Ans: the primary law of thermodynamics has following limitations.

(a) It doesn't indicate the direction of flow of warmth . Naturally heat flows from hot body to cold body but why heat doesn't be due cold body to hot body. First law of thermodynamics has no answer of this question.

(b) When a bullet strikes a target, K.E. of the bullet is converted into heat . But firsto law of thermodynamics fails to answer why heat developed within the target can't be converted back to energy of the bullet enabling it to fly back.

(c)First law of thermodynamics doesn't give the thought that to what extend the energy can be converted into heat and vice-versa. (This idea gives the efficiency of engine.)