Question

The dissolving of a solute into a solvent can be thought of as a three-step process. The drinking alcohol, ethanol \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{OH}\right)\), is infinitely soluble in water. However, another alcohol called pentanol, with the molecular formula \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2}-\mathrm{OH}\), is nearly insoluble in water. (a) Which of the three steps in the solubility model is most responsible for this difference in solubility? Describe the step you choose (specify its name). (b) Give a BRIEF explanation for your choice.

Short answer

(a) The most responsible step for the difference in solubility of ethanol and pentanol is the Formation of solute-solvent interactions (Solvation). (b) The longer carbon chain of pentanol reduces its ability to mix with polar water molecules, while ethanol has a shorter carbon chain, making its solvation process more effective and increasing its solubility.

Step-by-step solution

(a) Identify the most responsible step in the solubility model

The three steps in the solubility model are: 1. Separation of solute particles 2. Separation of solvent particles 3. Formation of solute-solvent interactions (Solvation) The most responsible step for the difference in solubility of ethanol and pentanol in water is the Formation of solute-solvent interactions (Solvation).

(a) Describe the chosen step (Solvation)

Solvation is the process where solvent molecules surround and interact with solute particles. This step involves the formation of new attractive forces between solute and solvent particles, which promotes the dissolving of the solute in the solvent. In the context of this problem, solvation is the formation of hydrogen bonds between the alcohol molecules (ethanol or pentanol) and water molecules.

(b) Explanation for the choice of Solvation

Both ethanol and pentanol have a hydroxyl group (-OH) that can form hydrogen bonds with water molecules, making them initially soluble. However, the difference between these two molecules lies in their carbon chain length. Ethanol has a shorter carbon chain with two carbon atoms, while pentanol has a longer carbon chain with five carbon atoms. The longer carbon chain of pentanol has more nonpolar character due to its increased hydrocarbon content. Although the hydroxyl group can still form hydrogen bonds with water, the predominant nonpolar character of the long carbon chain of pentanol reduces its ability to mix with the polar water molecules and reduces its solubility. On the other hand, ethanol has a shorter and less nonpolar carbon chain. As a result, its solvation process is more effective since it can easily form hydrogen bonds with water molecules and dissolve. Thus, the solvation step is most responsible for the difference in solubility between ethanol and pentanol.

Key concepts

Solvation

Solvation is a crucial process in chemistry, particularly when it comes to understanding how substances dissolve. It is the stage in which solvent molecules surround solute particles, forming interactions that allow the solute to disperse throughout the solvent. This interaction can be quite complex as it involves breaking previous bonds in both the solute and the solvent and then forming new ones.

In the case of our alcohols, ethanol and pentanol, both compounds have an -OH group capable of forming hydrogen bonds. Ethanol, with its short carbon chain, is more effectively 'solvated' by water due to its relatively polar nature, resulting in a good solubility. Pentanol, however, has a longer nonpolar carbon chain that doesn't interact as favorably with water, making its solvation and subsequent solubility much lower.

Notably, solvation is an endothermic process where energy is required to separate solvent and solute particles; yet, the process can also release energy when new bonds are formed. The balance of these energy changes determines the overall ease of solvation and, thereby, solubility.

Solubility Model

The solubility model helps us understand why some substances dissolve in solvents while others do not. It consists of three key steps: separation of solute particles, separation of solvent particles, and formation of solute-solvent interactions. For a substance to dissolve, interactions between solute and solvent in the third step must be strong enough to overcome the energy required for the first two steps.

This model predicts solubility based on the balance of forces acting in dissolution. Polar solvents like water are better at solvating polar solutes due to their ability to establish strong electrostatic interactions. Nonpolar solutes, on the other hand, are better dissolved by nonpolar solvents, with dispersion forces being the main type of interaction.

Pentanol's low solubility in water is explained through this model, as the nonpolar carbon chain resists the solvation step, outweighing the hydrogen bonding from the -OH group, leading to its near insolubility.

Hydrogen Bonding

Hydrogen bonding is a powerful intermolecular force that plays a fundamental role in solvation. It occurs when a hydrogen atom attaches to a highly electronegative atom, like oxygen or nitrogen, and the resulting partially positive hydrogen atom can interact with other electronegative atoms.

In our example, the hydroxyl (-OH) group is able to form hydrogen bonds with water, which is a highly polar solvent. Ethanol, with just two carbon atoms in its chain, allows these hydrogen bonds to account for a significant part of its interactions, ensuring it remains soluble. However, for pentanol, the longer carbon chain disrupts these hydrogen bonding interactions due to its nonpolar nature, overshadowing the -OH group's capacity to interact with water and hence reducing its solubility.

Polar and Nonpolar Molecules

The concepts of polar and nonpolar molecules are fundamental in understanding chemical interactions. Molecules are polar when they have a significant difference in electronegativity between atoms, resulting in a distribution of charge across the molecule. Water is a classic example of a polar molecule due to the difference in electronegativity between hydrogen and oxygen, leading to a dipole moment.

Nonpolar molecules, on the other hand, have a more uniform charge distribution, often due to symmetrical arrangement of atoms or bonds between atoms of similar electronegativity. The carbon chains in organic molecules like pentanol are nonpolar because carbon and hydrogen have relatively similar electronegativities.

The polar nature of ethanol's carbon chain is minimal, thereby enabling it to dissolve in water. In contrast, the significantly nonpolar carbon chain of pentanol impedes its interaction with water, explaining why pentanol's solubility in water is much lower than ethanol's.