The relationship between free energy (G), enthalpy (H), and entropy (S) is a foundational concept in biochemistry, providing insight into the spontaneity and directionality of chemical reactions within biological systems.
These three thermodynamic parameters are interconnected by the Gibbs free energy equation, which is pivotal for predicting the feasibility of biochemical reactions.
To understand the dynamics of this relationship in detail, let's break down each component and see how they interact within the context of the Gibbs free energy equation:
Gibbs Free Energy Equation
The equation that connects free energy, enthalpy, and entropy is expressed as: ΔG=ΔH−TΔS where:
ΔG is the change in free energy,
ΔH is the change in enthalpy,
T is the absolute temperature in Kelvin,
ΔS is the change in entropy.
Free Energy (G)
Free energy quantifies the amount of work a system can perform when a reaction occurs at constant temperature and pressure.
It essentially measures the usability of the system's energy.
The sign of ΔG determines the reaction's spontaneity:
A negative ΔG indicates a spontaneous reaction, which can proceed without any external energy input.
A positive ΔG suggests a non-spontaneous reaction, requiring energy to proceed.
Enthalpy (H)
Enthalpy represents the total heat content of a system, reflecting both its internal energy and the product of its pressure and volume.
Changes in enthalpy (ΔH) during a reaction indicate whether the system absorbs or releases heat:
A positive ΔH denotes an endothermic reaction, where the system absorbs heat from its surroundings.
A negative ΔH signifies an exothermic reaction, where the system releases heat to its surroundings.
Entropy (S)
Entropy measures the disorder or randomness within a system.
The change in entropy (ΔS) during a reaction reflects changes in the system's disorder:
A positive ΔS indicates that the disorder (or randomness) in the system has increased.
A negative ΔS suggests that the system has become more ordered.
Interplay Between H, S, and G
The Gibbs free energy equation beautifully illustrates how the balance between enthalpy and entropy influences the spontaneity of reactions:
Spontaneous reactions (ΔG<0) can occur when the system releases heat (ΔH<0) and/or the disorder increases (ΔS>0).
Non-spontaneous reactions (ΔG>0) require energy input to proceed and are characterized by the absorption of heat (ΔH>0) and/or a decrease in disorder (ΔS<0).
Temperature (T) plays a crucial role, modulating the effect of entropy changes on the free energy.
This interdependence is crucial in biological systems, where the precise control of reaction spontaneity underpins metabolic processes, energy storage, and the overall functioning of living organisms.