Download Polymorphs of Sulfur

CHEMISTRY 1b DISCUSSION PROBLEM
POLYMORPHS OF SULFUR
Elemental sulfur has two crystalline polymorphs:
a) orthorhombic sulfur with a density of 2.07 g/cm3
b) monoclinic sulfur with a density of 2.01 g/cm3
1) Which form has the greater standard entropy? Explain.
2) Sulfur reacts exothermically with oxygen to form sulfur dioxide. In
magnitude (i.e. absolute value), monoclinic sulfur has the larger enthalpy of
combustion. Which form has the lower enthalpy of formation, a signed
quantity?
3) Which form is stabler at low temperature?
4) The absolute value of the standard entropy and enthalpy changes for the
interconversion of the two forms are 1.090 J/K-mole and 402 J/mole,
respectively. Calculate the transition temperature, i.e. the temperature where
the 2 forms are in equilibrium.
5) Will the transition temperature increase or decrease with an increase in
pressure?
6) If one rapidly cools the high-temperature form below the transition
temperature, it initially retains the high-temperature structure and only
converts to the low-temperature form over a period of days. Suggest why
this rate might be slow.
7) Sulfur is soluble in carbon disulfide. Which polymorph has the higher
solubility at room temperature? [The dependence of solubility on crystal
structure is an important issue in drug design.]