# enthalpy change of sol more or less exo

QUESTION
The enthalpy changes of solution of the magnesium halides show a trend from MgF2 to
MgI2.
Explain why it is difficult to predict whether the enthalpy change of solution becomes
more exothermic or less exothermic down the group from MgF2 to MgI2. (4)

MARKSCHEME
Halide ion gets larger down the group
-Lattice enthalpy
Lattice enthalpy is less exothermic down group
OR halide ion has less attraction for Mg2+
-Hydration enthalpy
Hydration enthalpy is less exothermic down group
OR halide ion has less attraction for H2O
-Enthalpy of solution
Difficult to predict whether lattice enthalpy or
hydration enthalpy has bigger effect

Im struggling to understand this question. this is my thought process:

enthalpy change sol= sum of hydration enthalpies- lattice enthalpy

fluoride ion has a smaller ionic radius than iodide ion

there is a greater attraction between F- and Mg2+ than I- and Mg2+ so greater LE for MgF2 than for MgI2.

this results in a MORE exothermic enthalpy change of sol (since your taking away a larger number)

HOWEVER, this also results in a larger enthalpy change of hydration since there is greater attraction between ions and water in when MgF2 dissolves than when MgI2 dissolves.

this results in a LESS exothermic value (since you have a bigger enthalpy chnage of hydration)

it is difficult to predict which change has the greater effect

i know now that if the enthalpy change of hydration is larger, you get a more exothermic value but i don't understand why that is accroding to the formula: enthalpy change sol= sum of hydration enthalpies- lattice enthalpy
Original post by busybee87
QUESTION
The enthalpy changes of solution of the magnesium halides show a trend from MgF2 to
MgI2.
Explain why it is difficult to predict whether the enthalpy change of solution becomes
more exothermic or less exothermic down the group from MgF2 to MgI2. (4)

MARKSCHEME
Halide ion gets larger down the group
-Lattice enthalpy
Lattice enthalpy is less exothermic down group
OR halide ion has less attraction for Mg2+
-Hydration enthalpy
Hydration enthalpy is less exothermic down group
OR halide ion has less attraction for H2O
-Enthalpy of solution
Difficult to predict whether lattice enthalpy or
hydration enthalpy has bigger effect

Im struggling to understand this question. this is my thought process:

enthalpy change sol= sum of hydration enthalpies- lattice enthalpy

fluoride ion has a smaller ionic radius than iodide ion

there is a greater attraction between F- and Mg2+ than I- and Mg2+ so greater LE for MgF2 than for MgI2.

this results in a MORE exothermic enthalpy change of sol (since your taking away a larger number)

HOWEVER, this also results in a larger enthalpy change of hydration since there is greater attraction between ions and water in when MgF2 dissolves than when MgI2 dissolves.

this results in a LESS exothermic value (since you have a bigger enthalpy chnage of hydration)

it is difficult to predict which change has the greater effect

i know now that if the enthalpy change of hydration is larger, you get a more exothermic value but i don't understand why that is accroding to the formula: enthalpy change sol= sum of hydration enthalpies- lattice enthalpy

If you are trying to work it out based on the formula enthalpy change sol= sum of hydration enthalpies- lattice enthalpy, then consider that BOTH hydration and lattice enthalpy are decreasing but you don't know what rate each is decreasing at. If hydration was decreasing faster than lattice enthalpy, then the sum would be smaller. But if lattice enthalpy was decreasing faster, then the sum (enthalpy of solution) would increase instead.
Overall, you don't know which is decreasing by more, so you don't know weather enthalpy of solution will increase or decrease.
hope this helps with your understanding