The first law of thermodynamics tells us about the relationship between the heat absorbed and the work performed on or by a system.
It puts no restrictions on the direction of heat flow. However, the flow of heat is unidirectional from higher temperature to lower temperature.
In fact, all naturally occurring processes whether chemical or physical will tend to proceed spontaneously in one direction only. For example, a gas expanding to fill the available volume, burning carbon in dioxygen giving carbon dioxide.
But heat will not flow from colder body to warmer body on its own, the gas in a container will not spontaneously contract into one corner will not spontaneously contract into one corner or carbon dioxide will not form carbon and dioxide will not form carbon and dioxygen spontaneously.
These and many other spontaneously occurring changes show unidirectional change. We may ask 'what is the driving force of spontaneously occurring changes? What determines the direy of e spontaneous change? In this section, we shall establish some criterion for these processes whether these will take place or not.
Let us s first understand what do we mean by spontaneous reaction or change? You may think by your common observation that spontaneous reactants. Take the case of combination may be mixed at room temperature and left for many years without observing any perceptible change. Although the reaction is taking place between them, it is at an extremely slow rate. It is still called spontaneous reaction.
So spontaneity means 'having the potential to proceed without the assistance of external agency'. However, it doi not tell about the rate of the reaction or process. Another aspect of spontaneous reaction or process, as we see of spontaneous reaction or process, as we see is that these cannot reverse their direction on is that these cannot reverse their direction on their be own. We many summaries itas follows:
A spontaneous process is an irret process and may only be reversed by some external agency.
