Question

It is quite common for a solid to change from one structure to another at a temperature below its melting point. For example, sulphur undergoes a phase change from the rhombic crystal structure to the monoclinic crystal form at temperatures above $\mathbf{95}\mathbf{°}\mathbf{C}$

1. Predict the signs of $\mathbf{∆}\mathbf{H}\mathbf{}\mathbf{and}\mathbf{}\mathbf{∆}\mathbf{S}$for the process${\mathrm{S}}_{\mathrm{rhombic}}\to {\mathrm{S}}_{\mathrm{monoclinic}}$
2. Which form of sulphur has the more ordered crystalline structure (has the smaller positional probability)?

Short answer

Answer:

1. Entropy is positive as the randomness increases

Enthalpy is positive as the reaction is endothermic

b. Rhombic sulfur is more ordered lattice of sulfur

Step-by-step solution

Definition of enthalpy and entropy

The measure of total heat present in the thermodynamic system where the pressure is constant is defined as Enthalpy

The measure of disorder in a thermodynamic system is defined as Entropy

Sign of entropy and enthalpy

1. The process is${\mathrm{S}}_{\mathrm{rhombic}}\to {\mathrm{S}}_{{\mathrm{monoclinic}}^{-}}$

The monoclinic state of sulfur is more disordered in comparison to rhombic state to sulfur. Randomness increases. So the entropy is positive

The process is endothermic that is heat is absorbed in the reaction so enthalpy is positive

b. The process is${\mathrm{S}}_{\mathrm{rhombic}}\to {\mathrm{S}}_{{\mathrm{monoclinic}}^{-}}$

Rhombic lattice is more ordered in comparison to monoclinic lattice. In both the states the dimensions of cell are unequal

In case of rhombic state, the edges of unit cell are equal whereas the angles of unit cell are unequal in case of monoclinic state.

Rhombic sulfur is more ordered lattice of sulfur