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7.THERMOCHEMISTRY eLearn.Punjab

7.1 SPONTANEOUS AND NON-SPONTANEOUS REACTIONS

A process which takes place on its own without any outside assistance and moves from a non-
equilibrium state towards an equilibrium state is termed as spontaneous process or natural process.
It is unidirectional, irreversible and a real process. Some examples of spontaneous processes are
given below.
(i) Water flows from higher level to the lower level. The flow cannot be reversed without some
external aid.
(ii) Neutralization of a strong acid with a strong base is a spontaneous acid-base reaction.

NaOH(aq) + HCI(aq)  NaCl(aq) + H2 O(l)

(iii) When a piece of zinc is added to the copper sulphate solution, blue colour of the solution

disappears due to the spontaneous redox reaction.

CuSO4(aq) + Zn(s) → ZnSO4(aq) + Cu(s)

A reaction will also be called a spontaneous process, if it needs energy to start with, but once it
is started, then it proceeds on its own. Burning of coal and hydrocarbon in air are examples of such
spontaneous reactions. A piece of coal does not burn in air on its own rather the reaction is initiated
by a spark and once coal starts burning, then the reaction goes spontaneously to completion.

Non-spontaneous process is the reverse of the spontaneous process. It does not take place

on its own and does not occur in nature. Reversible processes constitute a limiting case between

spontaneous and non-spontaneous processes. Some non-spontaneous processes, can be made

to take place by supplying energy to the system from external source. Some examples of non-

spontaneous processes are given below.

(i) Pumping of water uphill.

(ii) Transfer of heat from cold interior part nof the refrigerator to the hot surroundings.

(iii) When nitrogen reacts with oxygen, nitric oxide is formed. This reaction takes place by the

absorbance of heat. Although, N2 and O2 are present in air, but they do not react chemically at

ordinary conditions.The reaction takes place when the energy is provided by lightning.

N2(g) + O2(g)  2NO(g) (Non-spontaneous reaction)

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