# Gibbs Free Energy : Definition of Gibbs Free Energy

Gibbs Free Energy
(G) – The energy combined with a chemical reaction can be used in chemical
work. The free energy of a system is the entirety of its enthalpy + the product
of the temperature (Kelvin) and the system’s entropy: The reaction’s free
energy.

**Equation** :

The change in Gibbs
Free Energy is caused by constant heat and pressure and defines as ΔG
= ΔH-TΔS. As the heat and pressure are unchangeable, the T remains
constant. ΔG = change in Final and initial Gibbs energy, ΔS = change
in final, and Enthalpy of the system.

**Symbol** :

The symbol is
symbolized as ‘G.’

To get an overview
of Gibbs’s energy and its general uses in chemistry, Gibbs Free Energy ,
denoted G, and combines enthalpy and entropy into a single value. The change in
free energy is ΔG, which is equal to the enthalpy’s sum plus the system’s
temperature and entropy product.

**Purpose :**

The Gibbs Free Energy is one of the essential thermodynamic
functions for the characterization of a system. It is a multiplier in realizing
outcomes such as the voltage of an electrochemical cell, and the equilibrium
constant for a reversible reaction.

**What does Gibbs’s free energy tell us?**

It is the maximum
amount of non-valuable work that can be extracted from a thermodynamically
closed system (can exchange heat and work with its surroundings, but no
matter). This maximum can be performed only in a completely reversible process.

**The physical significance:-**

According to the
first law of thermodynamics,

ΔU = q – w

Here, q is the
heat absorbed by the system, and w is the work done by the system, while Δu
is the change in initial energy. The work (w) includes two types of work, the
two types of works can be understood by taking an example of the electrolysis
of water to form Hydrogen and Oxygen, leading to an increase in
the system’s volume. The work non-expansion (non-mechanical) is done to
cause the decomposition of water while the work expansion is due to expansion
in the volume of the system. Thus, the electrical work is called
non-pressure-volume work or non-expansion work, or non-mechanical work. The
non-expansion work is also known as useful work.

The greater the free
energy change, the greater the amount obtained from the process. This relation
is useful in assessing the electrical work produced by electrochemical cells
and fuel cells.

Free Energy change
and electrical work are done cells :

The following
relation relates ΔG and emf of the cell €:

ΔG
= – nFE

Where,
F = faraday = 96500 coulomb

E = emf of the cell

n = Number of moles of celetrones involved in a balanced electrochemical
reaction

If the reactants and
products are their standard states, then

ΔG
degree = -nFE degree

When E degree =
standard emf of the cell.

In constant pressure
and temperature, the change of Gibbs Free Energy of any process :

The change of Gibbs
Free Energy in a process with constant pressure and temperature is ΔG
= ΔH – TΔS.

(Though T is
constant, so Δ(TS) = TΔS)……..(!)

This (1) equation
means the relation between the change of enthalpy, the entropy of the system,
and T under the constant pressure and temperature condition.

By the equation,
anyone can decide if this process is spontaneous. In constant pressure and
temperature in any chemical or physical change, the change of Gibbs Free Energy
is less than zero.

Then the process
will be spontaneous. In constant pressure, temperature, and pressure in any
chemical or physical change of Gibbs, free energy is more significant than
zero. This process will not be spontaneous, but the opposite process will be
spontaneous. Gibbs Free Energy is a state function.