Rhombic sulfur, illustration

The adsorption of dextrin molecules by sulfur has been reported to be similar to that observed for talc and coal. Hydrophobic interaction has been proposed to be the major adsorption mechanism (Brossard et al. 2008 Wisniewska 2005). In this study, a rhombic sulfur mineral was cut along the 111 plane to keep the integrity of each Sg molecule. Our simulation results, as illustrated in Figure 4.31, suggest that the behavior of the dextrin molecule at the sulfur surface is almost identical to its behavior at the graphite surface. 

PHYSICAL PROPERTIES Physical properties of chromic acid and specific chromates are provided for illustrative purposes, (chromic acid) dark-red, rhombic crystals or powder deliquescent (i.e., able to absorb atmospheric water vapor and become liquid) odorless true chromic acid, sulfuric acid, exists only in solution very soluble in water, soluble in sulfuric acid and most organic solvents MP (197.2°C, 387°F) BP (250°C, 482°F)(decomposes) DN (2.70 g/cm at 20°C) SG (2.70) VD (NA) VP (very low) (potassium chromate) lemon-yellow, rhombic crystals odorless solid soluble in water insoluble in alcohol, acetone, and cyanogen MP (975°C, 1787°F) BP (data not available) DN (2.73 g/cm at 18°C) SG (2.7 at 20°C) VD (NA) VP (approximately 0 mmHg at 20°C) (sodium dichromate) reddish to bright orange crystals odorless solid somewhat deliquescent anhydrous very soluble in water insoluble in alcohol MP (357°C, 674.6°F) BP (400°C, 752°F) (decomposes) DN (2.348 g/cm at 25°C) SG (2.34) VD (NA) VP (approximately 0 mmHg at 20°C). 

The difference and the similarity between allotropy and polymorphism can be illustrated by considering sulfur. The rhombic and monoclinic crystalline forms both consist of puckered Ss rings, and these two modifications can interconvert by heating and cooling. It is tempting to call this relationship crystal allotropy, but the correct term is polymorphism because both structures involve the same compound (i.e., atomic connectivity). When heated to above 160 °C, the Ss rings open by means of a free radical reaction to form polymeric chains. In contrast to crystal allotropy, the relationship between the polymeric chains and the Sg rings can be termed chemical allotropy. However, since polymorphism is the preferred term for crystal allotropy, chemical allotropy can be shortened to allotropy. 

The thermodynamic evidence for this conclusion is based on observations like the following. Sulfur undergoes a phase transition from its rhombic form to its monoclinic polymorph at 96 C (369 K) and the enthalpy of transition is -1-402 J mob. The entropy of transition is therefore -fl.09 J K mol at this temperature. We can also measure the molar entropy of each phase relative to its value at T = 0 by determining the heat capacity from T = 0 up to the transition temperature (Fig. 2.8). At this stage, we do not know the values of the entropies at 0. However, as we see from the illustration, to match the observed entropy of transition at 369 K, the molar entropies of the two crystalline forms must be the same at T=0. We cannot say that the entropies are zero at T = 0, but from the experimental data we do know that they are the same. This observation is generalized into the Third Law of thermodynamics ... 

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