Question

Choose the member of each set that you would expect to be more soluble in water. (a) ethyl chloride \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\right)\) or ethanol \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\right)\) (b) ammonia \(\left(\mathrm{NH}_{3}\right)\) or phosphine \(\left(\mathrm{PH}_{3}\right)\) (c) heptanoic acid \(\left(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{5} \mathrm{COOH}\right)\) or propionic acid \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\right)\) (d) iodine trichloride or sodium iodide

Short answer

Answer: (a) Ethanol (C2H5OH), (b) Ammonia (NH3), (c) Propionic acid (CH3CH2COOH), (d) Sodium iodide (NaI)

Step-by-step solution

(a) Ethyl chloride (C2H5Cl) or Ethanol (C2H5OH)

To determine which one is more soluble in water, we need to compare their polarity. Ethyl chloride has a polar C-Cl bond, but Ethanol has a polar O-H bond, which is much stronger. Additionally, ethanol can form hydrogen bonds with water molecules, while ethyl chloride cannot. Therefore, ethanol (C2H5OH) is more soluble in water.

(b) Ammonia (NH3) or Phosphine (PH3)

Ammonia and phosphine are both polar molecules due to the presence of polar N-H and P-H bonds respectively. However, the nitrogen in ammonia has a lone pair of electrons that can participate in hydrogen bonding with water, whereas phosphorus in phosphine does not have this ability. Therefore, ammonia (NH3) is more soluble in water.

(c) Heptanoic acid (CH3(CH2)5COOH) or Propionic acid (CH3CH2COOH)

Both heptanoic acid and propionic acid are carboxylic acids and can form hydrogen bonds with water due to the presence of the polar O-H bond in their carboxylic acid functional group. The solubility of carboxylic acids in water generally decreases with increasing chain length, which means that the acid with the shorter hydrocarbon chain will be more soluble. Propionic acid has a shorter hydrocarbon chain (with three carbon atoms) than heptanoic acid (with seven carbon atoms). Therefore, propionic acid (CH3CH2COOH) is more soluble in water.

(d) Iodine trichloride (ICl3) or Sodium iodide (NaI)

Iodine trichloride is a covalent compound with polar I-Cl bonds. However, in water, it can undergo hydrolysis forming hydrochloric acid (HCl) and hypoiodous acid (HOI). These products are soluble in water but increase its acidity. Conversely, Sodium iodide is an ionic compound composed of Na+ and I− ions. In water, these ions separate and become surrounded by water molecules, forming stable interactions through ion-dipole forces. As a result, sodium iodide (NaI) is more soluble in water than iodine trichloride.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that occurs between a hydrogen atom bonded to a highly electronegative atom, such as oxygen or nitrogen, and another electronegative atom with a lone pair of electrons. In the context of molecular solubility, hydrogen bonds greatly enhance the solubility of compounds in water.

For instance, ethanol \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) is more soluble in water than ethyl chloride \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\) primarily due to its ability to form hydrogen bonds. The -OH group in ethanol can donate a hydrogen atom to the lone pair of oxygen in a water molecule, creating a strong interaction that helps it dissolve.

Polarity of Molecules

The polarity of a molecule is determined by the distribution of electrical charge across its atoms. Polar molecules have a partial positive charge on one end and a partial negative charge on the other, enabling them to interact with water molecules, which are also polar.

Polarity is significant in understanding solubility because polar substances tend to be more soluble in polar solvents, like water. In the given exercise, ethanol and ammonia are examples of polar molecules that dissolve in water due to their polar bonds and the formation of hydrogen bonds.

Carboxylic Acids

Carboxylic acids are organic compounds containing a carboxyl group (\(–COOH\)). Their polarity and ability to form hydrogen bonds make them generally soluble in water.

However, as the length of the hydrocarbon chain in a carboxylic acid increases, its hydrophobic (water-repelling) character increases, and its solubility tends to decrease. Propionic acid \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\), having a shorter chain, is more soluble in water compared to heptanoic acid.

Molecular Solubility

Molecular solubility in water is influenced by molecular structure and interactions. Soluble molecules typically exhibit polar functional groups or charged species that can interact favorably with water molecules.

In the provided examples, ethanol and propionic acid are more soluble due to their smaller size and their ability to form strong interactions through hydrogen bonding and polarity, compared to their respective counterparts.

Ion-Dipole Forces

Ion-dipole forces play a crucial role in the solubility of ionic compounds in water. This type of interaction occurs between an ion and the partial charge on a polar molecule.

When ionic substances such as sodium iodide (\(\mathrm{NaI}\)) dissolve in water, they separate into ions that are surrounded and stabilized by water molecules. These interactions are much stronger than the interactions between molecules with only polar bonds, accounting for the high solubility of ionic compounds in water.