Define (a) the standard free-energy change, ∆G°, for a reaction, and (b) the standard free energy of formation, ∆G°f, of a substance.

Verified step by step guidance

Step 1: Define the standard free-energy change, \( \Delta G^\circ \), for a reaction. It is the change in Gibbs free energy when reactants in their standard states are converted to products in their standard states at a specified temperature, usually 298 K (25°C).

Step 2: Explain that the standard state refers to the pure form of a substance at 1 bar of pressure and a specified temperature, typically 298 K.

Step 3: Describe that \( \Delta G^\circ \) is used to predict the spontaneity of a reaction. If \( \Delta G^\circ < 0 \), the reaction is spontaneous under standard conditions; if \( \Delta G^\circ > 0 \), it is non-spontaneous.

Step 4: Define the standard free energy of formation, \( \Delta G^\circ_f \), of a substance. It is the change in Gibbs free energy when one mole of a compound is formed from its elements in their standard states.

Step 5: Clarify that \( \Delta G^\circ_f \) values are used to calculate \( \Delta G^\circ \) for reactions using the equation: \( \Delta G^\circ = \sum \Delta G^\circ_f (\text{products}) - \sum \Delta G^\circ_f (\text{reactants}) \).

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Standard Free-Energy Change (∆G°)

The standard free-energy change (∆G°) of a reaction is a thermodynamic quantity that indicates the spontaneity of a reaction under standard conditions (1 bar pressure, 1 M concentration, and a specified temperature, usually 25°C). A negative ∆G° value suggests that the reaction is spontaneous, while a positive value indicates non-spontaneity. It is calculated using the Gibbs free energy equation, which relates enthalpy, entropy, and temperature.

Standard Free Energy of Formation (∆G°f)

The standard free energy of formation (∆G°f) of a substance is the change in free energy when one mole of a compound is formed from its elements in their standard states. This value is crucial for calculating the overall free energy change of a reaction using Hess's law. The standard free energy of formation for elements in their most stable form is defined as zero, providing a reference point for other compounds.

Relationship Between ∆G° and Reaction Spontaneity

The relationship between standard free-energy change (∆G°) and reaction spontaneity is fundamental in thermodynamics. A negative ∆G° indicates that the reaction can proceed spontaneously in the forward direction, while a positive ∆G° suggests that the reverse reaction is favored. This relationship helps predict the direction of chemical reactions and is essential for understanding equilibrium and reaction mechanisms.