Problem 168 - Introductory Chemistry Atoms First

Question

Draw pictures illustrating the solute-solvent interactions and the solvent- solvent interactions for methanol, \(\mathrm{CH}_{3} \mathrm{OH}\), in water. Is this a case of like dissolves like? Explain.

Step-by-Step Solution

Answer

In the case of methanol (\(\mathrm{CH}_{3}\mathrm{OH}\)) dissolving in water, both solute and solvent have similar intermolecular forces, which is hydrogen bonding. The favorable solute-solvent interactions between methanol and water, as well as the solvent-solvent interactions within water and methanol, confirm that the "like dissolves like" principle applies in this scenario.

Step 1: Understand the "like dissolves like" principle

The "like dissolves like" principle states that polar solvents will dissolve polar solutes, and nonpolar solvents will dissolve nonpolar solutes. This is primarily due to the similarity in intermolecular forces within the solute and solvent, allowing them to interact favorably and dissolve one another.

Step 2: Draw and label the structures of methanol and water

Methanol (\(\mathrm{CH}_{3}\mathrm{OH}\)) has a structure where three hydrogen atoms are attached to a central carbon atom, which is connected to an oxygen atom that has a hydrogen atom attached to it. Water (\(\mathrm{H}_{2}\mathrm{O}\)) has a structure where two hydrogen atoms are attached to a central oxygen atom.

Step 3: Identify the dominant intermolecular forces in both molecules

In both methanol and water, the dominant intermolecular force is hydrogen bonding. Hydrogen bonding occurs when a hydrogen atom bonded to a highly electronegative atom (such as oxygen) is attracted to another highly electronegative atom. In this case, oxygen attracts hydrogen atoms in both molecules.

Step 4: Analyze solute-solvent and solvent-solvent interactions

When methanol and water mix, the hydrogen bonding between the molecules allows for favorable solute-solvent interactions. Methanol can form hydrogen bonds with water molecules, so the methanol-water (solute-solvent) interaction is favorable. In the solvent-solvent interactions (water-water and methanol-methanol), hydrogen bonding is again the dominant force. This means that the solvent-solvent interactions are also favorable.

Step 5: Determine if "like dissolves like" applies in this case

In the case of methanol dissolving in water, the principles of "like dissolves like" apply because both solute and solvent have similar intermolecular forces (hydrogen bonding). This allows the solute (methanol) to dissolve in the solvent (water), as favorable solute-solvent interactions exist. In conclusion, methanol dissolves in water following the "like dissolves like" principle, as both molecules exhibit hydrogen bonding as their primary intermolecular force, supporting favorable solute-solvent and solvent-solvent interactions.

Key Concepts

Methanol Structure

Methanol, with the chemical formula \(\mathrm{CH}_{3}\mathrm{OH}\), is an alcohol. Its structure is simple yet important to understand. Methanol contains:

• A methyl group (\(\mathrm{CH}_{3}\)), which is a carbon atom bonded to three hydrogen atoms.
• A hydroxyl group (\(\mathrm{-OH}\)), where an oxygen atom is bonded to a hydrogen atom.

Together, these groups form methanol, a polar molecule. The oxygen atom is more electronegative, pulling electrons towards itself. This creates a slightly negative charge on the oxygen and a slightly positive charge on the hydrogen attached to it. This polarity enables interaction through hydrogen bonding.

Water Structure

Water, with the formula \(\mathrm{H}_{2}\mathrm{O}\), is well-known for its unique properties. In a molecule of water:

• Two hydrogen atoms are bonded to a single oxygen atom.
• The configuration is bent, with a bond angle of about 104.5 degrees.

The polarity of water arises because the oxygen atom is more electronegative than hydrogen, leading to a partial negative charge on oxygen and a partial positive charge on the hydrogens. This polarity and bent shape facilitate strong hydrogen bonding, essential for water's role as a universal solvent.

Hydrogen Bonding

Hydrogen bonding is a crucial interaction in chemistry, especially in mixtures like methanol and water. It occurs when:

• A hydrogen atom, bonded to an electronegative atom like oxygen, attracts another electronegative atom.
• This attraction creates a bond that is stronger than Van der Waals forces but weaker than covalent bonds.

In methanol and water, the oxygen of one molecule can bond with the hydrogen of another, forming strong hydrogen bonds. These bonds are key to understanding why methanol dissolves in water effectively.

Solute-Solvent Interactions

When methanol is mixed with water, solute-solvent interactions occur predominantly through hydrogen bonding. Here's what happens:

• The hydrogen atoms in methanol form bonds with the oxygen atoms in water.
• Similarly, the hydroxyl group of methanol interacts with water molecules, strengthening the mix.

These interactions support the "like dissolves like" principle because both methanol and water are polar and capable of hydrogen bonding. This mutual attraction facilitates the dissolution of methanol in water, creating a homogenous solution. These favorable interactions ensure that both substances mix thoroughly.