation, the magnitude of the heat of adsorption can be considered as a simple criterion to distinguish between physical and chemical adsorption. 

Kekule nor Couper intended the structures they wrote to be interpreted as having any significance as a physical representation of the molecule. Like Butlerov after them, they were careful to distinguish between physical and chemical stracture. The caveat implied here was that the stmctnres drawn indicated the chemical locations of the atoms, and may or may not correspond to the physical positions of those same atoms in the molecule atoms close together "chemically" were not necessarily close together physically. In other words, the stmctnres were actnally maps of the linkages between the affiiuties of the individnal atoms in the molecnle. 

Should the isomerization route be shown to be important in some molecules, the question arises as to whether the term internal conversion should be used to describe this kind of relaxation. It may be worthwhile to distinguish between physical and chemical processes by confining the term internal conversion purely to the physical process, and coining a new term for the isomerization route. However, until the isomerization route has been definitely substantiated by careful quantitative work, this is a moot point. 

The difference is not merely practical, it is conceptual as well. R.D. Levine [46] distinguished between physical and chemical shapes. According to him, the physical shape corresponds to a hard spacefilling model, whereas the chemical shape describes how molecular reactivity depends on the direction of approach and distance of the other reagent. In terms of geometry representations, the chemical shape can be related to the average structures determined from the experiments and the physical shape to the hypothetical equilibrium structure. 

The main objective of absorption processes is the removal of one or more components from a gas stream using selective solvents. Figure 3 shows a typical absorption process in which, with the help of a selective solvent (absorbent), the undesired compounds (in this case, HjS, COj) are removed from the raw gas (in this case, natural gas) in a multistage countercurrent process. While the purified gas leaves the absorber (saturated with the selective solvent), the absorbent is regenerated in a second column (desorber) and is recycled to the absorber. In the case of absorption, one can distinguish between physical and chemical absorption processes. In the case of physical absorption, the absorber is operated at high pressures and low temperatures while for the desorber the opposite conditions are used. 

You will distinguish between physical and chemical properties. 

Processes of this type are distinguished between physical and chemical absorption or stripping. For physical processes, transfer of the solute between phases is by physical mechanisms without any chemical reactions. For chemical processes, the solute reacts with a component of the solvent, resulting in an increased capacity of the solvent for the solute. Conversely, stripping sometimes breaks the chemical bonds between solute and solvent. 

Skill 11.1g-Distinguish between physical and chemical changes and provide examples of each... 

SAMPLE PROBLEM 1.1 Distinguishing Between Physical and Chemical Change Problem Decide whether each of the following processes is primarily a physical or a chemical change, and explain briefly ... 

A number of criteria have been applied to distinguish between physical and chemical adsorption. In some cases the distinction is not dear. Table 5.7 lists several properties that can be used to distinguish between physisorption and chemisorption. The most.distinguishing characteristics are the degree of coverage and the specificity. 

Skill 19.2 Distinguish between physical and chemical changes. 

OBJECTIVES To learn to distinguish between physical and chemical properties. learn to distinguish between physical and chemical changes. 

Discuss why background correction is necessary in AA. Distinguish between physical and chemical interferences in AA. Explain what is meant by the term Smith Hieftje background correction and discuss how it differs from the kind of correction employed in the Perkin-Elmer Model 3110 AA. 

It is also possible in adsorption phenomena to distinguish between physical and chemical adsorption. Chemical adsorption or chemisorption is characterized by a simple electron transfer between the gas in physisoibed state and the solid. This transfer results in the forming of a reversible chemical bond between the two compounds (see Figure 1.1b). Once again, the appearance of the chemisorption process is directly related to the environment s thermodynamic conditions. 

SAMPLE PROBLEM 1.1 Distinguishing Between Physical and Chemical Change... 

To distinguish between physical and chemical properties, consider whether the substance changes composition while displaying the property. If it does not change composition, the property is physical if it does, the property is chemical. 

The Redlich-Peterson equation only differs from the Langmuir-Freundlich equation by the absence of exponent on at the numerator part of the equation (Ng et al, 2002). Meanwhile, the Dubinin-Radushkevitch (D-R) isotherm describes the adsorption on a single type of uniform pores and can be applied to distinguish between physical and chemical adsorption. This isotherm does not assume a homogeneous surface or a constant adsorption potential (Unlu and Ersoz, 2006). 

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