No, there is a natural tendency of a system to attain a state of the greater randomness i.e. more disordered state. For example,

(i) There is more randomness on mixing of two gases (which do not react chemically).

(ii) Evaporation of water. The evaporation of water results in increase of randomness because the molecules in the vapour state have more randomness than in the liquid state

   

(iii) Dissolution of ammonium chloride in water. Solid ammonium chloride has less randomness while in solution ammonium chloride particles move freely as  and hence randomness increases. 

  

If the randomness factor were the only criterion, then the process like liquefaction of gas or solidification of a liquid would not have been feasible since these were accompanied by a decrease in randomness. Hence the tendency of a system to acquire a state of maximum randomness is not the sole criterion for determining the spontaneity of the process.

The degree of randomness or disorderliness is expressed by a thermodynamic function called entropy. It may be defined as the property of a system which measures the degree of randomness or disorderliness in the system. It is denoted by S. It is a state function. It depends only on the initial and final state of the system and not on the path followed.

If S_A and S_B are the entropies at state A and B, then entropy change ∆S is given by



For a chemical reaction, 



For a reversible process at equilibrium, the change in entropy may be expressed as,



Where q_rev represents the total heat absorbed reversibly and isothermally at temperature T.

 A small change in entropy can be represented by dS since entropy is a state function. Thus, we may write


Units of entropy: The entropy change is expressed in terms of heat divided by absolute temperature. Thus entropy is expressed in terms of calories per degree i.e. cal deg^–1 . This unit is called entropy unit i.e. e.u. In S.I. units, it is expressed in Joules per degree Kelvin i.e. JK^–1 . ∆S is an extensive property and therefore its value depends on upon the amount of the substance involved. Therefore, units of entropy will be JK^–1 mol^–1.

Physical significance: Entropy has been regarded as a measure of disorder or randomness of a system. Thus when a system goes from a more orderly to less orderly state, there is an increase in its randomness and hence entropy of the system increases. Conversely, if the change is one in which there is an increase in orderliness, there is a decrease in entropy. For example, when a solid changes to a liquid, an increase in entropy takes place, because with the breaking of the orderly arrangement of the molecules in the crystal to the less orderly liquid state, the randomness increases. The process of vaporisation produces an increase in randomness in the distribution of molecules, hence an increase in entropy. When two gases are mixed, the molecules of the gases intermix to achieve more randomness.

Thus, this concept of entropy (measure of randomness) has led to the conclusion that all substances in their normal crystalline state at absolute zero temperature would be in the condition of maximum orderly arrangement, because all motion has essentially ceased at ‘0 K.’ In other words, entropy of a substance at 0 K is minimum.

Suppose heat is absorbed by the system reversible and the heat is lost by the surroundings also reversibly (process occurs under complete reversible condition).

If q_rev is the heat absorbed by the system reversibly, then the heat lost by the surroundings will also be q_rev. If the process takes place isothermally at T kelvin, then Entropy change of the system

 

Entropy change of the surroundings



Hence the total entropy change for the combined system and surroundings will be:



Hence in a reversible process, the net entropy change for the combined system and the surroundings is zero i.e. there is no net change in entropy. 

The overall tendency of a process to occur can be expressed on the resultant of two tendencies namely:
(i) the tendency to acquire a state of minimum energy, and
(ii) the tendency to acquire a state of maximum randomness or disorder.

The overall tendency of a process to take place by itself is called the driving force.
It should be noted that:
(i) the two tendencies act independent of each other,
(ii) the two tendencies may work in the same direction or opposite direction in a process and
(iii) the driving force is the resultant of the magnitude of the two tendencies. When the two tendencies act in the opposite direction, the tendency with the greater magnitude determines whether the process is feasible or not. For example,
(a) Evaporation of water. Evaporation of water is endothermic, therefore, energy factor opposes the process. But it is favoured by randomness factor.

Since the process is known to take place, randomness factor must be greater than energy factor.

(b) The reaction between hydrogen and oxygen to form water. It is an exothermic reaction, therefore, favours the process, but randomness factor opposes the reaction.

As the reaction takes place, the energy factor must be greater than the randomness factor.