Calculate Delta G of Reaction Using Δ G 0

What is ΔG of Reaction?

The Gibbs free energy change (ΔG) measures the energy available to do work in a chemical reaction. It combines enthalpy (ΔH) and entropy (ΔS) changes:

• ΔG = ΔH - TΔS

Where:

• ΔG = Gibbs free energy change (kJ/mol)
• ΔH = Enthalpy change (kJ/mol)
• T = Absolute temperature (K)
• ΔS = Entropy change (J/mol·K)

Standard Gibbs free energy (δ G 0) provides ΔG values under standard conditions (25°C, 1 atm pressure).

How to Calculate ΔG of Reaction

To calculate ΔG of a reaction using δ G 0 values:

1. Write the balanced chemical equation
2. Find δ G 0 values for each reactant and product
3. Multiply each δ G 0 by its stoichiometric coefficient
4. Sum the products and reactants separately
5. Calculate ΔG by subtracting the sum of reactants from the sum of products

The Formula

The complete formula for calculating ΔG of reaction using δ G 0 values is:

This formula accounts for:

• Stoichiometric coefficients (ν) of each species
• Standard Gibbs free energy values (δ G 0)
• The direction of the reaction (products minus reactants)

Worked Example

Let's calculate ΔG for the reaction:

2H₂(g) + O₂(g) → 2H₂O(g)

Given δ G 0 values:

• H₂(g): -0.000 kJ/mol
• O₂(g): 0.000 kJ/mol
• H₂O(g): -237.1 kJ/mol

Calculation:

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

Interpreting Results

Interpret ΔG values as follows:

• ΔG < 0: Spontaneous reaction (energy released)
• ΔG = 0: Reaction at equilibrium
• ΔG > 0: Non-spontaneous reaction (energy required)

Note that ΔG depends on temperature and pressure conditions. Standard conditions provide a useful starting point but may not reflect real-world scenarios.