Common anion rule

Common anion rule a general guideline, which states that the lattice anion controls the position of the valence band edge of a semiconductor... 

The Common Anion Rule When the anion (the electron accepting atom such as As in GaAs and InAs) is in common across a semiconductor heterojunction, the change in the conduction band edge is greater than the change in the valence band edge across the semiconductor heterojunction. Mathematically, AEv[Pg.105]

Increasing chemical differences between atoms in heteropolar compounds lead to more ionic bonds, to an increasing connection of the valence band with the anion, and the conduction band with the cation. This is the basis of the common cation and common anion rules. 

What is the physical basis in bonding theory as described in this chapter for the common cation and common anion rules Would you expect these rules to be more obvious when comparing GaP with GaAs or comparing CdS with CdTe Explain briefly. 

Given a cation and anion in aqueous solution, we can determine if a precipitate will form according to some common solubility rules. 

The general rule is that if the anion X- is an effective base—that is, if HX is a weak acid—the salt MX will show increased solubility in an acidic solution. Examples of common anions that are effective bases are OH-, S2-, CO32-, C2042-, and Cr042-. Salts involving these anions are much more soluble in an acidic solution than in pure water. 

Rooney has recently revived work on this monomer in an investigation of its polymerisation by trityl hexafluoroantimonate - He used a spectroscopic stop-flow apparatus to follow initiation and an adiabatic calorimeter to measure rates of polymerisation. Propagation was shown to compete effectively with initiation to the point that some initiator was often present at the end of the polymerisations. These observations cast some doubts on the assumption made in the paper by the Liverpool school discussed above. A kinetic analysis of the initiation reaction showed it to be bimolecular, with a rate constant of about 130 sec at 20°C. The determination of the propagation rate constant was less strai tforward despite the fact that further monomer-addition experiments seemed to rule out any appreciable termination. The kp values fluctuated considerably as the initial catalyst concentration was varied, a fact which induced Rooney to propose that the empirical constant was a composite function of kp and kp. Experiments with a common-anion salt supported this proposal and their kinetic treatment led to the individual values of kp = 6 x 10 sec and kp = 5 x 10 sec. It is difficult to assess the reliability of these values in view of the following statement by the author the reaction at a 5 x 10 M concentration of initiator, thought to proceed exclusively through paired ions. .. . This statement is certainly incorrect as far as the initiator is concerned for which the proportion of ion pairs for a concentration 5 x 10 M at 20°C is only about 20% in methylene chloride However, the experiments... 

This simple relationship was derived before as equation 5.24, and was first used by Bauman and Eichorn in 1947 to predict selectivity sequences for simple monovalent cations from mean ionic activity coefficient data for pure aqueous electrolyte solutions containing a common anion. The inaccessibility of resin phase activity coefficients to direct measurement always remains a problem with thermodynamic equilibrium treatments. Therefore Glueckauf and others developed weight swelling and isopiestic water vapour sorption techniques to determine osmotic coefficients of pure salt forms of a resin, from which the mean ionic activity coefficients of mixed resinates could be computed using a modified form of Harned s Rule. Such studies predicted selectivity coefficient values which were in fair agreement with experiment and also demonstrated the fixed ion of the resin to be osmotically inactive. 

Some empirical rules can also be deduced regarding the behavior of the common anion. 

Based on the Wagner method [16], the required character of conductivity can be achieved by the introduction of admixture into the basis oxide, which has a common anion with the basis oxide, and the cation has less valence. Type and quantity of the defects are stipulated by the admixture and its concentration. As a rule, the majority of well-known solid electrolytes with pure oxygen-ionic conductivity have a fluorite CaFj crystalUne structure [17]. 

Preferential adsorption of one type of ions onto the particle surface coupled with the formation of a diffuse layer of the counterions (ions of opposite charge) leads to electrostatic stabilization due to repulsion between the double layers (Chapter 4). The valence and radius of the counterions can modify the repulsion between the particles and so can influence the stability of the suspension. Counterions with higher valence are more effective for causing flocculation Schulze-Hardy rule), while for ions of the same valence, the smaller ions are more effective. For monovalent cations, the effectiveness of flocculation is in the order Li > Na > K+ > NH4, while for divalent cations, Mg " > Ca " > Sr " > Ba. This sequence is known as the Hofmeister series. For common anions, the effectiveness of flocculation is in the order 804 > Cl > NOJ. 

The neat resin preparation for PPS is quite compHcated, despite the fact that the overall polymerization reaction appears to be simple. Several commercial PPS polymerization processes that feature some steps in common have been described (1,2). At least three different mechanisms have been pubUshed in an attempt to describe the basic reaction of a sodium sulfide equivalent and -dichlorobenzene these are S Ar (13,16,19), radical cation (20,21), and Buimett s (22) Sj l radical anion (23—25) mechanisms. The benzyne mechanism was ruled out (16) based on the observation that the para-substitution pattern of the monomer, -dichlorobenzene, is retained in the repeating unit of the polymer. Demonstration that the step-growth polymerization of sodium sulfide and /)-dichlorohenzene proceeds via the S Ar mechanism is fairly recent (1991) (26). Eurther complexity in the polymerization is the incorporation of comonomers that alter the polymer stmcture, thereby modifying the properties of the polymer. Additionally, post-polymerization treatments can be utilized, which modify the properties of the polymer. Preparation of the neat resin is an area of significant latitude and extreme importance for the end user. 

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