Question

You dissolve a salt in water. How would you determine if the reaction is exothermic or endothermic?

Answer 1

Hint: In the above question, it is asked if we dissolve a salt in water then what kind of reaction takes place, exothermic or endothermic. The type of reaction depends upon the difference between lattice energy and hydration energy. If it is negative, then the process is exothermic and if it is positive, then the process is endothermic.

Complete step-by-step answer:If we dissolve salt in water, then two exchanges of energy take place:
Energy added to the solution to pull the ions away from each other which is known as Lattice energy. The second one is the energy that is released when the water molecules surround the ion, which is commonly termed as hydration energy.
The reaction is exothermic or endothermic depending upon lattice and hydration energy, that is, net energy is equal to difference in lattice energy and hydration energy.
For example, if we dissolve table salt in water then lattice energy is 779kJ/mol and the hydration energy is 774 kJ. The net energy for 1 mole of table salt is $779 - 774 = + 5$kJ.
Hence, dissolving table salt in water is endothermic.

Note:There is only a type of chemical process that is either exothermic or endothermic, that is, dissolving of salts in water. A salt is a compound which is made up of positively charged and negatively charged ions which are held together in a solid state because the positive and negative charges attract each other. If the solution is cold, the reaction is endothermic, but if the solution is hot, the reaction is exothermic.