Question

Sample problems 3: How does the entropy of the system change when the following occur: a) a so vaporizes; ce) a solid dissolves in water; d) a gas liquefies? as 2. The normal boiling point of methanol (CHsOH) is 64.7 Co, and its molar enthalpy of vaporization is Hap -71.8 kJ/mol. A) When CHoOH 1) boils at its normal boiling point, does its entropy increase or decrease? B) Calculate the value of AS when 1.00 mol of CHOH() is vaporized at 64.7 C 3. The element cesium (Cs) freezes at 28.4 Co, and its molar enthalpy of fusion is ???"209 kJ/mol. a) When molten cesium solidifies to Cs (s) at its normal melting point, is AS positive or negative? b) Calculate the value of AS when 15.0 g of Cs (1) solidifies at 28.4 Co

Answer 1

Entropy represents the randomness of a system. Entropy being the highest in gases followed by liquids and then solids. Melting of a solid changes solid to liquid state. Hence, entropy increases vaporizing liquid changes liquid to gas. hence, entropy increases. Solid dissolving in liquid enables free movement of its particles inside the solvent, thereby increasing analomness. Hence, entropy increases. Liquification involves changing from gas to liquid. Hence, entropy decreases. Entropy of C H_3 O H on boiling increases as it involves phase change from liquid to gaseous state. Delta S_vaporisation = Delta H_vaporisation/T_vaporisation Delta H_ vaporization = 71.8 K J/mol T_ vaporization = 64.7 degree C = (64.7 + 273.15) K = 337.85 K Delta S_ vaporization = 71.8 K J/mol times 10^3 J/1 kg/337.85 K Delta S_ vaporization = 212.5 J mol^-1 K^-1 Therefore Delta S for 1 mole = 212.5 J mol^-1 K^-1 times 1 mol = 212.5 J K^-1