Boiling point entropy change

Equation (3.7) gives a simple procedure for evaluating the entropy change accompanying a change of state. At the normal boiling point of a liquid, for example, the heat is absorbed reversibly and equals the heat of vaporization AH,. Since T is constant, the entropy of vaporization is AH,/T. For benzene, for example, AS, = (30.8 k J mol" )/353 = 87 J K mol. ... 

We can expect the entropy to increase when a solid melts and its molecules become more disordered. Similarly, we can expect an even greater increase in entropy when a liquid vaporizes, because its molecules then occupy a much greater volume and their motion is highly chaotic. In this section, we develop expressions for the change in entropy at the transition temperature for the prevailing pressure. For instance, if the pressure is 1 atm, then these expressions are applicable only at the normal melting point, Tf (the f stands for fusion), the temperature at which a solid melts when the pressure is 1 atm, or the normal boiling point, Th, the temperature at which a liquid boils when the pressure is 1 atm. 

The entropy of vaporization, ASvap, is the change in entropy per mole of molecules when a substance changes from a liquid into a vapor. The heat required per mole to vaporize the liquid at constant pressure is equal to the enthalpy of vaporization (A//vap, Section 6.11). It then follows from Eq. 1, by setting = AH, that the entropy of vaporization at the normal boiling point is... 

The standard entropy of vaporization of benzene is approximately 85 J-K -mol-1 at its boiling point, (a) Estimate the standard enthalpy of vaporization of benzene at its normal boiling point of 80.°C. (b) What is the standard entropy change of the surroundings when 10. g of benzene, CfiHfi, vaporizes at its normal boiling point ... 

The graph should be similar to that for 2 Mg(s) + 02(g) —> 2 MgO(s). We expect a positive slope with slight changes in the slope after the melting point(839 °C) and boiling points(1484 °C). owing mainly to changes in entropy. The plot will always below the AG° line for 2 Mg(s) + 02(g) — 2 MgO(s) at all temperatures. 

At its boiling point, what is the entropy change of benzene from a liquid to vapor per gram Use Table 6.1. 

If palladium dichloride is heated, it decomposes, and at 1195°K its chlorine pressure reaches 1 atmosphere. This decomposition point is determined by an equation similar to that for the boiling point. In this case, too, a gas, with a very high entropy, is formed from a solid that always has a relatively low entropy. Here, again, the change in entropy per mol of gas formed is almost independent of the decomposing compound and its decomposition products. 

For very many liquids, the entropy of vaporization at the normal boiling point is approximately 21 cal/mole °C water is not typical. The units for changes in entropy are the same as those for molar heat capacity, and care must be used to avoid confusion. When referring to an entropy change, a cal/mole °C is often called an entropy unit, abbreviated e.u. In order to avoid later misunderstanding, note now that this method of calculating AS from A HIT is valid only under equilibrium conditions. For transitions, for example, this method can be used only at temperatures where the two phases in question can coexist in equilibrium with each other. 

The function describing the change in entropy, as a function of temperature, involves the use of a prescription that contains a formula specific to a particular phase. At each phase transition temperature the function suffers a finite jump in value because of the sudden change in thermodynamic properties. For example, at the boiling point 7b the sudden change in entropy is due to the latent heat of evaporation (see Figure 2.8). 

Similarly, we can estimate the change of entropy for vaporization using the molar heat of vaporization AHv at boiling point Tb... 

At the normal melting point or boiling point of a substance the two states of matter present at that temperature and at 1 atm pressure are in equilibrium. That is, the two states can coexist indefinitely if the system is isolated (left totally undisturbed). Recall that a reversible process can occur only at equilibrium. Thus, since a change of state from solid to liquid at the substance s melting point is a reversible process, we can calculate the change in entropy for this process by using the equation... 

These enthalpies and entropies are of course a function of the structure of the compound. The thermodynamic quantities for vaporization were discussed previously (page 14), and it was seen that this factor varied in a reasonably predictable way with a change in structure. The thermodynamic quantities for sublimation are the sum of those for vaporization and for fusion, and it is now necessary to consider the latter. These are not as simple a function of the structure as is the boiling point, because they depend on the crystal structure which is possible with the compound and on short-range attractive forces which operate in the crystal. Certain generalizations may, however, be made. 

Calculate the entropy change when 3.00 mol benzene vaporizes reversibly at its normal boiling point of 80.1°C. The molar enthalpy of vaporization of benzene at this temperature is 30.8 kj moPh... 

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