Limitations of the first law of thermodynamics are discussed below:
1. No restriction on the direction of the flow of heat: the first law establishes definite relationship between the heat absorbed and the work performed by a system. The first law does not indicate whether heat can flow from a cold end to a hot end or not. For example: we cannot extract heat from the ice by cooling it to a low temperature. Some external work has to be done.
2. Does not specify the feasibility of the reaction: first law does not specify that process is feasible or not for example: when a rod is heated at one end then equilibrium has to be obtained which is possible only by some expenditure of energy.
3. Practically it is not possible to convert the heat energy into an equivalent amount of work.
To overcome this limitations, another law is needed which is known as second law of thermodynamics.
The second law of thermodynamics helps us to predict whether the reaction is feasible or not and also tell the direction of the flow of heat.
It also tells that energy cannot be completely converted into equivalent work.