Spontaneous and Nonspontaneous Process.
Spontaneous Process. “The process takin place without external action/aid is called a spontaneous process“. A spontaneous change continues until a system reaches a state of equilibrium and no further change occurs.
- The flow of water from a higher level to a lower level.
- The flow of charge between two-point due to the potential difference.
- The expansion of gas in a vacuum.
Spontaneous processes are not thermodynamically reversible without external agency.
Natural process, which can occur is called a spontaneous process. The reverse process can not occur are called nonspontaneous processes. Thus, nonspontaneous processes are the processes which occur by the intervention of external agency.
Explanation. Both the spontaneous and nonspontaneous processes are possible but the only spontaneous process occurs naturally. Nonspontaneous processes require the system to be acted upon in some way. For example, sodium metal can react with chlorine under normal conditions (natural condition i.e, 25 C and 1 atm.) to form NaCl,
Thermodynamically speaking we can force the second reaction to occur by using energy properly, e.g., if NaCl is melted and electricity is passed, Na(s) and Cl2(g) are produced.
The criterion of Spontaneity:
In the search for a criterion for a spontaneous process, the following two factors were recognized.
Change in energy.
Change in amount of disorders.
Explanation: In mechanical systems, we observe a decrease in potential energy in a spontaneous change, e.g., ball rolls downhill and water flows to a lower level. For a chemical system, a spontaneous change occurs in the direction in which the enthalpy of the system decreases, i.e., an exothermic reaction is spontaneous.
However, there is some endothermic reaction which is also spontaneous, e.g., melting of ice at room temperature, the evaporation of liquid ether from an open beaker and solution formation of NH4NO3 in water. The spontaneous nature of endothermic processes can be explained with the help of entropy, i.e., a measure of disorders. In melting, evaporation, and dissolution of NH4NO3, the entropy increases and makes these processes spontaneous, i.e., possible to occur naturally. In terms of energy changes and entropy changes, spontaneous processes can be defined as:
- The process in which the energy content of the system decreases, tend to occur spontaneously.
- The process in which the disorder of the system increases, tend to occur spontaneously.