Spontaneity
A spontaneous process which can take place by itself, speed of spontaneous reaction varies from very slow to fast.
Example
Dissolution of Na Cl in water.
CO gets oxidized to CO2 on its own.
A process which can neither take place by itself or by initiation is called non Spontaneous.
Driving forces for spontaneous process: (i) Tendency for minimum energy state. (ii) Tendency for maximum randomness
Entropy (s)
 It is the measure of degree of randomness or disorderliness of a system.
 Entropy is minimum in solid state and maximum in gaseous state.
 ΔS =S_{final}  S_{initial}
 $\Delta S=\frac {\Delta H}{T}$ Where H is enthalpy and T is temperature

Entropy in the states of matter is like
Gas>Liquid>Solid

Entropy Unit is J/K moles
Second Law of thermodynamics
The Entropy of universe is continuously increasing due to spontaneous processes taking place in it.
ΔS
_{system} +ΔS
_{Surrounding} > 0
S
_{total} > 0
If S
_{total} < 0,process is not spontaneous
A spontaneous process cannot be reversed
Gibb’s Free Energy (ΔG)
It is equal to the minimum amount of useful work done by the system.
Gibbs Functions (G) is defined as the thermodynamic quantity of the system, the decrease in whose value during a process is equal to the useful work done by the system. It is a extensive property
G= HTS
The change in Gibbs energy is given by
Mathematically: 
ΔG=ΔH TΔS
Where G is the Gibbs function
Δ
H is the enthalpy change of a reaction,
TΔ
S is the energy which is not available to useful work. So Δ
G is the net energy available to do useful work and is thus a measure of the ‘free energy’
For a process ifΔ G = ve, reaction is spontaneous
Δ G= 0, reaction is at eq
^{b}
Δ G = +ve, reaction is non – spontaneous
Effects of Temperature on Δ G
Gibbs Energy change in a Reaction
Δ
_{r} G
^{0} =Δ
_{r} G
^{0} (p)  Δ
_{r} G
^{0} (r)
Relation between G and Equilibrium Constant
G = 2.303 RT log K
_{c}
Where K
_{c} = equilibrium constant
Entropy change in a reaction
Δ
_{r} S
^{0} =Δ
_{r} S
^{0} (p)  Δ
_{r} S
^{0} (r)
Third law of Thermodynamics
The Entropy of a perfectly crystalline substance is Zero at zero Kelvin.
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