   # What is the standard free energy change, ∆ G °, for a reaction? What is the standard free energy of formation, Δ G f ∘ for a substance? How are Δ G f ∘ values used to calculate Δ G rxn ∘ ? How can you use Hess’s law to calculate Δ G rxn ∘ ? How can you use ∆H ° and ∆ S ° values to calculate Δ G rxn ∘ ? Of the functions ∆ H°, ∆ S °, and ∆ G °, which depends most strongly on temperature? When ∆ G ° is calculated at temperatures other than 25°C，what assumptions are generally made concerning ∆ H° and ∆ S °? ### 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 6RQ
Textbook Problem
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## What is the standard free energy change, ∆G°, for a reaction? What is the standard free energy of formation, Δ G f ∘ for a substance? How are Δ G f ∘ values used to calculate Δ G rxn ∘ ? How can you use Hess’s law to calculate Δ G rxn ∘ ? How can you use ∆H° and ∆S° values to calculate Δ G rxn ∘ ? Of the functions ∆H°, ∆S°, and ∆G°, which depends most strongly on temperature? When ∆G° is calculated at temperatures other than 25°C，what assumptions are generally made concerning ∆H° and ∆S°?

Interpretation Introduction

Interpretation: The answers of various questions based upon Gibbs free energy and Hess law 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.

### Explanation of Solution

The standard free energy (ΔGrxn0)  is the difference in the sum of the free energies of all the products and all the reactants.

The standard free energy of formation ΔGf0 is the change in energy during the formation of 1 mole of a substance from its constituent elements keeping all the reactants and products in their standard states.

The values of ΔGf0 are used to calculate ΔGrxn0 .

Consider a general reaction.

Cdiamond(s)+O2(g)CO2(g)ΔG10=397kJ (1)

Cgraphite(s)+O2(g)CO2(g)ΔG20=394kJ (2)

The equation (2) is reversed. The resultant reaction is,

Cdiamond(s)Cgraphite(s)

The ΔGrxn0 is calculated as,

ΔGrxn0=ΔG20ΔG10=397kJ+394kJ=3kJ

Therefore, Hess law is used to calculate ΔGrxn0

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