Vaporization entropy changes

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. ... 

Self-Test 7.14A Calculate the entropy change of the surroundings when 1.00 mol H20(l) vaporizes at 90°C and I bar. Take the enthalpy of vaporization of water as 40.7 kj-mol. ... 

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 ... 

A change in entropy is equal to the heat supplied reversibly to a system divided by the temperature at which the transfer takes place, entropy of vaporization (AS ) The entropy change per mole accompanying vaporization (the conversion of a substance from the liquid state into the vapor state). 

Continuing the process we may now change the pressure on the gas to the vapor pressure of the glass as given by (12) and then carry out a reversible condensation. These steps will be seen to involve the entropy change... 

C14-0121. Apiece of dry ice at T = 195 K is dropped into a beaker containing water at T = 273.15K.You may assume that all of the resulting CO2 vapor escapes into the atmosphere, (a) Describe the process that takes place at the molecular level, (b) Calculate the overall entropy change if 12.5 g of diy ice undergoes this process. 

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

The next set of examples show an entropy of adsorption roughly equal to the entropy change on losing the degree of translational freedom normal to the surface, i.e., in the adsorbed state the molecules are equivalent to a two-dimensional gas or vapor. The data for a variety of different adsorbates and adsorbents is given in Table V. The isotherms obtained by Armbruster (20) for the adsorption of CO and N2 on silver were not S-shaped, and they could be fitted to equations of the Langmuir type. The amount of adsorbate required to saturate the surface was given for each substance at both temperatures. Armbruster calculated the heats of adsorption by the method of Brunauer, Emmett and Teller (22) and there is some doubt about the validity of such heats. 

This is a strongly endothermic process, but it becomes possible at high temperature due to a favorable entropy change - formation of the random vapor state from solid reactants. Such reactions provide another reason for the lower flame temperatures achieved when organic binders are added to oxidizer/metal mixtures [3]. 

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. 

Self-Test 7.11B Calculate the entropy change in the surroundings when, 1.00 mol NH3(g) condenses at — 30°C. Take the enthalpy of vaporization of ammonia as 23.4 kJ-moC1. 

E° = E°(cathode) — °(anode). standard enthalpy of combustion AHc° The change of enthalpy per mole of substance when it bums (reacts with oxygen) completely under standard conditions, standard enthalpy of formation AH° The standard reaction enthalpy per mole of compound for the compound s synthesis from its elements in their most stable form at 1 atm and the specified temperature, standard entropy of fusion ASfus° The standard entropy change per mole accompanying fusion (the conversion of a substance from the solid state to the liquid state), standard entropy of vaporization ASvap° The standard entropy change per mole accompanying vaporization (the conversion of a substance from the liquid state to the vapor state). 

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