   # What is the third law of thermodynamics? What are standard entropy values, S° , and how are these S° values (listed in Appendix 4) used to calculate ∆ S ° for a reaction? How would you use Hess’s law to calculate ∆ S ° for a reaction? What does the superscript ° indicate? Predicting the sign of ∆ S ° for a reaction is an important skill to master. For a gas-phase reaction, what do you concentrate on to predict the sign of ∆ S °? For a phase change, what do you concentrate on to predict the sign of ∆ S °? That is, how are S solid ∘ , S liquid ∘ , and S gas ∘ related to one another? When a solute dissolves in water, what is usually the sign of ∆ S ° for this process? ### Chemistry: An Atoms First Approach

2nd Edition
Steven S. Zumdahl + 1 other
Publisher: Cengage Learning
ISBN: 9781305079243

#### Solutions

Chapter
Section ### Chemistry: An Atoms First Approach

2nd Edition
Steven S. Zumdahl + 1 other
Publisher: Cengage Learning
ISBN: 9781305079243
Chapter 16, Problem 5RQ
Textbook Problem
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## What is the third law of thermodynamics? What are standard entropy values, S°,  and how are these S° values (listed in Appendix 4) used to calculate ∆S° for a reaction? How would you use Hess’s law to calculate ∆S° for a reaction? What does the superscript ° indicate?Predicting the sign of ∆S° for a reaction is an important skill to master. For a gas-phase reaction, what do you concentrate on to predict the sign of ∆S°? For a phase change, what do you concentrate on to predict the sign of ∆S°? That is, how are S solid ∘ , S liquid ∘ , and S gas ∘ related to one another? When a solute dissolves in water, what is usually the sign of ∆S° for this process?

Interpretation Introduction

Interpretation: The answers of various questions based upon entropy changes are to be stated.

Concept introduction: Thermodynamics is associated with heat, temperature and its relation with energy and work. It helps us to predict whether a process will take place or not. But it gives no information about the time required for the process. The terms associated with thermodynamics are system, surrounding, entropy, spontaneity and many more.

To determine: The answers of various questions based upon entropy changes

### Explanation of Solution

Third law describes the properties of a system at absolute zero temperature.

The third law states that at absolute zero temperature, the molecular motion in a substance ends. This means that the entropy of a perfectly crystalline substance is zero because there is no randomness.

The term Sο is the standard entropy value of a substance.

The term Sο from the standard entropy data indicates the standard entropy value of substances in their standard state. The standard temperature is 25οC and standard pressure is 1atm.

The standard entropy values are used to calculate the standard entropy change ΔSο for a reaction.

Consider a reaction given below.

Cdiamond(s)+O2(g)CO2(g)ΔS1ο=XJ/K (1)

Cgraphite(s)+O2(g)CO2(g)ΔS2ο=YJ/K (2)

Where,

• ΔS1ο is the standard change in entropy for equation (1).
• ΔS2ο is the standard change in entropy for equation (2).

Subtract equation (2) from equation (1) and the resultant equation is,

Cdiamond(s)Cgraphite(s)

The value of ΔSο for the above reaction is calculated by subtracting ΔS2ο from ΔS1ο

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