Answer: NaOH (s) → NaOH (aq) is an exothermic process because energy is released when solid NaOH dissolves in water to form an aqueous solution.
NH4Cl (s) → NH4Cl (aq) is also an exothermic process for the same reason as NaOH above.
CaCl2 (s) → CaCl2 (aq) is also an exothermic process because energy is released when solid CaCl2 dissolves in water to form an aqueous solution.
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l) is an exothermic process because energy is released as the acid-base reaction occurs and the products are formed.
NaCl (s) → NaCl (aq) is not an endothermic or exothermic process since there is no change in the state of the substance (solid to aqueous solution) and therefore no energy is either absorbed or released.
Explanation: Endothermic and exothermic refer to processes that either require energy to be absorbed or release energy to the surroundings.
In the given reactions, NaOH, NH4Cl, and CaCl2 dissolve in water to form aqueous solutions, and in doing so, they release energy to the surroundings, making these processes exothermic.
The reaction between HCl and NaOH also releases energy as the products are formed, and so it is also exothermic.
However, in the case of NaCl, there is no change in the state of the substance, and so no energy is either absorbed or released, making it neither endothermic nor exothermic.