what happens to the water of hydration when the solid dissolves

Learning Objective

• Predict whether a given ionic solid volition dissolve in h2o given the lattice energy and heat of hydration

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Fundamental Points

• In order to dissolve an ionic solid, water molecules must break upward the interactions betwixt all of the ions in the solid. To do this, they orient themselves such that they effectively reduce the localized charge on the ions. This is called hydration.
• Hydration of ions is a thermodynamically favorable process, and every bit such can release heat. This is why it is called the "heat of hydration."
• The heat of hydration (H_hydration) offsets the lattice energy(H_{lattice energy}) of an ionic solid to allow for solution formation to occur typically when H_hydration > H_{lattice free energy}.

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Terms

• oestrus of hydrationThe oestrus that is released by hydration of one mole of ions at a constant pressure. The more than the ion is hydrated, the more heat is released.
• ionAn atom or group of atoms begetting an electrical charge, such equally the sodium and chlorine atoms in a salt solution.
• thermodynamicsThe science of the conversions between heat and other forms of free energy.

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The Energetics of Solution Germination

Solubility depends on dissolution of the solute into the solvent and, like all chemical reactions, is governed by the laws of thermodynamics. This particular process is a change of country from the solute'due south starting state, either solid, liquid or gas, to a dissolved state (termed aqueous when the solvent is water), which is a distinct physical state and thus is considered a chemical reaction. In order for any chemical reaction to go along, it must be thermodynamically favorable. Many factors influence how thermodynamically favorable a given reaction is, including the oestrus of hydration, or hydration free energy released when water solvates, or surrounds, an ion, and the amount of free energy required to overcome the bonny forces between solute molecules, known as lattice free energy.

Solvent-Solute Interactions

Since the coulombic forces that bind ions and highly polar molecules into solids are quite strong, nosotros might look these solids to be insoluble in most solvents. The attractive interactions betwixt ionic molecules are chosen the lattice free energy, and they must be overcome for a solution to form. Ionic solids are insoluble in the majority of non-aqueous solvents, but they tend to have high solubility specifically in water.

The key factor that determines solubility is the interaction of the ions with the solvent. The electrically-charged ions undergo ion-dipole interactions with water to overcome strong coulombic allure, and this produces an aqueous solution. The water molecule is polar; it has a partial positive charge on the hydrogens while oxygen bears a partial negative charge. This dipole arises from the disparity in electronegativity present in the O-H bonds within the h2o molecule. Furthermore, the two lone pairs on the oxygen in water too contribute to the stabilization of any positively charged ions in solution.

Hydrated Na+Water cationWater molecules surroundings and stabilize a cation via interaction with the partial negative charge on the oxygen ends.

Every bit a result, ions in aqueous solutions are ever hydrated; that is, they are quite tightly bound to water molecules through ion-dipole interactions. The number of water molecules contained in the primary hydration vanquish, which completely encompasses the ion, varies with the radius and charge of the ion.

Lattice Energy

The dissolution of an ionic solid MX in h2o tin be thought of as a sequence of two processes:

[latex]ane) MX (s) \to M^+ (1000) + X^-(g) [/latex] [lattice energy]

[latex]2.) M^+ (m) + X^-(g) \to Thou^+ (aq) + X^-(aq)[/latex] [heat of hydration]

The first reaction (ionization) is e'er endothermic; it takes a lot of work to break upward an ionic crystal lattice into its component ions. Lattice energy is defined every bit the free energy that is released when one mole of ionic solid is formed from gaseous ions, and it increases with increasing atomic charge and decreasing diminutive size (radii). The greater the value of a compound'south lattice energy, the greater the force required to overcome coulombic attraction. In fact, some compounds are strictly insoluble due to their loftier lattice energies that cannot exist overcome to form a solution.

Estrus of Hydration (H_hydration) vs Lattice Energy

The hydration pace in the second reaction is ever exothermic (H_hydration < 0) as H₂O molecules are attracted into the electrostatic field of the ion. The heat (enthalpy) of solution (H_solution) is the sum of the lattice and hydration energies ( H_solution = H_hydration + H_{lattice energy}). From this relationship, we tin clearly see that the processes of overcoming the lattice free energy and hydrating the ions are in contest with one another.

The value of H_solution is dependent upon the magnitudes of H_hydration and H_{lattice energy} of the solute. Favorable conditions for solution formation typically involve a negative value of H_solution; this arises because the hydration procedure exceeds the lattice free energy in the solute. As oft happens for a quantity that is the sum of two large terms having reverse signs, the overall dissolution process can be either endothermic or exothermic. H_solution is merely one of the factors determining solution formation, but information technology is typically the major consideration in solution formation because of the role that enthalpy plays in about thermodynamic considerations.

The average time an ion spends in a hydration beat is virtually two to four nanoseconds, which is nigh two orders of magnitude longer than the lifetime of an private H₂O–H₂O hydrogen bail. The relative strengths of these ii intermolecular forces is apparent: ion-dipole interactions are stronger than hydrogen bond interactions.

In case you were wondering where we got the term "heat of hydration," it has to practice with the fact that some solutions are highly exothermic when formed. A hot solution results when the heat of hydration is much greater than the lattice free energy of the solute.

Enthalpy diagram for the dissolution procedureThe enthalpy diagram showing exothermic solution formation. Observe that H_solution is lower in free energy than the starting solute/solvent enthalpies. An endothermic process, on the other manus, would show H_solution as positive, and it would be higher in energy than the starting solute/solvent enthalpies.

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Source: https://courses.lumenlearning.com/introchem/chapter/solutions-and-heats-of-hydration/

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