NH4Cl or ammonium chloride combines with H2O or water to form aqueous ammonium chloride. This process requires heat energy as the product, aqueous ammonium chloride, has a higher enthalpy than the reactants, ammonium chloride and water. TutorVista explains that when ammonium chloride crystals are dissolved in a beaker of water, the reaction between ammonium chloride and water takes up heat from the surroundings so that the beaker becomes colder.
Another example of an endothermic reaction is the combination of sodium thiosulphate, known as Na2S2O3 5H2O, with water to form aqueous sodium thiosulphate, known as Na2SO3. The reaction between one mole of oxygen and one mole of nitrogen to form 2 moles of nitric oxide is also an endothermic reaction.
An exothermic reaction, on the other hand, is one which releases heat into the surroundings. An exothermic reaction produces heat energy because the enthalpy of the product is lower than that of the reactants. Besides heat energy, chemical reactions can also produce other forms of energy, such as electrical energy. For example, the reaction between Zn and CuSO4 in a Daniel cell produces electrical energy.Learn more about Chemistry