Diamond molar entropies

C14-0135. S ° of graphite is 3 times larger than S ° of diamond. Explain why this is so. (You may need to review the structures and properties of graphite and diamond in Section 11-.) Buckminsterfullerene is a solid that consists of individual molecules with formula Cgo. Is the molar entropy of buckminsterfullerene larger or smaller than that of graphite How about the entropy per gram Explain. 

We can understand some of the differences in standard molar entropies in terms of differences in structure. For example, let s compare the molar entropy of diamond, 2.4 J-K 1-mol, with the much higher value for lead, 64.8 JKr -mol-1. The low entropy of diamond is what we should expect for a solid that has rigid bonds at room temperature, its atoms are not able to jiggle around as much as the atoms of lead, which have less directional bonds, can. Fead also has much larger atoms... 

Diamond is the hardest solid known. It has a high density and index of refraction, the highest mp ( 4000°C), thermal conductivity (25 W/cm °C at room temperature, more than six times that of copper), and lowest molar entropy (2.4 J mol K 1) of any element. 

A rule to bear in mind is that, at the same pressure, temperature and particle number, the entropy of a body will be greater, the heavier the atoms and the weaker the bonding forces. Diamond, which consists of atoms that are rather light and very firmly linked in four directions, has an unusually low entropy per mole. Lead, on the other hand with its heavy, loosely bound atoms, is rather rich in entropy. The characteristics of iron lie somewhere in between it has a medium value of molar entropy. Using the example of water, the table shows how entropy increases by transition from a solid to a liquid state and even more by transition from a liquid to a gaseous state. 

Relative Standard Entropies Allotropes As mentioned previously, some elements can exist in two or more forms—called allotropes— in the same state of matter. For example, the allotropes of carbon include diamond and graphite— both solid forms of carbon. Since the arrangement of atoms within these forms is different, their standard molar entropies are different ... 

The values of all standard molar entropies are positive. Remember that elements have positive standard molar entropy values. Do not get this muddled with the case of enthalpies, where the elements in their standard states have entropy values of zero. The entropy values are compared to a theoretically perfect crystal. The Third Law of Thermodynamics states that All perfect crystals have the same entropy at a temperature of absolute zero . The nearest we can get to this is a perfect diamond weighing 12 g cooled to as low a temperature as possible. 

Graphite and diamond are both forms of carbon. Their standard molar entropies are fraphite = 5-70 J mol-, = 2.40 J K-1 mol-1... 

Suggest why the standard molar entropy of graphite is greater than that of diamond. 

Stream Information. Directed arcs that represent the streams, with flow direction from left to right wherever possible, are numbered for reference. By convention, when streamlines cross, the horizontal line is shown as a continuous arc, with the vertical line broken. Each stream is labeled on the PFD by a numbered diamond. Furthermore, the feed and product streams are identified by name. Thus, streams 1 and 2 in Rgure 3.19 are labeled as the ethylene and chlorine feed streams, while streams 11 and 14 are labeled as the hydrogen chloride and vinyl-chloride product streams. Mass flow rates, pressures, and tempera-mres may appear on the PFD directly, but more often are placed in the stream table instead, for clarity. The latter has a column for each stream and can appear at the bottom of the PFD or as a separate table. Here, because of formatting limitations in this text, the stream table for the vinyl-chloride process is presented separately in Table 3.6. At least the following entries are presented for each stream label, temperature, pressure, vapor fraction, total and component molar flow rates, and total mass flow rate. In addition, stream properties such as the enthalpy, density, heat capacity, viscosity, and entropy, may be displayed. Stream tables are often completed using a process simulator. In Table 3.6, the conversion in the direct chlorination reactor is assumed to be 100%, while that in the pyrolysis reactor is only 60%. Furthermore, both towers are assumed to carry out perfect separations, with the overhead and bottoms temperatures computed based on dew- and bubble-point temperatures, respectively. 

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