Ammonium nitrate -
When ammonium nitrate is dissolved in water, it produces an endothermic reaction, i.e. energy is taken in by the system from its surroundings in the form of heat. When the nitrate ion is separated from the ammonium ion by the polar water molecules, an endothermic reaction takes place.
However, when the separated ions become hydrated in the solution, an exothermic reaction reaction takes place - heat is given off, instead of absorbed. But since more energy is required to separate the ions than to hydrate these ions, the overall reaction is indeed endothermic.
Ammonium chloride -
The same thing happens when ammonium chloride is dissolved in water. The endothermic separation of the ions will dictate the overall reaction, since the exothermic hydration of the ions will gfive off less energy. As a result, dissolving ammonium chloride in water will be an endothermic process.
Calcium chloride -
This time the reaction will be exothermic. The dissociation of calcium chloride is exothermic due to the energy released in the form of heat when anhydrous calcium chloride absorbs water to become hydrated calcium chlolride.
Sodium carbonate -
Again, this reaction will be exothermic, since more energy is released when sodium hydroxide and carbonic acid are formed, than when
Sodium acetate -
This reaction is exothermic because of the bond-forming that takes place during the crystallization of the supersaturated solution of sodium acetate in water.
Magnesium sulfate -
This reaction is endothermic, since more energy is required to break up the magnesium sulfate lattice into individual ions, than is released during the solvation process.