When ammonium chloride is added to barium hydroxide solution, following reaction takes place:
2 NH4Cl(aq) + Ba(OH)2(aq) 2 NH4OH (aq) + BaClā2(aq)
The reaction above is double displacement reaction in which ions are exhanged to form a more stable compound. Thus, as bond breaking and making process is leading to the formation of much more stable compounds, the energy is absorbed and the process is endothermic.