Q12.121CP - Chemistry: Molecular Nature Of Matter And Change

Question

Bismuth is used to calibrate instruments employed in high-pressure studies because it has several well-characterized crystalline phases. Its phase diagram (right) shows the liquid phase and five solid phases that are stable above 1 katm (1000 atm) and up to  $\mathbf{\text{300°C}}$ .

(a) Which solid phases are stable at  $\mathbf{\text{25°C}}$?

(b) Which phase is stable at 50 katm and $\mathbf{\text{175°C}}$ ?

(c) As the pressure is reduced from 100 to 1 katm at  $\mathbf{\text{200°C}}$, what phase transitions does bismuth undergo?

(d) What phases are present at each of the triple points?

Step-by-Step Solution

Answer

1. The solid phases that are stable at temperature  $\text{25°C}$ is Phase I, II and III.
2. The phase stable at 50 katm and temperature  $\text{175°C}$ is Phase IV.
3. The phase transition when the pressure is reduced from 100 to 1 katm at temperature $\text{200°C}$ is a phase I, IV and V.
4. The phase present at each of the triple points is phase IV.

Step 1: Definition

The phase diagram is the diagram showing the change in the phase of the molecule do the change in the temperature and pressure shown in the two-dimensional temperature at one axis and pressure at another axis.

 

The triple point is the point at which all the phases co-exist and after that temperature the phase transition. 

Step 2: Subpart (a) The solid phases are stable at 25°C .

The solid phases that are stable at temperature $25°\mathrm{C}$ is Phase I, II and III.

The solid phases are stable at the lower temperature as there is less effect of pressure. As the temperature increases, the pressure also increases which causes the phase transition.

Step 3: Subpart (b)The phase that is stable at 50 k atm and 175°C .

The phase stable at 50 katm and temperature $\text{175°C}$   is Phase IV.

The solid phases are stable at the lower temperature as there is less effect of pressure but at this high temperature $\text{175°C}$ , the pressure is also applied and there is the transition of phases in phases I, II, III and V.

As the temperature increases, the pressure also increases which causes the phase transition. Phase IV is stable at even that high temperature.

Step 4: Subpart (c) The phase transitions that bismuth undergoes when the pressure is reduced from 100 to 1 katm at 200°C .

The phase transition when the pressure is reduced from 100 to 1katm at temperature is a phase I, IV and V. 

The solid phases are stable at the lower temperature as there is less effect of pressure. As the pressure decreases, phases I, IV and V again become stable.

Step 5: Subpart (d) The phases that are present at each of the triple points.

The phase present at each of the triple points is phase IV.

Phase IV is stable at a triple point which shares the boundary with all the phases.

The change in temperature and pressure suddenly changes the phase.