Question

(i) The element, S (sulfur), undergoes a phase transition from a rhombic form to a monoclinic form at 95.4°C. If ΔHtransition=0.38 kJ mol-1, calculate ΔStransition under the above conditions. Show all working. (ii) Monoclinic sulfur is known to melt at 119°C. Given ΔHfusion=1.23 kJ mol-1, calculate, showing all working, ΔSfusion. (iii) Given the S occurs as S8 in solid or liquid forms, convert the values of the enthalpy and entropy of fusion above to those expected for S8.

Answer 1

Part a

Entropy of transition

ΔStransition = ΔH transition / T transition

= ( 0.38 kJ/mol) / (95.4 + 273.15)K

= 0.001031 kJ/mol·K x 1000J/kJ

= 1.031 J/mol·K

Part b

ΔS fusion = ΔH fusion / T fusion

= ( 1.23 kJ/mol) / (119 + 273.15)K

= 0.00314 kJ/mol·K x 1000J/kJ

= 3.14 J/mol·K

Part c

The above values are calculated on the basis of 1 mol

For S8, moles = 8

Multiply by 8

ΔH transition = 8 x 0.38 = 3.04 kJ/mol

ΔStransition = 8 x 1.031 = 8.248 J/mol·K

ΔHfusion = 1.23 x 8 = 9.84 kJ/mol

ΔS fusion = 8 x 3.14 = 25.12 J/mol·K