Question

Use the accompanying phase diagram for sulphur to answer the following questions. (The phase diagram is not to scale.)

a. How many triple points are in the phase diagram?

b. What phases are in equilibrium at each of the triple

points?

c. What phase is stable at room temperature and

1.0 atm pressure?

d. Can monoclinic sulphur exist in equilibrium with sulphur

vapor?

e. What are the normal melting point and normal boiling

point of sulphur?

f. Which is the denser solid phase, monoclinic or

rhombicsulphur?

Short answer

The given sulfur phase diagram shows different phase changes for different allotropic forms of sulfur, such as rhombic and monoclinic. In this, rhombic sulfur, monoclinic sulfur, liquid sulfur, and vapor sulfur are the phases are included.

Step-by-step solution

Step 1:The triple points in the given phase diagram

There arethree triple points involved in this phase diagram.

Three triple points represent the transition temperature between rhombic to monoclinic at ${95.5}^0\mathrm{C}$, the melting point of monoclinic sulfur ${120}^0\mathrm{C}$, and the existence of the systems at ${115}^0\mathrm{C}$ and $1290\mathrm{atm}$ .

Phases are in equilibrium at each of the triple points

The phase change occurs at the point where one form of sulfur changes to another form. Therefore the three phases that coexist in equilibrium at this triple point are

${\mathrm{S}}_{\mathrm{R}}\leftrightarrow {\mathrm{S}}_{\mathrm{M}}\leftrightarrow {\mathrm{S}}_{\mathrm{V}}$

${\mathrm{S}}_{\mathrm{M}}\leftrightarrow {\mathrm{S}}_{\mathrm{L}}\leftrightarrow {\mathrm{S}}_{\mathrm{V}}$

${\mathrm{S}}_{\mathrm{R}}\leftrightarrow {\mathrm{S}}_{\mathrm{M}}\leftrightarrow {\mathrm{S}}_{\mathrm{L}}$

Where ${\mathrm{S}}_{\mathrm{R}}$= rhombic sulfur phase

${\mathrm{S}}_{\mathrm{M}}$= monoclinic sulfur phase

${\mathrm{S}}_{\mathrm{V}}$= vapor sulfur phase

${\mathrm{S}}_{\mathrm{L}}$= liquid sulfur phase

The phase is stable at room temperature and1.0 atm pressure 

At room temperature and 1.0 atm, therhombic sulfur phase (SR)is stable on heating. It transforms into monoclinic sulfur. This temperature is calledtransition temperature.

Rhombic sulfur has an octahedral structure consisting 8 atoms which makes this allotrope most stable than monoclinic.

The monoclinic sulfur exists in equilibrium with sulfur vapor 

At thesublimation curve, the monoclinic sulfur exists in equilibrium with sulfur vapor. Monoclinic sulfur exists in equilibrium with vapor sulfur at the temperature of ${95.5}^0\mathrm{C}$ to ${115}^0\mathrm{C}.$

The triple point lies at which the three phases exist, i.e., liquid, monoclinic, and vapor.

The normal melting point and normal boiling point of sulfur

From the phase diagram, sulfur's normal melting and boiling points are ${112.8}^0\mathrm{C}$ and${444.6}^{0}C$.

Monoclinic sulfuris denser than rhombic sulfurdue to the high pressure at constant temperature applied, which causes the density of the phase to increase.