Firstly, First law of thermodynamics does not inform about the direction of the flow of heat. The first law of thermodynamics does allow heat flow from a cold body to hot body, a fact which is never observed in nature.
Secondly , this law allows heat engines to have 100% efficiency, running of water uphill etc which are not seen in nature.
Thus, the first law of thermodynamics is inadequate in showing why heat behaves in the way that it does. These limitations are overcome by the second law of thermodynamics. It 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.