Question

Apply your knowledge of polarity and solubility to predict whether solvation is possible in each situation shown in Table \(14.9 .\) Explain your answers. \(\begin{array}{ll}{\text {Solute}} & {\text {Solvent}} \\ {\text { solid } \mathrm{MgCl}_{2}} & {\text { liquid } \mathrm{H}_{2} \mathrm{O}} \\ {\text { liquid } \mathrm{NH}_{3}} & {\text { liquid } \mathrm{C}_{6} \mathrm{H}_{6}} \\\ {\text { gaseous } \mathrm{H}_{2}} & {\text { liquid } \mathrm{H}_{2} \mathrm{O}} \\ {\text { liquid } \mathrm{I}_{2}} & {\text { liquid } \mathrm{Br}_{2}} \end{array}\)

Short answer

1. Solvation of MgCl2 in water is possible due to the polar nature of both the ionic compound and the water. 2. Solvation of NH3 in benzene is not possible because NH3 is polar while benzene is nonpolar. 3. Solvation of gaseous H2 in water is not possible because H2 is nonpolar while water is polar. 4. Solvation of liquid I2 in liquid Br2 is possible because both I2 and Br2 are nonpolar diatomic molecules.

Step-by-step solution

1. Solid MgCl2 in liquid H2O

MgCl2 is an ionic compound that dissociates into Mg2+ and 2Cl- ions when it dissolves in water. Water is a highly polar solvent with the ability to solvate both positively charged and negatively charged ions because of its polar nature and hydrogen bonding capabilities. Therefore, solvation of MgCl2 in water is possible.

2. Liquid NH3 in liquid C6H6

NH3 is a polar molecule with a lone pair of electrons on the nitrogen atom, while C6H6 (benzene) is a nonpolar molecule. As "like dissolves like," a polar solute like NH3 would not dissolve well in a nonpolar solvent like benzene. Therefore, solvation of NH3 in benzene is not possible.

3. Gaseous H2 in liquid H2O

H2 is a nonpolar molecule with no polar bonds, while H2O is a highly polar solvent. Since the solute H2 and the solvent H2O have different polarities, solvation is not favored in this case. Therefore, solvation of gaseous H2 in water is not possible.

4. Liquid I2 in liquid Br2

Both I2 and Br2 are nonpolar diatomic molecules. Since "like dissolves like," solvation is possible between nonpolar solutes and nonpolar solvents. Therefore, solvation of liquid I2 in liquid Br2 is possible.

Key concepts

Polarity

When it comes to understanding chemicals and their behaviors, polarity is a fundamental concept. Polarity refers to the distribution of electrical charge over the atoms in a molecule. A molecule is polar if it has a positive end and a negative end, a result of uneven charge distribution. Water (H_2O) is a classic example: it has a bent shape with a slightly negative oxygen atom and slightly positive hydrogen atoms.

This inherent polarity of molecules like water makes them excellent solvents for other polar substances. Polar solvents tend to dissolve polar solutes, because the positive and negative ends of the solute can attract the opposite ends of the solvent molecules, facilitating what we call solvation. This mismatch in polarity between solvents and solutes can prevent solvation, as seen in the textbook example where the nonpolar NH_3 does not readily dissolve in the nonpolar solvent benzene (C_6H_6).

Solubility

Solubility is directly linked with polarity. It is a measure of how well a solute can dissolve in a solvent to form a homogeneous mixture, often referred to as a solution. High solubility occurs when the interactions between the solute and solvent are strong. This principle is exemplified by the saying 'like dissolves like', which means solutes and solvents with similar polarities tend to mix well.

For instance, table salt (NaCl), an ionic compound, is highly soluble in water because the polar water molecules can surround the charged ions of salt and allow them to separate and disperse evenly. On the contrary, oil, which is nonpolar, does not mix with water as their inherent polarities are different, leading to low solubility and thus, oil remains separate.

Intermolecular Forces

The forces that hold molecules together in a liquid or solid are called intermolecular forces. There are several types of intermolecular forces, with varying strengths and properties. For instance, hydrogen bonds are strong forces present in water molecules, where a hydrogen atom from one water molecule bonds with the oxygen atom of another. Dipole-dipole interactions occur between polar molecules, while London dispersion forces are weak and exist in both polar and nonpolar molecules.

In the context of the provided exercise, MgCl_2 dissolves in water due to ionic bonds between water molecules and the magnesium and chloride ions. Conversely, benzene can't dissolve ammonia because the weak London forces in benzene cannot overcome ammonia's stronger dipole-dipole and hydrogen bonding with itself.

Ionic and Molecular Compounds

Compounds are either ionic or molecular based on the bond types that hold their atoms together. Ionic compounds, such as MgCl_2, are composed of positively and negatively charged ions held together by ionic bonds, which are strong electrostatic attractions. These compounds typically have high melting points and conduct electricity when dissolved in water.

Molecular compounds, on the other hand, are made up of molecules, which are groups of atoms held together by covalent bonds. These compounds, like I_2 and Br_2, can be polar or nonpolar. They often have lower melting points than ionic compounds and do not conduct electricity in solution. When we consider solvation, ionic compounds typically dissolve in polar solvents due to the charge interactions, whereas molecular compounds dissolve based on the principle of polarity match.