Table of Contents
Spontaneous and Non-Spontaneous Process:
Spontaneous Process:
A process which takes place on its own under the given set of conditions is called a spontaneous or natural process. They are also called feasible processes. Thus, the tendency of a process to occur naturally is called the spontaneity. The spontaneous processes occur with or without the initiation. Spontaneous processes need not to be instantaneous. Spontaneous processes include both physical and chemical reactions.
Some examples of spontaneous process are:
- Formation of clouds in the sky.
- Flow of water from river towards the sea.
- Flow of heat from a hot body to a colder body.
- Melting of ice at room temperature.
- Evaporation of water during summer.
- Intermixing of two or more gases.
Characteristics of a Spontaneous Process:
- A spontaneous change is unidirectional i.e. it takes place in only one direction. For the change to occur in reverse direction work has to be done.
- For a spontaneous change to occur, time is no factor. A spontaneous process may take place rapidly or very slowly.
- When the system is not in an equilibrium state, a spontaneous change has to take place. The change will continue until the system attains an equilibrium state. Once the state of equilibrium is reached, the system does not undergo any spontaneous change.
Types of Spontaneous Process:
(i) Spontaneous processes that need no initiation. Example-
- Common salt dissolves in water of its own.
- Formation of clouds in the sky.
- Flow of water from river towards the sea.
- Flow of heat from a hot body to a colder body.
- Melting of ice at room temperature.
- Evaporation of water from lakes, ponds etc.
- Carbon monoxide is oxidized to carbon dioxide of its own.
(ii) Spontaneous processes that need some initiation. Example-
- A candle made of wax burns only when ignited.
- The electrolysis of water can take place on passing electric current.
2H2O (l) ——-electric current———-> 2H2 (g) + O2 (g)
- Calcium carbonate evolves CO2 upon heating at 1273 K.
CaCO3(s) ——–Heat———> CaO (s) + CO2 (g)
Non-Spontaneous Process:
A process which can not take place on its own accord is called a non-spontaneous or unnatural process. All non-spontaneous process needs the energy to take place.
Some examples of non-spontaneous process are:
- Flow of water from sea to the river.
- Flow of heat from a cold body to a hotter body.
- Evaporation of water in winter.
- Melting of ice at zero temperature i.e. at a temperature below 0°C.
- Separation of a mixture of gases.
- Hydrolysis of NaCl in water to get back HCl and NaOH.
Driving Force in a Spontaneous Process:
The force which is responsible for the spontaneity of a process is called driving force in a spontaneous process. There are two factors responsible for spontaneous reaction or processes-
Tendency to acquire minimum energy:
Most of the changes that take place in nature are meant to obtain minimum energy or maximum stability. In general, a state of higher energy is less stable while that of lower energy is more stable. Most of the spontaneous processes are of exothermic in nature i.e. they are accompanied by the decrease in energy. Thus the driving force in spontaneous processes decreases the energy of the products as compared to the reactants.
Example-
C (s) + O2 (g) —————-> CO2 ; ΔH = -393.5 kJ
N2 (g) + 3H2 (g) —————–> 2NH3 (g) ; ΔH = -92 kJ
All these reactions are spontaneous because they are accompanied by the decrease in energy. The products have less energy and more stability than the reactants.
Limitations of Energy Factor:
There are many spontaneous processes, which are accompanied by the increase in energy i.e. they are endothermic in nature.
Example-
(i) Evaporation of water is endothermic in nature but is spontaneous.
H2O (l) ————> H2O (g) ; ΔH = +44.0 kJ
(ii) Melting of ice is also of endothermic nature but is spontaneous.
H2O (s) ————> H2O (l) ; ΔH = +5.86 kJ
Thus, the tendency to have minimum energy cannot be the only or sole criteria for the spontaneous process or reaction.
Tendency to have maximum randomness or disorder:
The tendency to have greater or maximum randomness or disorder is another factor, which can make a process spontaneous. The energy absorbed in the endothermic process increase the kinetic energy of the product species compared with the reactants. This brings about an increase in their free movement or randomness.
Example-
(i) In the evaporation of water, the H2O molecules in the gaseous state have greater disorder or randomness than the molecules in the liquid state.
(ii) When ice changes to water, there is an increase in free movement or randomness of the molecules.
Thus, a process can be spontaneous in a particular direction in case there is an increase in the randomness of the species as a result of the change.
Limitations of the randomness factor:
The randomness factor has also certain limitations. There are some spontaneous processes, which take place with the decrease in randomness or disorder.
For example- in the liquefication of gas or in the solidification of a liquid, the randomness of the particles decreases but still these processes are spontaneous.
Thus, randomness or disorder alone cannot be the sole criteria for the spontaneity of a process.
Hence, tendency to decrease overall energy and tendency to increase randomness are the driving force for spontaneity.
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