The Standard Free Energy of Formation

Oct 13, 2025 by wbrum.org 114 views

Understanding the Basics of Free Energy Change

Free energy change, denoted by the symbol ΔG, is a measure of the energy change that occurs during a chemical reaction. It is a crucial concept in chemistry, as it helps us predict the spontaneity of a reaction. In this article, we will focus on calculating the standard free energy change of a chemical reaction using the given standard free energy of formation values.

The Standard Free Energy of Formation

The standard free energy of formation, denoted by the symbol ΔGf, is the change in free energy that occurs when one mole of a substance is formed from its constituent elements in their standard states. The standard free energy of formation values are typically given in units of kJ/mol.

Calculating the Standard Free Energy Change

To calculate the standard free energy change of a chemical reaction, we need to use the following equation:

ΔGrxn = Σ(ΔGf, products) - Σ(ΔGf, reactants)

where ΔGrxn is the standard free energy change of the reaction, ΔGf, products is the standard free energy of formation of the products, and ΔGf, reactants is the standard free energy of formation of the reactants.

Calculating the Standard Free Energy Change of the Given Reaction

Let's consider the following reaction:

2 C2H2(g) + 5 O2(g) → 4 CO2(g) + 2 H2O(g)

We are given the standard free energy of formation values for C2H2 and CO2:

ΔGf, C2H2 = 209.2 kJ/mol ΔGf, CO2 = -394.4 kJ/mol

To calculate the standard free energy change of the reaction, we need to calculate the standard free energy of formation of the products and the reactants.

Calculating the Standard Free Energy of Formation of the Products

The products of the reaction are 4 CO2(g) and 2 H2O(g). We can calculate the standard free energy of formation of the products by multiplying the standard free energy of formation of each product by the number of moles of that product.

ΔGf, products = 4(ΔGf, CO2) + 2(ΔGf, H2O)

Since we are not given the standard free energy of formation value for H2O, we need to calculate it using the following equation:

ΔGf, H2O = ΔGf, H2(g) + 1/2ΔGf, O2(g)

We can look up the standard free energy of formation values for H2(g) and O2(g) in a reference book or online resource.

Calculating the Standard Free Energy of Formation of the Reactants

The reactants of the reaction are 2 C2H2(g) and 5 O2(g). We can calculate the standard free energy of formation of the reactants by multiplying the standard free energy of formation of each reactant by the number of moles of that reactant.

ΔGf, reactants = 2(ΔGf, C2H2) + 5(ΔGf, O2)

Calculating the Standard Free Energy Change of the Reaction

Now that we have calculated the standard free energy of formation of the products and the reactants, we can calculate the standard free energy change of the reaction using the following equation:

ΔGrxn = Σ(ΔGf, products) - Σ(ΔGf, reactants)

Substituting the values we calculated earlier, we get:

ΔGrxn = 4(-394.4 kJ/mol) + 2(ΔGf, H2O) - 2(209.2 kJ/mol) - 5(0 kJ/mol)

Simplifying the equation, we get:

ΔGrxn = -1576.8 kJ/mol + 2(ΔGf, H2O) - 418.4 kJ/mol

To calculate the standard free energy change of the reaction, we need to calculate the standard free energy of formation of H2O.

Calculating the Standard Free Energy of Formation of H2O

We can calculate the standard free energy of formation of H2O using the following equation:

ΔGf, H2O = ΔGf, H2(g) + 1/2ΔGf, O2(g)

We can look up the standard free energy of formation values for H2(g) and O2(g) in a reference book or online resource.

Standard Free Energy of Formation Values for H2(g) and O2(g)

The standard free energy of formation values for H2(g) and O2(g) are:

ΔGf, H2(g) = 0 kJ/mol ΔGf, O2(g) = 0 kJ/mol

Calculating the Standard Free Energy of Formation of H2O

Substituting the values we calculated earlier, we get:

ΔGf, H2O = 0 kJ/mol + 1/2(0 kJ/mol) ΔGf, H2O = 0 kJ/mol

Calculating the Standard Free Energy Change of the Reaction

Now that we have calculated the standard free energy of formation of H2O, we can calculate the standard free energy change of the reaction using the following equation:

ΔGrxn = Σ(ΔGf, products) - Σ(ΔGf, reactants)

Substituting the values we calculated earlier, we get:

ΔGrxn = -1576.8 kJ/mol + 2(0 kJ/mol) - 418.4 kJ/mol ΔGrxn = -1995.2 kJ/mol

Therefore, the standard free energy change of the reaction is -1995.2 kJ/mol.

Conclusion

In this article, we calculated the standard free energy change of a chemical reaction using the given standard free energy of formation values. We used the following equation to calculate the standard free energy change:

ΔGrxn = Σ(ΔGf, products) - Σ(ΔGf, reactants)

We calculated the standard free energy of formation of the products and the reactants, and then used these values to calculate the standard free energy change of the reaction. The standard free energy change of the reaction is -1995.2 kJ/mol.

References

"Thermodynamic Properties of Pure Substances" by I. M. Abdulagatov and A. M. R. G. Sayeed
"Standard Free Energy of Formation Values" by the National Institute of Standards and Technology (NIST)

Note

Q: What is the standard free energy change of a chemical reaction?

A: The standard free energy change of a chemical reaction, denoted by the symbol ΔGrxn, is a measure of the energy change that occurs during the reaction. It is a crucial concept in chemistry, as it helps us predict the spontaneity of a reaction.

Q: How do I calculate the standard free energy change of a chemical reaction?

A: To calculate the standard free energy change of a chemical reaction, you need to use the following equation:

ΔGrxn = Σ(ΔGf, products) - Σ(ΔGf, reactants)

Q: What is the standard free energy of formation of a substance?

A: The standard free energy of formation of a substance, denoted by the symbol ΔGf, is the change in free energy that occurs when one mole of the substance is formed from its constituent elements in their standard states.

Q: How do I calculate the standard free energy of formation of a substance?

A: To calculate the standard free energy of formation of a substance, you need to look up the standard free energy of formation values in a reference book or online resource.

Q: What are the standard free energy of formation values for common substances?

A: The standard free energy of formation values for common substances can be found in reference books or online resources. Some common substances and their standard free energy of formation values are:

H2(g): 0 kJ/mol
O2(g): 0 kJ/mol
CO2(g): -394.4 kJ/mol
H2O(g): -237.1 kJ/mol
C2H2(g): 209.2 kJ/mol

Q: How do I calculate the standard free energy change of a reaction with multiple products and reactants?

A: To calculate the standard free energy change of a reaction with multiple products and reactants, you need to calculate the standard free energy of formation of each product and reactant, and then use the following equation:

ΔGrxn = Σ(ΔGf, products) - Σ(ΔGf, reactants)

Q: What is the significance of the standard free energy change of a reaction?

A: The standard free energy change of a reaction is a measure of the energy change that occurs during the reaction. It is a crucial concept in chemistry, as it helps us predict the spontaneity of a reaction.

Q: Can the standard free energy change of a reaction be negative or positive?

A: Yes, the standard free energy change of a reaction can be negative or positive. A negative standard free energy change indicates that the reaction is spontaneous, while a positive standard free energy change indicates that the reaction is non-spontaneous.

Q: How do I determine the spontaneity of a reaction based on the standard free energy change?

A: To determine the spontaneity of a reaction based on the standard free energy change, you need to look at the sign of the standard free energy change. If the standard free energy change is negative, the reaction is spontaneous. If the standard free energy change is positive, the reaction is non-spontaneous.

Q: What are some common applications of the standard free energy change of a reaction?

A: The standard free energy change of a reaction has many common applications in chemistry, including:

Predicting the spontaneity of a reaction
Determining the equilibrium constant of a reaction
Calculating the Gibbs free energy of a reaction
Understanding the thermodynamics of a reaction

Q: Where can I find more information on the standard free energy change of a reaction?

A: You can find more information on the standard free energy change of a reaction in reference books, online resources, and academic journals. Some common resources include:

"Thermodynamic Properties of Pure Substances" by I. M. Abdulagatov and A. M. R. G. Sayeed
"Standard Free Energy of Formation Values" by the National Institute of Standards and Technology (NIST)
"Chemical Thermodynamics" by John W. Moore and Richard A. Brown