Gibbs Free Energy...

Gibbs Free Energy and
its importance
GROUP MEMBERS
Agha Danish Mehdi 3031
Muhammad Bilal 3012
Muneeb Usman 3013
Bahzad Hameed 3014
Contents
• Gibbs Free energy
•Need to determine Gibbs free energy
•Concept of Enthalpy
•Concept of Entropy
•Importance of Gibbs free energy
•Effect of Temprature
•Standard Gibbs free energy
Gibbs Free Energy
Gibbs Free Energy is a measure of chemical free energy of the system which
is stored in a system and utilize to do useful work.
Mathematically, It is defined as :
G=H–TS
Where,
G = Gibbs Free Energy
H = Enthalpy ( Heat content )
S = Entropy ( Disorder)
T = Temperature
CNTD…
As Enthalpy and Entropy are state functions, Gibbs free energy is also a
state function.
We cannot measure the Value of G but we can measure the Value of
∆G. At Isothermal and Isobaric conditions, Gibbs free energy can be
measured as:
∆G = ∆H – T ∆S
Gibbs Free Energy Change= Total heat Change for System - Energy lost in
disordering the system
As we know , systems with low energy state are more stable .So ,All
chemical systems tend naturally toward states of minimum Gibbs Free
Energy to get more stability.
Why we need to determine Gibbs Free
Energy?
Spontaneous Reactions:
A reaction is said to be spontaneous if it occurs without being driven
by some outside force.
Non-Spontaneous Reactions:
A non-spontaneous reaction is a reaction which cannot occur without
the input from any external source.
To check whether the reaction is spontaneous or not, different
methodology were used in past as follow :
(1) Enthalpy (2) Entropy
Enthalpy
Enthalpy is heat content which is released or absorbed during chemical
reaction.
To check reaction is spontaneous or not . firstly, enthalpy ∆H was used.
We said that if
(1) Reaction is exothermic (release heat) i.e. ∆H is ( -ve ),the
reaction was recognized as spontaneous reaction.
(2) Reaction is endothermic (absorbed heat) ∆H is (+ve ), the
reaction was recognized as non-spontaneous.
But its not tells us 100% spontaneity as it has drawback as some
endothermic reaction which are spontaneous in nature.
ENTROPY
Entropy is a thermodynamic function that describes the
randomness and disorder of molecules based on the number of
different arrangements available to them in a given system or reaction.
∆S was used to check whether a reaction is spontaneous or non-
spontaneous:-
(1) If Randomness increases i.e. ∆S is (+ve) ,reaction was recognized
as spontaneous.
(2) If randomness decreases i.e. ∆S is (-ve) ,reaction was recognized
as non-spontaneous.
This method was also failed as some exceptions were observed.
Gibbs Free Energy
So, finally, Gibbs Free Energy was introduced by Willard Gibbs to
check whether a reaction is spontaneous or non-spontaneous.
It gives us 100% results about that the reaction is spontaneous or
not.
∆G = ∆H – T ∆S
∆ is used because these all are state functions except T(In Isothermal
conditions).
The final conditions was that, If
(1) ∆G<0 i.e ∆G is negative, the reaction is said to be spontaneous.
(2) ∆G>0 i.e. ∆G is positive, the reaction is said to be non-spontaneous.
Importance of Gibbs Free
Energy
Gibbs Free Energy is used to determine whether a reaction is
spontaneous or non-spontaneous.
It is used to know free energy present in the system.
Gibbs Free Energy is also used to find Heat, Temperature and Entropy of
a system.
There are four conditions are used to check whether the

reactions are spontaneous or not.
Gibbs Free Energy
Firstly, if reaction is Exothermic (negative ∆H ) and
Entropy Increases ( positive ∆S ),Then ∆G must be negative.These
reactions are always spontaneous as
∆G = ∆H – T ∆S
∆G =(-ve) – (+ve)
∆G = -ve – ve
∆G = -ve (always spontaneous)
Gibbs Free Energy
Secondly, If reaction is Endothermic (i.e. positive ∆H ) and Entropy
decreases ( i.e. negative ∆S ) ,then ∆G must be positive.
These reactions always not spontaneous.
∆G = ∆H - T∆S
∆G = +ve – (-ve)
∆G = +ve + ve
∆G = +ve (always spontaneous)
Gibbs Free Energy
Thirdly, If Reaction is endothermic ( i.e.positive ∆H )and Entropy
increases ( i.e. positive ∆H )
Then reaction will be Spontaneous only if the value of entropy is greater
than enthalpy.
∆S> ∆H
∆G= +ve– (+ve)[∆S has greater value]
∆G = +ve –Ve(as ∆S> ∆H so –ve would be greater)
As ∆G = 40KJ – 70KJ = -30 kJ
Hence, reaction is spontaneous.
Gibbs Free Energy
Fourthly , If Enthalpy is decreases ( negative ∆H ) and Entropy decreases
( negative ∆S ) then reaction is spontaneous only if the value of Enthalpy
Is greater than entropy. i .e.
∆H> ∆S
∆G = -ve[this has greater value] – (-ve)
∆G = -ve +ve ( as ∆H > ∆S so)
∆G = -ve
E.G
∆G = -80KJ – (-63KJ) = -80KJ + 63KJ = -17KJ
EFFECT OF TEMPRATURE
If conditions are not isothermal , then for spontaneous reactions
∆G< 0
As ∆G = ∆H - T ∆S so
0<∆H - T ∆S => -∆H< -T∆S
T∆S> ∆H => T> ∆H/ ∆S
Inverse condition for non-spontaneous reaction .
If the T = ∆H/ ∆S i.e. ∆G= 0 then reaction is neither spontaneous nor
non-spontaneous but in equilibrium
Standard Gibbs Free energy
When we measured ∆G at temperature 298K and 1 atm pressure ,the
free energy change said to be standard Gibbs free energy.
∆G◦ = ∆H◦ - T ∆S◦
∆GR◦ can be measured as :
∆G◦R = ∆G◦p - ∆G◦r
Standard Gibbs free energy has great importance as it has

standard conditions for reactions and give fix value across that
conditions.
Thanks For attention 

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There are four conditions are used to check whether the

Standard Gibbs free energy has great importance as it has

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