Diamond entropy

Harkins then estimated Tc for diamond to be about 6700 K and, using Eq. HI-10, found the entropy correction at 25°C to be negligible so that the preceding values also approximate the room temperature surface free energies. These... 

C(s, diamond) < Ff20(s) < H20(1) < H20(g). Diamond, being a covalently bound monatomic solid, has less disorder than H20(s), which is held together by weaker interniolecular forces. The three phases of water increase in entropy in changing from a solid to a liquid to a gas. 

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. 

By convention, the standard Gibbs function for formation AfG of graphite is assigned the value of zero. On this basis, AfG gg of diamond is 2900 J mol Entropies and densities also are listed in Table 8.2. Assuming that the entropies and densities are approximately constant, determine the conditions of temperature and pressure under which the manufacture of diamonds from graphite would be thermodynamically and kinetically practical [2]. 

In 1907. Einstein showed that at extremely low temperatures, the atoms of a solid don t have sufficient energy to jump to the first quantized energy level, which is a relatively large jump. The solid, therefore, may be exposed to small increments of heat without any increase in thermal motion. This lowers the ability of the solid to absorb heat, which means that its entropy is also lower. For practically all materials, this quantum effect only occurs at extremely low temperature. A dramatic exception is diamond, which because of this quantum effect resists the absorption of energy even at room temperature (Table 9.2). Diamond is special for many reasons, including its status as a room-temperature "quantum solid"... 

The enthalpy of combustion of diamond is -94 50 kcal/mole The enthalpy of combustion of graphite is -94 05 kcal/mole What is the standard enthalpy and entropy of transition from diamond to graphite0... 

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... 

J-K-1-mol-1 AG° = +2.900 kj-mol-1. Graphite has a delocalized structure similar to that of benzene, whereas, in diamond, all the carbon atoms are bonded to four other carbon atoms in a very rigid lattice. The delocalization gives graphite a higher entropy. 

Values of AG°f at 25°C for some common substances are listed in Table 17.3, and additional values are given in Appendix B. Note that AG°f for an element in its most stable form at 25°C is defined to be zero. Thus, solid graphite has AG°f = 0 kj/mol, but diamond, a less stable form of solid carbon at 25°C, has AG°f = 2.9kJ/mol. As with standard enthalpies of formation, AH°f, a zero value of AG°f for elements in their most stable form establishes a thermochemical "sea level," or reference point, with respect to which the standard free energies of other substances are measured. We can t measure the absolute value of a substance s free energy (as we can the entropy), but that s not a problem because we are interested only in free-energy differences between reactants and products. 

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. 

Thermodynamics, as noted earlier, deals with the effects of heat and entropy on equilibrium and is useful for predicting the extent to which a reaction will occur. But like an obstreperous child avoiding chores, thermodynamics may tell you the reaction will take place, but not when. For example, thermodynamics predicts that graphite is a more stable form of pure carbon than diamond, so all diamonds will eventually turn to graphite. But before you start adjusting your investment portfolio, be advised that this transition may take billions of years. 

First-order solid-state amorphization occurs due to an entropy catastrophe [39] causing melting of superheated graphite and decompressed diamond below Pg when the entropy of the ordered crystal would exceed the entropy of the disordered liquid. This condition is resolved with the occurrence of a kinetic transition to a (supercooled) glass whereby the exact kinetic conditions during carbon transformation will be critically Pg-depen-dent [39]. It is important to consider the crystal to liquid transition and the effect of a superheated crystal whereof the ultimate stability is determined by the equality of crystal and liquid entropies [40]. When this condition is met, a solid below its Pg will melt to an amorphous solid, particularly... 

Negative. Diamond, being a harder solid, would be expected to have more restricted atomic motions within the crystal. Diamond is denser and has less entropy on that account also. 

Face-centered cubic crystal lattice. Burns when heated with a hot enough flame (over 800, oxygen torch), df 3.513. rt 2 4173. Hardness — 10 (Mohs scale), Sp heat at 100°K 0.606 cal/g-atom/ K. Entropy at 298.I6 K 0,5684 cal/g-atom/ K. Band gap energy 6.7 ev. Dielectric constant 5.7. Electron mobility —1800 cm1/v-sec. Hole mobility 1200 cmz/v-sec. Can be pulverized in a steel mortar. Attacked by laboratory -type cleaning soln (potassi um dichromate + coned HiSO ), In the jewelry trade the unit of weight for diamonds is one carat — 200 mg. Ref Wall Street J. 164, no. 36, p 10 (Aug 19, 1964),... 

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