Summary of Thermochemistry: Gibbs Free Energy

TOPICS

What is Gibbs Free Energy and how does it determine the spontaneity of a reaction?
How do enthalpy and entropy influence Gibbs Free Energy?
What thermodynamic conditions of temperature and pressure affect spontaneity?

Gibbs Free Energy (ΔG): ΔG = ΔH - TΔS
- Where ΔG is the Gibbs free energy, ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy.
Spontaneity Condition:
- Spontaneous reactions: ΔG < 0
- Non-spontaneous reactions: ΔG > 0
- Equilibrium: ΔG = 0

Gibbs Free Energy (ΔG): Amount of energy capable of doing work during a reaction at constant temperature and pressure. It is the fundamental criterion for the spontaneity of reactions.
Spontaneity of Reactions: Refers to a reaction that occurs naturally without the need for additional external energy.
Enthalpy (H): Represents the heat exchanged in a process at constant pressure.
Entropy (S): Measure of disorder or randomness in a system.
Exergonic: Reaction that releases energy to the environment (negative ΔG).
Endergonic: Reaction that absorbs energy from the environment (positive ΔG).

The spontaneity of a reaction is not related to the reaction's speed but to its natural tendency to occur without external intervention.
The relationship between enthalpy and entropy is crucial. Reactions that release heat (negative ΔH) and increase disorder (positive ΔS) tend to be spontaneous.
Temperature (T) is a determining factor in the spontaneity of a reaction. At high temperatures, entropy has a greater influence.

Definition and Calculation of ΔG: Explanation of the formula ΔG = ΔH - TΔS and how it reflects spontaneity.
Signs of ΔG:
- Negative ΔG (exergonic): indicates that the reaction occurs spontaneously.
- Positive ΔG (endergonic): indicates that the reaction is non-spontaneous.
- ΔG equal to zero: indicates that the reaction is at equilibrium, with no tendency to proceed or reverse.
Factors Affecting ΔG:
- Changes in enthalpy (ΔH), which can be influenced by chemical bonds formed or broken.
- Changes in entropy (ΔS), which can be affected by the complexity of molecules or the state of matter.
- Temperature (T), which can favor endothermic or exothermic processes.

Example of Exergonic Reaction:
- Combustion of hydrocarbons (such as gasoline): ΔH is negative due to heat release; ΔS is positive due to the production of gases from liquids; ΔG is negative, indicating spontaneity.
Example of Endergonic Reaction:
- Photosynthesis: ΔH is positive because energy is absorbed; ΔS can be negative or slightly positive due to the formation of more organized substances from CO₂ and water; ΔG is positive under standard conditions, indicating non-spontaneity without solar energy.
Chemical Equilibrium:
- When the reaction reaches equilibrium, ΔG is zero, and no more useful work is being done, meaning there is no tendency to move forward to products or revert to reactants without an external change.

Gibbs Free Energy (∆G) is the thermodynamic criterion for evaluating the spontaneity of a reaction, considering the energy available to do work.
Enthalpy (∆H) refers to the heat involved in a chemical reaction at constant pressure, signaling exothermic or endothermic processes.
Entropy (∆S) is a measure of energy dispersion or disorder, essential for predicting changes in spontaneity with temperature variation.
Temperature (T) is a crucial component in the ∆G formula, which can alter the direction of spontaneity based on entropy influence.
The formula ∆G = ∆H - T∆S combines these variables, allowing the calculation of Gibbs free energy and predicting whether a reaction will occur spontaneously or not.

Reactions with negative ∆G are spontaneous, releasing free energy to the environment and are called exergonic.
Reactions with positive ∆G are non-spontaneous without the addition of external energy, classified as endergonic.
Chemical equilibrium is achieved when ∆G is equal to zero, with no net transformation from reactants to products.
Understanding how enthalpy, entropy, and temperature influence ∆G is crucial to predict the direction and spontaneity of chemical reactions.
The practical application of ∆G calculation strengthens the ability to predict the behavior of reactions under different thermodynamic conditions.