You probably have actually practical suffer with the process of solvation: a heavy is dumped in water, and also after part stirring the solid appears to “disappear” into the water. Predicting as soon as solvation in water will occur, and also to what degree, is of an excellent practical interest. To construct this predictive power, we require a firm understanding of what walk on in ~ the molecular level as soon as a solute is liquified in water. Ultimately, an knowledge of the stabilizing interactions the promote solvation will help us see once such interaction are feasible for a given solute. We’ll ultimately trace aqueous solubility back to the structure of the solute ~ above the molecular level, since the intermolecular pressures that promote solubility depend on particular structural elements.

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Ionic Solutes

Ionic solutes contain one or more cations together with one or an ext anions. In the hard form, many ionic compounds room crystalline in ~ the molecule level—they consists a frequently repeating array of cations and anions, held together by solid ionic bonds. As soon as an ionic compound dissolves in water, this continuous lattice is broken apart together water molecule squeeze their method between the ions.

The dipole the water is a beautiful enhance to the optimistic and negative charges in the cation and also anion of one ionic compound. Water molecules deserve to surround the ion so that the end of the dipole with opposite charge is dealing with each ion. In the process, a big number the ion-dipole interactions change the ionic bonds the were current in the hard ionic solute (note the separating the ions successfully breaks ionic bonds). Return the ionic bonds are regularly strong, the sheer number of ion-dipole forces that show up upon solvation can overcome the ionic bonds. Thermodynamically, the free energy released because of ion-dipole pressures is greater than the cost-free energy took in to separate the ions (and water molecule hydrogen bonding with one another).

The diagram listed below illustrates the solvation the NaCl at the molecule level. Notification that a very huge number the ion-dipole interactions organize several water molecule close to every ion. Salary attention also to the leading orientation that water molecules about each ion.

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For one interactive variation of this image, inspect out this link.

Covalent Solutes

Covalent compounds lack complete formal charges, but many polar covalent compound are nonetheless soluble in water. Covalent compounds in the solid kind are identified by far weaker intermolecular pressures than ionic compounds; generally, dipole-dipole interactions are their primary forces. As soon as a polar covalent solute is inserted in water, dipole-dipole interaction or hydrogen bonds through far more numerous water molecules deserve to replace solute-solute interactions. As above, solute-water interactions should release more free energy than is forced to rest solute-solute interactions and also hydrogen bonds in water.

Glucose is a nice instance of a polar covalent compound that is dissolve in water. Water is able to hydrogen bond with the hydroxyl teams in glucose, as displayed in the figure below. Because that an interactive picture of the resolution of sucrose (a related compound), inspect out this link. To compare the solvation processes of NaCl and also sucrose in ~ the molecular level!

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Note the requirement that the covalent solute it is in polar. Polar covalent bonds are essential because the origins of solubility space stabilizing dipole-dipole or hydrogen bonding interactions. Both that these forces boil down to electrostatic attraction between opposite partial charges; consequently, solvation is not thermodynamically favorable unless these charges space present. Non-polar covalent compounds interact only very weakly with water, via London dispersion pressures (and London pressures with molecule of their own kind are normally stronger). Because that example, the hydrocarbons benzene and also hexane, shown below, are not dissolve in water.

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Writing dissolution Equations

It’s worth noting at this suggest that solvation the ionic solutes causes the ions to rest apart, while solvation that covalent compounds walk not cause the cleavage of any kind of bonds per se in ~ solute molecules. This observation has necessary implications because that the method we create chemical equations for dissolution. Because that an ionic solute, we show the ion of the solute splitting apart—here, it’s vital to acknowledge polyatomic ions within a larger ionic compound.

$$mathrmNa_3PO_4(s) ightarrow 3 : mathrmNa^+(aq) + mathrmPO_4^3-(aq)$$

Note the all three sodium ions space written as different from the phosphate ion, and also the equation is well balanced overall. The “(aq)” phase designator is shorthand for the molecular-level photo we’ve just seen: a species surrounded by water molecules, which connect with the types via ion-dipole forces, dipole-dipole forces, or hydrogen bonding.

Dissolution equations for covalent solutes room written similarly—note the conversion from the “(s)” designator to “(aq).” However, the covalent solute go not split apart in any type of way, therefore to create a dissolved equation because that a covalent solute we need only change (s) with (aq).

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$$mathrmC_6H_12O_6(s) ightarrow mathrmC_6H_12O_6(aq)$$

Loaded right into the (aq) price is a wealth of information around the molecular level. Water is an implicitly reactant in dissolution equations, in the feeling that it shows up on the commodities side, concealed in the meaning of (aq).