Question

which change in H ( enthalpy) associated with the dissolution of lithium chloride are exothermic: 1) change H : energy associated with the separation of water molecules 2) change in H : energy associated with the separation of LiCl ions 3) change in H: energy associated with the formation of water - ion interactions 4) change in Hsolution: the enthalpy of solution

please, explain your choices/answers. for example, I do know that 4)the choice is true.

Answer 1

Let's understand the steps involved in the process of dissolution.

Step 1 : breaking of bonds between water molecules. : In order to break the bonds, energy has to be absorbed. Therefore option 1 ""change H : energy associated with the separation of water molecules"" is endothermic because energy is absorbed.

Step 2 : breaking of ionic bonds of the salt. : This also involves breaking of bonds so energy is absorbed. Therefore option 2 ""change in H : energy associated with the separation of LiCl ions"" is endothermic.

Step 3 : Solvation of ions : Here water molecules approach the ions which are formed and temporary bonds are formed between water molecules and ions. This is called solvation of ions. Since bonds are formed, energy is released in this step. Therefore option 3 ""change in H: energy associated with the formation of water - ion interactions"" is exothermic

Depending on the amount of energy absorbed and released , the final dissolution process becomes endothermic or exothermic.

In case of LiCl, the solvation energy of Li is very high, that is when water molecules approach Li+ ions, a large amount of energy is released. Therefore the dissolution of LiCl becomes exothermic

The correct options are therefore option 3 & 4