How to find the standard free energy of formation?

Make sure the chemical reaction is under standard conditions.Use the Gibbs free energy equation.Subtract the reactants’ Gibbs free energy of formation by the products’ Gibbs free energy of formation.

Free Energy of Formation

Q: How to calculate free energy of formation?

You calculate the free energy of formation using the Gibbs free energy equation.

Q: What substance has zero Gibbs free energy of formation?

Substances in their natural state have zero Gibbs free energy of formation, such as Br2 (g) and I2 (s).

Q: What does free energy of formation mean?

The free energy of formation refers to the Gibbs free energy change when 1 mole of a substance is formed.

Q: What is the standard free energy of formation for nh4hs?

The standard free energy formation is -0.62 kJ/mol. This means that since G < 0 the reaction is exergonic and spontaneous. An exergonic reaction means that energy is released to the surroundings, as the bonds being made are stronger than the ones being broken.

Chemical Analysis

Chemical Reactions

Chemistry Branches

History of Chemistry

Inorganic Chemistry

Ionic and Molecular Compounds

Kinetics

Making Measurements

Nuclear Chemistry

Organic Chemistry

Physical Chemistry

The Earths Atmosphere

TABLE OF CONTENTS :

TABLE OF CONTENTS

Lerne mit deinen Freunden und bleibe auf dem richtigen Kurs mit deinen persönlichen Lernstatistiken

Jetzt kostenlos anmelden

Nie wieder prokastinieren mit unseren Lernerinnerungen.

Jetzt kostenlos anmelden

Have you ever tried calculating the energy of a chemical reaction? If so, then you’ve probably already dealt with heat changes (enthalpy) and disorder of systems (entropy). Scientists usually use the Gibbs free energy equation to combine these two values into a single formula to be able to describe the formation of compounds from their constituent elements at the standard state.

In this article, we will learn all about the Gibbs free energy of formation: what it is, what is tells us, and how to calculate it.

This article is about the free energy of formation.
First, we’ll define the free energy of formation.
Then, we’ll relate it to Gibbs Free Energy and Enthalpy.
Next, we’ll look at a free energy formation chart and equation.
Lastly, we’ll go over examples of free energy of formation, featuring water and ammonia.

The Free Energy of Formation Definition

Free energy, G, is the energy available to do work.

The (standard) Gibbs free energy of formation (ΔG_f°) refers to the free energy change when 1 mole of a substance is formed from its component elements in their reference states.

The reference state is the most thermodynamically stable state at standard temperature (1 bar) and standard temperature (25 °C). It is used as a reference point to calculate certain properties under a given set of conditions.

The formula for the Gibbs free energy of formation is:

$$\Delta G^\circ_f=\Sigma \Delta G^\circ_{f\,(products)}- \Sigma \Delta G^\circ_{f\,(reactants)}$$

The "Σ" symbol is called a summation symbol. What this means is that all the GIbbs free energy values for the products/reactants are added. For example, this would look like: $$\Sigma \Delta G^\circ_{f\,(products)}=\Delta G^\circ_{f\,a}+\Delta G^\circ_{f\,b}$$ Where a and b are products

For example, here is the Gibbs free energy of liquid water:

$$2H_{2\,(g)} + O_{2\,(g)} \rightarrow 2H_2O_{(l)}:\,\,\,\Delta G^\circ_f=-228.6\frac{kJ}{mol}$$

Meanwhile, the standard enthalpy, or heat of formation (ΔH_f°), is the heat absorbed or released when 1 mole of a substance is formed from its respective elements in their standard states (Standard elements in their reference form have a free energy of formation of zero).

Both the standard enthalpy of formation (ΔH_f°), and entropy of formation (S°), relate to Gibbs free energy, as we use the Gibbs free energy equation to calculate both entropy and enthalpy together.

Enthalpy is the total change in heat within a system when pressure is constant. While entropy is the randomness or disorder of a system.

For more information regarding enthalpy and Gibbs free energy, please refer to our “Enthalpy” and “Gibbs Free Energy” articles.

The Free Energy of Formation Equation

Now that we understand each term in the free energy of formation equation, we can begin to understand the equation itself.

$$ \Delta G^\circ = \Delta H^\circ - T \Delta S^\circ $$

Where

$ \Delta G^\circ $ is the change in free energy
$ \Delta H^\circ $ is the change in enthalpy or heat
$T$$ \Delta S^\circ $ is the change in entropy (S) and temperature (T)

The free energy of formation basically refers to the change in Gibbs Free Energy when 1 mole of product is formed.

An important question regarding this equation is:

What does $\Delta G$'s sign say about the equation?

$\Delta G$ tells you the direction of a chemical reaction and if it's spontaneous or not.

A spontaneous reaction is a reaction that proceeds on its own without addition of external energy. The reaction favors the formation of products.

A non-spontaneous reaction requires an addition of external energy. The reaction does not favor the formation of products

$\Delta G$ > 0 exergonic reaction = spontaneous

$\Delta G$ = 0 the system is at equilibrium

$\Delta G$ < 0 endergonic reaction = not spontaneous

At equilibrium, there is no net change in the concentration of products and reactants.

$$\Delta G^\circ=\Sigma \Delta G^\circ_{products}- \Sigma \Delta G^\circ_{reactants}$$

You find the free energy of formation in a reaction by subtracting the sum of the free energy of formation for each reactant from the sum of the free energy of formation for each product.

On top of knowing about the Gibbs free energy signs, we should also understand what exergonic and endergonic reactions are:

Free Energy of Formation Exergonic vs Endergonic Reaction Vaia Fig. 1 - Exergonic vs. Endergonic reaction. Daniela Lin, Vaia Originals.

Exergonic means that energy is released to the surroundings, as the bonds being made are stronger than the ones being broken.

In contrast, endergonic signifies that the energy is absorbed from the surroundings since the bonds being broken are stronger than the ones being made.

Exergonic and endergonic reactions are similar to exothermic and endothermic reactions. Exergonic/endergonic is a measure of both enthalpy and entropy (i.e., free energy), while exothermic/endothermic is only a measure of enthalpy.

The Free Energy of Formation Table and Chart

The free energy of formation table and chart is given to establish a better understanding of how the Gibbs free energy equation, the free energy of formation, and enthalpy play a role when it comes to chemical reactions.

The chart below contains some of the most common compounds and their free energies of formation. Later on, we’ll go over some examples utilizing this chart.

Fig. 2 - The Free Energy of Formation Table and Chart of the most common compounds. Daniela Lin, Vaia Originals.

Some common free energy data are displayed in alphabetical order above. Notice how the elements in their standard states have a free energy formation of zero.

Free Energy of Formation Examples

Free Energy of Formation of Water

Calculate the free energy of formation for 5 moles of water.

To calculate the free energy formation of water we first write out the reaction for the formation of 1 mole of water:

$H_2 (g)+ \frac {1} {2} O_2 (g) \longrightarrow H_2O (l) $

$ \Delta G^\circ_f $ = -237.129 kJ/mol according to our table from earlier.

If we instead form 5 moles of water then our equation becomes:

$5H_2 (g)+ \frac {5} {2} O_2 (g) \longrightarrow 5H_2O (l) $

$ \Delta G^\circ_f $ = 5 x -237.129 kJ/mol= -1,185.7 kJ/mol

The free energy of the formation of water is already given by our table. But notice that for 5 moles of water we just multiply by 5 and for decomposition we just reverse the sign.

For the decomposition of water, the sign would just change to +237.129 kJ/mol as decomposition is the opposite of formation.

Gibbs Free Energy of Formation of Ammonia

Calculate the free energy of formation of ammonia.

To calculate the free energy of ammonia we first write out the reaction for the formation of ammonia:

$N_2 (g) + 3H_2 (g) \longrightarrow 2NH_3 (g) $

$ \Delta G^\circ_f $ = -16.45 kJ/mol for $NH_3$ according to our table from earlier.

$ \Delta G^\circ_f $ = 0 for both $N_2$ and $3H_2$ as they're both in their elemental standard states.

Since there are 2 moles of $NH_3$ formed in our balanced equation, we need to multiply -16.45 by 2, which makes the answer:

$ \Delta G^\circ_f $ = -32.90 kJ/mol.

The free energy of the formation of ammonia is already given in our table. But notice for nitrogen and hydrogen gas the free energy of formation is zero due to them being in their standard elemental states.

We’ve reached the end of the article. Now you should understand what the free energy of formation is, why it works, and how to apply it. For more practice, please head over to our flashcards section!

Free Energy of Formation - Key takeaways

The free energy of formation refers to the energy change when 1 mole of a substance is formed.
The standard enthalpy or heat of formation is the heat absorbed or released when 1 mole of a substance is formed from its respective elements in their standard states.
Both the standard enthalpy of formation and free energy of formation relate to Gibbs free energy, as we use Gibbs free energy equation to calculate both entropy and enthalpy together.
Exergonic means that energy is released to the surroundings, as the bonds being made are stronger than the ones being broken. In contrast, endergonic signifies that energy is absorbed from the surroundings because the bonds being broken are stronger than the ones being made.

References

Libretexts. (2022, April 3). 16.14: The Free Energy. Chemistry LibreTexts.
Libretexts. (2022, February 14). Gibbs (Free) Energy. Chemistry LibreTexts.

Frequently Asked Questions about Free Energy of Formation

You calculate the free energy of formation using the Gibbs free energy equation.

Substances in their natural state have zero Gibbs free energy of formation, such as Br₂ (g) and I₂ (s).

The free energy of formation refers to the Gibbs free energy change when 1 mole of a substance is formed.

The standard free energy formation is -0.62 kJ/mol. This means that since G < 0 the reaction is exergonic and spontaneous. An exergonic reaction means that energy is released to the surroundings, as the bonds being made are stronger than the ones being broken.

Make sure the chemical reaction is under standard conditions.
Use the Gibbs free energy equation.
Subtract the reactants’ Gibbs free energy of formation by the products’ Gibbs free energy of formation.

Final Free Energy of Formation Quiz

Free Energy of Formation Quiz - Teste dein Wissen

Question

What refers to the energy change when 1 mole of a substance is formed?

Show answer

Answer

Enthalpy of formation.

Show question

Question

What is the heat absorbed or released when 1 mole of a substance is formed from its respective elements in their standard states?

Show answer

Answer

Standard Enthalpy.

Show question

Question

Why do both the standard enthalpy of formation and free energy of formation relate to Gibbs free energy?

Show answer

Answer

They relate to Gibbs free energy, as we use the Gibbs free energy equation to calculate both entropy and enthalpy together as one value.

Show question

Question

Exergonic means that energy is released to the surroundings, which means what for the bonds being broken and formed?

Show answer

Answer

Exergonic means the bonds being made are stronger than the ones being broken.

Show question

Question

What's the difference between enthalpy and entropy?

Show answer

Answer

Enthalpy is the total change in heat within a system when pressure is constant. While entropy is the randomness or disorder of a system.

Show question

Question

What does it mean when a reaction's ΔG = 0?

Show answer

Answer

It means the system is at equilibrium and neither the forward nor backward reaction are favored.

Show question

Question

Which substances have a Gibbs free energy formation of zero?

Show answer

Answer

Any element that is in its standard constituent state, such as hydrogen gas.

Show question

Question

What's the free energy of formation for 7 moles of I₂ (g)?

Show answer

Answer

Since iodine gas is in its standard state the Gibbs free energy of formation is 0 meaning 7 moles = 0 kJ/mol.

Show question

Question

What does it mean when we say that a reaction isn't spontaneous?

Show answer

Answer

It means the reaction is endergonic.

Show question

Question

What does it mean when ΔG < 0?

Show answer

Answer

ΔG < 0 means a reaction is spontaneous and exergonic.

Show question

Question

What does it mean when ΔG > 0?

Show answer

Answer

ΔG > 0 means a reaction is not spontaneous and endergonic.

Show question

Question

What do we mean when we say something is in standard state?

Show answer

Answer

The elements or reactants are at standard temperature (25 °C) and standard pressure (1 bar).

Show question

Question

How does Gibbs free energy (ΔG) relate to endergonic or exergonic reactions?

Show answer

Answer

Spontaneous reactions occur when ΔG < 0 is exergonic.

Non-spontaneous reactions occur when ΔG > 0 is endergonic.

Show question

More about Free Energy of Formation

How would you like to learn this content?

Creating flashcards

Studying with content from your peer

Taking a short quiz

94% of StudySmarter users achieve better grades.

How would you like to learn this content?

Creating flashcards

Studying with content from your peer

Taking a short quiz

Free chemistry cheat sheet!

Everything you need to know on . A perfect summary so you can easily remember everything.

Email Address*

Select your language

Free Energy of Formation

Free Energy of Formation

Free Energy of Formation

The Free Energy of Formation Definition

The Free Energy of Formation Equation

The Free Energy of Formation Table and Chart

Free Energy of Formation Examples

Free Energy of Formation of Water

Gibbs Free Energy of Formation of Ammonia

Free Energy of Formation - Key takeaways

References

Frequently Asked Questions about Free Energy of Formation

How to calculate free energy of formation?

What substance has zero Gibbs free energy of formation?

What does free energy of formation mean?

What is the standard free energy of formation for nh4hs?

How to find the standard free energy of formation?

Final Free Energy of Formation Quiz

Free Energy of Formation Quiz - Teste dein Wissen

More explanations about Physical Chemistry

Discover the right content for your subjects

Biology

Business Studies

Combined Science

Computer Science

Economics

English

English Literature

Environmental Science

Geography

History

Human Geography

Law

Macroeconomics

Marketing

Math

Microeconomics

Physics

Politics

Psychology

Sociology

No need to cheat if you have everything you need to succeed! Packed into one app!

Study Plan

Quizzes

Flashcards

Notes

Study Sets

Documents

Study Analytics

Weekly Goals

Smart Reminders

Rewards

Magic Marker

Smart Formatting

Join millions of people in learning anywhere, anytime - every day

This is still free to read, it's not a paywall.

You need to register to keep reading

This is still free to read, it's not a paywall.

You need to register to keep reading

Start learning with Vaia, the only learning app you need.

StudySmarter bietet alles, was du für deinen Lernerfolg brauchst - in einer App!