Gibb’s Free Energy

Core Concept

Gibbs Free Energy is the maximum amount of work that a system can perform at constant temperature and pressure.

The sign of $\Delta G$ determines spontaneity:

$\Delta G < 0$ (Negative): The reaction is spontaneous (favorable).
$\Delta G > 0$ (Positive): The reaction is non-spontaneous (unfavorable).
$\Delta G = 0$: The system is at equilibrium.

Key Tips

ΔG predicts spontaneity: ΔG < 0 means spontaneous.
Temperature plays a critical role in determining ΔG when ΔS ≠ 0.
Gibbs Free Energy is linked to equilibrium constants and the feasibility of reactions.

Test Yourself

Assorted Multiple Choice

A certain chemical reaction is found to have a standard enthalpy change ($\Delta H^\circ$) of $-85.0\text{ kJ/mol}$ and a standard entropy change ($\Delta S^\circ$) of $-120.0\text{ J/(mol}\cdot\text{K)}$. If the standard Gibbs free energy change ($\Delta G^\circ$) is calculated to be $-49.2\text{ kJ/mol}$ at $298\text{ K}$, what does this indicate about the thermodynamic nature of the reaction under standard conditions?

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Gibb's Free Energy

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The Gibbs Free Energy Equation

The change in Gibbs Free Energy (ΔG) is given by: ΔG = ΔH − TΔS

Where:

ΔG: Change in Gibbs Free Energy.
ΔH: Enthalpy change (J/mol).
T: Temperature (Kelvin).
ΔS: Entropy change (J/K·mol).

What Does ΔG Tell Us?

Spontaneous Reaction: ΔG < 0 (negative)
- The reaction occurs without external energy input.
Non-Spontaneous Reaction: ΔG > 0 (positive)
- The reaction requires energy input to proceed.
Equilibrium: ΔG=0
- The system is in a state of balance.

Factors Affecting Gibbs Free Energy

Enthalpy (ΔH):

Represents heat changes in a reaction.
Exothermic reactions (ΔH<0) tend to favor spontaneity.

Entropy (ΔS):

Represents disorder or energy dispersal.
Reactions that increase entropy (ΔS>0) tend to be spontaneous.

Temperature (T):

High temperatures amplify the effect of TΔS.
For reactions with ΔS>0, higher temperatures favor spontaneity.

Standard Gibbs Free Energy (ΔG)

Definition: Gibbs Free Energy change under standard conditions (25°C, 1 atm, 1 M concentrations).
Formula: $\Delta G^\circ = \sum \Delta G^\circ_{\text{products}} - \sum \Delta G^\circ_{\text{reactants}}$

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