Question

Sulfur is allotropic -- two crystalline forms of the element are stable under conditions not far from standard. The transition temperature from rhombic to monoclinic sulfur is 95.4 ˚C; the enthalpy of transition is 0.38 kJ per mol of sulfur atoms. Monoclinic sulfur melts at 119 ˚C; the enthalpy of fusion is 1.23 kJ per mol of sulfur atoms.

(a) Calculate the entropy change accompanying each of these phase transitions per mole of S atoms.

(b) Consider then that these stable forms of crystalline sulfur contain S8 molecules, and calculate the entropy changes per mol of this species. (The latter values are more representative of typical magnitudes of entropies of solid-state transition and fusion.)

Answer 1

We kow that, entropy change is calculated as

where S = entropy change

H = enthalpy change

T = Temperature

So,

(a)

                           =

                           = 1.03 x 10^-3 kJ K^-1 mol^-1

                           = 1.03 J K^-1 mol^-1

Entropy of fusion:

                 =

                 = 3.13 x 10^-3 kJ K^-1 mol^-1

                 = 3.13 J K^-1 mol^-1

(b) For S₈ molecule, each of the values obtained in part (a) should be multiplied by 8.

So, = 8 x 1.03 J K^-1 mol^-1 = 8.24 J K^-1 mol^-1

and = 8 x 3.13 J K^-1 mol^-1 = 25.04 J K^-1 mol^-1