Question

The Born-Haber cycle was developed as a means to explain the concept of lattice energy for ionic compounds, but it can also be applied to describe the energetics of solution formation. It is especially useful in the case of dissolution of ionic solids in water.
We can envision solution formation as being the net result of a three-step process:
(1)The solvent expands, making space for the solute particles;
(2)The solute’s IMF have to be overcome, freeing the individual ions or molecules from each other; and
(3)Solute and solvent particles (molecules, ions) attract each other forming the solution.
Consider the case of an ionic solid, MX dissolving in water, as shown by the equation:
$M{X_{(s)}} \to M_{aq}^ + + X_{aq}^ - $
Select the choice below that is incorrect concerning the formation of this solution.
A.Step (1) and (2) are both endothermic.
B.Step (3) must be exothermic, or a solution will not be found.
C.The overall enthalpy of solution, $\Delta {H_{solution}}$ must be negative (exothermic).
D.The entropy of solution,$\Delta {S_{solution}}$ , must be positive

Answer 1

Hint: A reaction that releases energy is an exothermic reaction; its enthalpy change is negative. The enthalpy of the products is less than that of the reactants. Energy is released to the surroundings. Opposite is true for endothermic reactions.

Complete Step by step answer: In this case of an ionic solid, MX dissolved in water,
$M{X_{(s)}} \to M_{aq}^ + + X_{aq}^ - $
When energy is given, the ionic solid MX dissociates. The overall enthalpy of the solution will be positive as the reaction utilizes energy.
Consider ionic solid ($NaCl$ ) which dissociates into corresponding $N{a^ + }$ and $C{l^ - }$ ions in water when energy is applied in the form of heat or electricity in three steps.
(1) The solvent that is, water expands, making space for the solute particles, $NaCl$.
(2) The $NaCl$ solid will utilize the energy provided to overcome it’s intermolecular forces of attraction and frees individual ions from each other, forming $N{a^ + }$ and $C{l^ - }$ ions.
(3) Solute and solvent particles attract each other, forming solutions by
The $N{a^ + }$ ions are attracted to the oxygen side of the water.
-The $C{l^ - }$ions are attracted to the hydrogen side of water.
-This is due to electronegativity difference.
−In the above reaction step 1 and step 2 are endothermic reactions as they utilize energy and $\Delta H$ is positive.
−Step 3 is exothermic as this involves release of energy.
−The overall enthalpy of solution $\Delta H$ solution is positive as there is more utilization of heat than emission of heat.
−Entropy of solution $\Delta S$ solution is positive because Positive enthalpy decreases the entropy of the surroundings because energy is absorbed. When salt dissolves, the entropy of water and salt increases.
The increase in the entropy of the water and the salt is greater than the decrease in the entropy of surroundings.
The result is the increase of overall change in entropy and is positive, thus salt dissolves in water.

So the incorrect option is (c).

Note: Increasing the temperature will increase the entropy. Changes in volume will lead to changes in entropy. The larger the volume the more ways there are to distribute the molecules in that volume.