Question

Explain how spontaneity ties in with the second law of thermodynamics. That is, how spontaneity relates to whether or not a process is reversible or irreversible.

Answer 1

A spontaneous process is just like a natural process and that occurs without requiring any external energy or any other additional thing to process to happen and process is thermodynamically favourible . It is natural , feasible

This spontaniety of a process is directly related to laws of thermodynamics in which it ties up with 2nd law of thermodynamics according to which in any spontanous process , there is always increase in entropy of system or state of entropy of system always increase over time .

So from this law we can say that if there is increase in entropy of a system that means $\Delta$S is greater than zero and it increases over time . ( Entropy is represented by S )

To decide whether a process is spontaneous or not we use Gibbs free energy relation which is given by

$\Delta$G = $\Delta$H - T$\Delta$S , where H is enthalpy , T is temperature and G is gibbs free energy

A reaction will be spontaneous if free energy change is negative that is $\Delta$G = - ve

So according to 2nd law , entropy increases over time that means $\Delta$S increases which implies that T$\Delta$S will also increase and eventually $\Delta$G will become more less and process or reaction will be more spontaneous .

So tha's how spontaneity ties with 2nd law of thermodynamics .

Now Reversible process is one which can be reversed at any time and can go back to it's previous state and irreversible process is one which can't be reversed over time and it's direction can not be reversed.

Generally Spontaneous process are considered irreversible in nature because according to thermodynamics spontaniety means reaction occuring without any external support or asset and they are occuring of their own which means they do not prefer to get reversed or go back in previous state . So spontaneous process are generally irreversible .

This can be understood by taking example of Ice melting into liquid water which is occuring natrurally so this process is spontaneous and now we cannot convert water into ice until temperature is reduced or external support is given so this is irreversible process but made reversible by applying some external support .