Problem 34 - Chemistry The Central Science

Question

Which of the following in each pair is likely to be more soluble in water: (a) cyclohexane \(\left(\mathrm{C}_{6} \mathrm{H}_{12}\right)\) or glucose \(\left(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}\right)\) (Figure 13.12 ); (b) propionic acid \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\right)\) or sodium propionate \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COONa}\right) ;\) (c) HCl or ethyl chloride \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\right) ?\) Explain in each case.

Step-by-Step Solution

Answer

The more soluble compounds in water are: (a) glucose, due to its polar nature and ability to form hydrogen bonds with water molecules; (b) sodium propionate, as it dissociates into ions, leading to high solubility as a result of ion-dipole interactions; and (c) HCl, as it forms ion-dipole interactions with water upon dissociation into H+ and Cl- ions.

Step 1: Pair (a): Cyclohexane (C6H12) and Glucose (C6H12O6)

To compare the solubility of cyclohexane and glucose in water, let's look at the molecular structures and the presence of polar groups or hydrogen bonding. Cyclohexane is a nonpolar hydrocarbon with no polar groups, whereas glucose is a polar molecule containing multiple hydroxyl groups, which can form hydrogen bonds with water molecules. This makes glucose more soluble in water than cyclohexane.

Step 2: Pair (b): Propionic acid (CH3CH2COOH) and Sodium propionate (CH3CH2COONa)

In this case, we are comparing the solubility of a weak acid (propionic acid) and its salt (sodium propionate). Propionic acid can form hydrogen bonds with water due to the presence of the acidic -COOH group, whereas sodium propionate, being a salt, will dissociate into CH3CH2COO- and Na+ ions, leading to a higher solubility in water due to the resulting ion-dipole interactions. Thus, sodium propionate is more soluble in water than propionic acid.

Step 3: Pair (c): HCl and Ethyl Chloride (CH3CH2Cl)

Finally, we will compare the solubility of HCl and ethyl chloride in water. HCl is a strong acid that will dissociate upon dissolution in water into H+ and Cl- ions, leading to high solubility due to ion-dipole interactions. Ethyl chloride, on the other hand, is a nonpolar molecule with a small polar component due to the C-Cl bond. However, it cannot form hydrogen bonds with water molecules. Consequently, HCl is more soluble in water than ethyl chloride due to its ability to form ion-dipole interactions. In summary, based on the polar nature of the molecules and their ability to form hydrogen bonds or ion-dipole interactions with water, the more soluble compounds in water are: (a) glucose, (b) sodium propionate, and (c) HCl.

Key Concepts

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole attraction between molecules. It occurs when a hydrogen atom, which is covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, interacts with another electronegative atom. This interaction results in a strong bond that significantly affects the molecule's properties.

Glucose is a prime example, featuring multiple hydroxyl groups (OH). Each hydroxyl group can form hydrogen bonds with water molecules, greatly enhancing glucose's solubility in water. This is because the hydrogen bonds help to stabilize the glucose molecules as they mix into the water, making the overall solution more energetically favorable.

• Hydrogen bonds are stronger than regular dipole interactions, but weaker than covalent bonds.
• They play a crucial role in many biological processes, including DNA base pairing.
• Glucose vs. Cyclohexane: Glucose forms many hydrogen bonds due to its structure, while cyclohexane does not, explaining glucose's higher solubility.

Polar Molecules

Polar molecules have an uneven distribution of electron density, leading to a molecule with a slight electrical dipole moment where one end is slightly positive and the other is slightly negative. This polarity allows the molecule to interact with other polar molecules, such as water.

Water, a highly polar solvent, is excellent at dissolving other polar substances. This is evident when comparing substances like propionic acid and sodium propionate. While propionic acid is polar and can interact with water through hydrogen bonds, sodium propionate ionizes completely, demonstrating even stronger interactions with water's dipoles due to ion formation.

• Polar substances tend to dissolve well in polar solvents ("like dissolves like").
• Sodium propionate vs. Propionic acid: Sodium propionate's ionic nature enhances solubility beyond the hydrogen bonding capacity of propionic acid.
• Polarity often dictates solubility and reactivity in chemical processes.

Ion-Dipole Interactions

Ion-dipole interactions occur between ions and polar molecules. These interactions are vital because they often lead to the dissolution of salts and similar compounds in water. The strength of these interactions depends on the charge of the ion and the size of the polar molecule's dipole moment.

For instance, when HCl dissolves in water, it dissociates into H⁺ and Cl^- ions. These ions interact strongly with the polar water molecules, forming ion-dipole bonds, which make HCl highly soluble in water.

• Ion-dipole forces are crucial in many chemical reactions and solutions.
• They are stronger than hydrogen bonds due to the full charge of ions.
• HCl vs. Ethyl Chloride: HCl's dissociation into ions provides strong ion-dipole interactions, unlike ethyl chloride, which lacks such capability.