Question

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

Short answer

In conclusion: 1. Glucose (C6H12O6), being a polar molecule, is more likely to be soluble in water than cyclohexane (C6H12), which is nonpolar. 2. Sodium propionate (CH3CH2COONa), an ionic substance, is likely to be more soluble in water than propionic acid (CH3CH2COOH). 3. HCl, which can dissociate into ions, is more likely to be soluble in water than ethyl chloride (CH3CH2Cl), which does not form ions.

Step-by-step solution

Identify the polarity of each molecule

Cyclohexane is a nonpolar molecule, consisting of only carbon and hydrogen atoms forming a ring. In contrast, glucose is a polar molecule with multiple hydroxyl groups (-OH) attached to its carbon skeleton.

Determine solubility in water

Since water is a polar solvent, glucose, being a polar molecule, is more likely to be soluble in water than cyclohexane, which is nonpolar. Pair (b): Propionic acid (CH3CH2COOH) or sodium propionate (CH3CH2COONa)

Identify the properties of each molecule

Both propionic acid and sodium propionate are polar molecules. However, sodium propionate is an ionic substance, as it can dissociate into ions (CH3CH2COO- and Na+).

Determine solubility in water

Ionic substances usually dissolve well in polar solvents like water due to the ion-dipole interactions that stabilize the ions. Thus, sodium propionate is likely to be more soluble in water than propionic acid. Pair (c): HCl or ethyl chloride (CH3CH2Cl)

Identify the properties of each molecule

HCl is a polar molecule and can even dissociate into ions (H+ and Cl-) when in contact with water. In comparison, ethyl chloride is a polar molecule but does not dissociate into ions.

Determine solubility in water

As HCl can dissociate into ions in the presence of water, it is more likely to be soluble in water than ethyl chloride, which does not form ions. In conclusion: 1. Glucose is likely to be more soluble in water than cyclohexane. 2. Sodium propionate is likely to be more soluble in water than propionic acid. 3. HCl is likely to be more soluble in water than ethyl chloride.

Key concepts

Polarity of Molecules

When it comes to understanding the solubility of different substances in water, the concept of polarity is fundamental. Water is a polar solvent, which means it has a partial positive charge on one side and a partial negative charge on the other, much like a magnet with north and south poles. This inherent polarity allows water to engage more effectively with other polar substances.

Polar molecules, such as glucose (\(\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6\))), have regions of positive and negative charge due to uneven distribution of electrons. This uneven charge distribution in polar molecules enables them to form hydrogen bonds and other types of electrostatic interactions with water molecules, making them more soluble. In contrast, nonpolar molecules like cyclohexane (\(\mathrm{C}_6\mathrm{H}_{12}\)), which have a more even distribution of electrons and no distinct poles, do not interact as favorably with water.

Thus, molecules with higher polarity, such as glucose with its many hydroxyl (-OH) groups, are typically more soluble in water compared to their nonpolar counterparts.

Ionic Substances

Some compounds, known as ionic substances, consist of positively and negatively charged ions held together by strong electrostatic forces, referred to as ionic bonds. An excellent example of an ionic substance is sodium propionate (\(\mathrm{CH}_3\mathrm{CH}_2\mathrm{COONa}\)). Unlike covalent compounds, where atoms share electrons, ionic compounds form when atoms transfer electrons, resulting in fully charged ions.

The water's polarity is particularly adept at disrupting ionic bonds because the partial negative charge on the oxygen atoms is attracted to the cations, and the partial positive charge on the hydrogen atoms is attracted to the anions. This interaction leads to the phenomena known as dissociation where the ionic compound breaks apart into individual ions when dissolved in water. As a result, ionic substances tend to have high solubility in water.

Sodium propionate, when placed in water, dissociates into sodium (Na+) and propionate (CH3CH2COO-) ions. The dissolution of these ions into the surrounding water molecules enhances its solubility compared to propionic acid, which although polar, is not ionic and so does not dissociate to the same extent.

Dissociation into Ions

Dissociation into ions in an aqueous solution can significantly influence the solubility of a substance. When certain polar compounds, particularly ionic salts and some acids and bases, dissolve in water, they separate into their constituent ions. This process is a critical factor in determining the solubility of a substance in water.

Hydrochloric acid (HCl) is a prime example of a substance that dissociates into ions when it dissolves in water, producing hydrogen (H+) and chloride (Cl-) ions. This complete dissociation is characteristic of strong acids, which is one reason why HCl is more soluble in water than many other substances. Even though both HCl and ethyl chloride (CH3CH2Cl) are polar molecules, HCl's ability to ionize makes it far more soluble.

The ions formed during dissociation are surrounded by and stabilized by water molecules because of the water's polar nature, which is conducive to the formation of a hydration shell around each ion. This shell helps to keep the ions separated and dispersed in solution, increasing the solubility of the substance.