   Chapter 17, Problem 49E

Chapter
Section
Textbook Problem

# It is quite common for a solid to change from one structure to another at a temperature below its melting point. For example, sulfur undergoes a phase change from the rhombic crystal structure to the monoclinic crystal form at temperatures above 95°C.a. Predict the signs of ∆H and ∆S for the process Srhombic(s) → Smonoclinic(s).b. Which form of sulfur has the more ordered crystalline structure (has the smaller positional probability)?

(a)

Interpretation Introduction

Interpretation: It is given that, sulfur undergoes a phase change from the rhombic crystal structure to the monoclinic crystal form at temperatures above 95°C . The sign of ΔS and ΔH is to be predicted for this change. The form of sulfur that has the more ordered crystalline structure is to be identified.

Concept introduction: Entropy is the measure of degree of disorder or randomness. The value of ΔS is calculated using the formula,

ΔS=ΔHT

Explanation

The stated process is,

Srhombic(s)Smonoclinic(s)

Phase change occurs at higher temperature. But, higher the temperature, higher will be the movement (randomness) of atoms. It means that entropy is increasing in this process. Therefore, the sign of ΔS is positive.

Given

Temperature is 95°C

(b)

Interpretation Introduction

Interpretation: It is given that, sulfur undergoes a phase change from the rhombic crystal structure to the monoclinic crystal form at temperatures above 95°C . The sign of ΔS and ΔH is to be predicted for this change. The form of sulfur that has the more ordered crystalline structure is to be identified.

Concept introduction: Entropy is the measure of degree of disorder or randomness. The value of ΔS is calculated using the formula,

ΔS=ΔHT

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