The first of the thermodynamics tells us about the relationship between y 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 temperatures to heat is unidirectional from higher temperatures 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 flow colder body to warmer body on its own, the gas in a container will not spontaneous contract inty one corner or carbon dioxide will not form carbon and dioxide will not form carbon and dioxide spontaneous occurring changes show unidirectional change.
We may ask 'what is the driving force of spontaneously occurring changes? What determines the direction of a spontaneous change? In this direction of a spontaneous change? In this section, we shall establish some criterion for these processes whether these will take place or not.
Let us first understand what do we mean by spontaneous react or change? You may think by by your common observation that spontaneous reaction is one which occurs immediately when contact is made between the reactions. Take the case of combination of hydrogen and oxygen.
These gases 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 spontaneous means 'having the potential to proceed without the assistance to external agency'.
However, it does not tell about the rate of the reaction or process. as we see is that these cannot reverse their direction on is that these cannot reverse their direction on their own. We may summaries it as follows:
A spontaneous process is an irreversible process and may only be reversed by some external agency.
