Calculate The Delta G for The Following Reaction

This calculator helps you determine the Gibbs free energy change (ΔG) for chemical reactions using standard free energies of formation. ΔG is a key thermodynamic property that indicates whether a reaction is spontaneous, non-spontaneous, or at equilibrium.

How to Calculate ΔG

The Gibbs free energy change for a reaction can be calculated using the standard free energies of formation (ΔG°f) for the reactants and products. The formula accounts for the stoichiometry of the reaction and the number of moles of each substance involved.

ΔG = Σ(n × ΔG°f)products - Σ(n × ΔG°f)reactants

Where:

ΔG is the Gibbs free energy change for the reaction
n is the stoichiometric coefficient for each substance
ΔG°f is the standard free energy of formation for each substance

To use this calculator:

Enter the balanced chemical equation for your reaction
Input the standard free energies of formation for all reactants and products
Specify the stoichiometric coefficients for each substance
Click "Calculate" to determine ΔG

The ΔG Formula

The complete formula for calculating ΔG is:

ΔG = Σ(n_products × ΔG°f_products) - Σ(n_reactants × ΔG°f_reactants)

This formula accounts for:

The number of moles of each product and reactant (n)
The standard free energy of formation for each substance (ΔG°f)
The stoichiometry of the reaction

Note: Standard free energies of formation are typically measured at 25°C and 1 atm pressure. For accurate results, use values from reliable thermodynamic databases.

Worked Example

Let's calculate ΔG for the following reaction:

Example Reaction

2H₂(g) + O₂(g) → 2H₂O(l)

Standard free energies of formation:

H₂(g): -228.57 kJ/mol
O₂(g): 0 kJ/mol
H₂O(l): -285.83 kJ/mol

Using the formula:

ΔG = [2 × (-285.83)] - [2 × (-228.57) + 1 × 0] ΔG = [-571.66] - [-457.14 + 0] ΔG = -571.66 + 457.14 ΔG = -114.52 kJ

The calculation shows that this reaction has a negative ΔG, indicating it is spontaneous under standard conditions.

Interpreting Results

The value of ΔG provides important information about the reaction:

ΔG < 0: The reaction is spontaneous and will proceed as written
ΔG = 0: The reaction is at equilibrium
ΔG > 0: The reaction is non-spontaneous as written and requires energy input

Additional considerations when interpreting ΔG results:

Temperature effects: ΔG is temperature-dependent
Concentration effects: ΔG changes with reactant/product concentrations
Phase changes: Different phases can affect ΔG values

Remember that ΔG provides information about spontaneity, not reaction rate. A spontaneous reaction may proceed slowly if it has a high activation energy.

FAQ

What is the difference between ΔG and ΔG°?

ΔG is the Gibbs free energy change for a reaction under specific conditions, while ΔG° refers to the standard Gibbs free energy change at standard conditions (25°C and 1 atm pressure).

How do I find standard free energies of formation?

Standard free energies of formation can be found in thermodynamic databases, chemistry handbooks, or reliable scientific sources. Common values are available for many common substances.

What units are used for ΔG?

ΔG is typically expressed in kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol). The calculator uses kJ/mol by default.

Can ΔG be negative for an endothermic reaction?

Yes, a reaction can have a negative ΔG even if it's endothermic. The spontaneity depends on both enthalpy (ΔH) and entropy (ΔS) changes, not just ΔH alone.