Enthalpies of solution and hydration

Question 1

Define standard enthalpy change of solution

Answer 1

the enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form a very dilute solution
ΔHsolꝋ can be exothermic (negative) or endothermic (positive)

Question 2

Give an example of enthalpy change of solution

Answer 2

KCl(s) + aq → K+(aq) + Cl-(aq)

Question 3

Define standard enthalpy change of hydration

Answer 3

the enthalpy change when 1 mole of specified gaseous ion dissolves in sufficient water to form a very dilute solution
Hydration enthalpies are exothermic

Question 4

Give an example of enthalpy change of hydrtion

Answer 4

Mg2+(g) + aq → Mg2+(aq)

Question 5

ΔHsolꝋ =

Answer 5

-ΔHlattꝋ + ΔHhydꝋ

Question 6

ΔHhydꝋ =

Answer 6

ΔHsolꝋ + ΔHlattꝋ
Note: ΔHhydꝋ can also be found adding the ΔHhydꝋ values of both anions and cations together

Question 7

What are the factors affecting enthalpy change of hydration

Answer 7

ionic charge and radius

Question 8

How does ionic charge affect enthalpy change of hydration

Answer 8

ΔHhydꝋ is more exothermic for ions with larger ionic charges
Ions with large ionic charges have a greater charge density resulting in stronger ion-dipole attractions between the water molecules and the ions in the solution
Therefore, more energy is released when they become hydrated and ΔHhydꝋ becomes more exothermic

Question 9

How does ionic radius affect enthalpy change of hydration

Answer 9

ΔHhydꝋ becomes more exothermic with decreasing ionic radii
Smaller ions have a greater charge density resulting in stronger ion-dipole attractions between the water molecules and the ions in the solution
Therefore, more energy is released when they become hydrated and ΔHhydꝋ becomes more exothermic

Question 10

Solubility of Sulphates in Group 2

Answer 10

-lattice enthalpy and enthalpy of hydration decreases down the group
-enthalpy of hydration decreases down the group by relatively larger values compared to lattice enthalpy
-that is why the solubility of the Group 2 sulfates decreases down the group because ΔHsol gets more endothermic

Question 11

Solubility of Hydroxides in Group 2

Answer 11

-lattice enthalpy and enthalpy of hydration decreases down the group
-lattice enthalpy decreases down the group by relatively larger values compared to enthalpy of hydration
-that is why the solubility of the Group 2 hydroxides increases down the group because ΔHsol gets more exothermic