Calculate Δg Rxn at 298 K for The Following Reaction

What is ΔG rxn?

The Gibbs free energy change (ΔG rxn) measures the energy available to do useful work in a chemical reaction. At 298 K (25°C), this value is particularly important because it represents standard conditions for many thermodynamic calculations.

ΔG rxn is calculated using the standard Gibbs 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.

How to Calculate ΔG rxn at 298 K

To calculate ΔG rxn, you need:

• The balanced chemical equation
• Standard Gibbs free energies of formation (ΔG°f) for all reactants and products
• The number of moles of each substance in the reaction

The calculation involves summing the ΔG°f values for the products and subtracting the sum of the ΔG°f values for the reactants, then multiplying by the stoichiometric coefficients.

Formula

Where:

• ΔG rxn = Gibbs free energy change for the reaction (kJ/mol)
• n, m = stoichiometric coefficients
• ΔG°f = standard Gibbs free energy of formation (kJ/mol)

All ΔG°f values should be for the same temperature (typically 298 K).

Example Calculation

Consider the reaction: 2H₂ + O₂ → 2H₂O

Standard Gibbs free energies of formation:

• H₂: -228.57 kJ/mol
• O₂: 0 kJ/mol
• H₂O: -237.13 kJ/mol

Calculation:

This negative value indicates the reaction is spontaneous under standard conditions.

Interpreting Results

The sign of ΔG rxn indicates the spontaneity of the reaction:

• ΔG rxn < 0: Reaction is spontaneous and exergonic
• ΔG rxn = 0: Reaction is at equilibrium
• ΔG rxn > 0: Reaction is non-spontaneous and endergonic

Magnitude of ΔG rxn indicates the driving force of the reaction. Larger absolute values indicate stronger spontaneity or non-spontaneity.