Question

Choose the member of each set that you would expect to be more soluble in water. Explain your answer. (a) chloromethane, \(\mathrm{CH}_{3} \mathrm{Cl}\), or methanol, \(\mathrm{CH}_{3} \mathrm{OH}\) (b) nitrogen triiodide or potassium iodide (c) lithium chloride or ethyl chloride, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\) (d) ammonia or methane

Short answer

a) Chloromethane or Methanol b) Nitrogen Triiodide or Potassium Iodide c) Lithium Chloride or Ethyl Chloride d) Ammonia or Methane Answer: a) Methanol b) Potassium Iodide c) Lithium Chloride d) Ammonia

Step-by-step solution

Problem (a) - Chloromethane or Methanol

Chloromethane (\(\mathrm{CH}_{3} \mathrm{Cl}\)) is a slightly polar molecule due to the electronegativity difference between carbon and chlorine atoms. Methanol (\(\mathrm{CH}_{3} \mathrm{OH}\)), on the other hand, is polar and can form hydrogen bonds with water due to the presence of an -OH group. Hydrogen bonding increases the solubility of substances in water. Between the two, methanol (\(\mathrm{CH}_{3} \mathrm{OH}\)) is expected to be more soluble in water due to its ability to form hydrogen bonds with water molecules.

Problem (b) - Nitrogen Triiodide or Potassium Iodide

Nitrogen triiodide is a covalent compound, whereas potassium iodide is an ionic compound. In general, polar water molecules can more effectively solvate ions than nonpolar molecules. This means that potassium iodide, an ionic compound, will have higher solubility in water than the covalent nitrogen triiodide. Therefore, potassium iodide is expected to be more soluble in water than nitrogen triiodide.

Problem (c) - Lithium Chloride or Ethyl Chloride

Lithium chloride is an ionic compound composed of lithium and chloride ions. Ethyl chloride (\(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\)) is a slightly polar molecule with lesser polarity than lithium chloride. As water is a polar solvent and can effectively solvate ions, lithium chloride is expected to be more soluble in water than ethyl chloride.

Problem (d) - Ammonia or Methane

Ammonia is a polar molecule and can form hydrogen bonds with water molecules due to the presence of nitrogen-containing polar bond. Methane is a nonpolar molecule with no hydrogen bonding ability. Based on the polarity and ability to form hydrogen bonds, ammonia is expected to be more soluble in water than methane.

Key concepts

Polar Molecules

Polar molecules have an uneven distribution of electrons, creating a partial electrical charge. They occur when atoms in a molecule have different electronegativities, causing a dipole moment. This means one side of the molecule is slightly negative, and the other is slightly positive. Water is a well-known polar molecule due to its bent shape and the oxygen atom being more electronegative than hydrogen.

• These molecules can dissolve in water, as the positive end of one polar molecule attracts the negative end of another.
• Polar molecules are hydrophilic, meaning they "like" water, and hence they are often more soluble.

Examples include methanol and ammonia. Methanol, with its -OH group, and ammonia, with its nitrogen, can interact with water molecules effectively, leading to higher solubility.

Hydrogen Bonding

Hydrogen bonding is a special type of attraction in polar molecules, involving a hydrogen atom. It's stronger than a typical dipole-dipole interaction but weaker than covalent or ionic bonds. These bonds occur when hydrogen is bonded directly to small, highly electronegative atoms like oxygen, nitrogen, or fluorine. This bond isn’t just about attraction; it’s about creating a bridge between molecules.

• Essential for the high solubility of molecules like methanol and ammonia in water.
• Gives water its unique properties, such as high boiling point and surface tension.

In the example of methanol ( \( \mathrm{CH}_3 \mathrm{OH} \) ), the oxygen atom in the hydroxyl group attracts electrons, making hydrogen slightly positive. This allows it to create strong interactions with the oxygen of water molecules, increasing solubility.

Ionic Compounds

Ionic compounds form when atoms transfer electrons, creating ions. These ions, typically between metals and nonmetals, have full positive and negative charges that create an electrostatic attraction, forming strong ionic bonds. In water, which is polar, the charged ends of the water molecules can surround the ions and pull them apart, dissolving the compound.

• Ionic compounds like potassium iodide and lithium chloride are usually very soluble in water.
• The solvation process involves hydration, where water molecules form a hydration shell around ions.

Potassium iodide serves as a prime example, showing that water’s polar nature effectively solvates and dissolves ionic compounds, unlike covalent compounds such as nitrogen triiodide.

Covalent Compounds

Covalent compounds are formed when atoms share electron pairs. These compounds can be polar, like water, or nonpolar, like methane. Water interacts with covalent polar molecules through dipole interactions and possibly hydrogen bonding, but its interaction with nonpolar covalent molecules is much weaker.

• Nonpolar covalent compounds, such as methane, do not interact well with water and therefore, have low solubility.
• Covalent bonds in these molecules don't break in water, unlike ionic bonds.

For instance, nitrogen triiodide, though covalent, doesn't dissolve in water as effectively as ionic compounds due to its lack of charge separation that water can interact with.