Problem 2 - Chemistry and Chemical Reactivity

Question

What type of forces must be overcome when solid \(\mathrm{I}_{2}\) dissolves in methanol, \(\mathrm{CH}_{3} \mathrm{OH}\) ? What type of forces must be disrupted between \(\mathrm{CH}_{3} \mathrm{OH}\) molecules when I \(_{2}\) dissolves? What type of forces exist between \(I_{2}\) and \(\mathrm{CH}_{3} \mathrm{OH}\) molecules in solution?

Step-by-Step Solution

Answer

Overcome dispersion forces in I_2 and hydrogen/dipole forces in methanol. Form dispersion forces between I_2 and methanol.

Step 1: Identify the Forces in Solid Iodine

Solid iodine ( I_2 ) is held together by weak van der Waals forces, specifically London dispersion forces, because iodine is non-polar. These weak forces must be overcome for I_2 to dissolve.

Step 2: Determine the Forces in Methanol

Methanol ( CH_3OH ) molecules are held together by hydrogen bonding, as well as dipole-dipole interactions because it is a polar molecule with an -OH group. These forces need to be disrupted for I_2 to dissolve.

Step 3: Identify the Forces in the Solution

In the methanol solution, between I_2 and CH_3OH molecules, induced dipole-induced dipole interactions (dispersion forces) occur because the non-polar I_2 can be polarized by the polar methanol molecules.

Key Concepts

London Dispersion Forces

London dispersion forces are one of the three main types of intermolecular forces. They occur in all molecules, whether they are polar or non-polar. These forces are particularly significant in non-polar molecules. They arise from temporary shifts in the electron density within atoms or molecules, creating instantaneous dipoles.
These dipoles can induce dipoles in nearby atoms or molecules, leading to an attraction known as London dispersion forces. These forces are typically very weak compared to other intermolecular forces, such as hydrogen bonding, and they become more significant in larger atoms or molecules.
For example, solid iodine (\(\mathrm{I}_{2}\)) is held together by London dispersion forces. As iodine molecules are non-polar, they rely on these weak intermolecular forces for interaction. To dissolve iodine in a solvent like methanol, these forces must be overcome.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that plays a critical role in the properties of molecules. These bonds occur when hydrogen is directly bonded to a small, highly electronegative atom, such as nitrogen, oxygen, or fluorine. In methanol (\(\mathrm{CH}_{3}\mathrm{OH}\)), hydrogen bonding is particularly evident due to the -OH group.
The hydrogen atom in the hydroxyl group can form a strong hydrogen bond with the oxygen of nearby methanol molecules. This strong force must be disrupted when iodine dissolves in methanol. Hydrogen bonding is crucial because it significantly affects the boiling and melting points of compounds and their solubility in different solvents.

• It is stronger than London dispersion forces but weaker than covalent or ionic bonds.
• It gives methanol its higher boiling point compared to compounds lacking such interactions.

Dipole-Dipole Interactions

Dipole-dipole interactions occur in molecules that are polar, meaning they have a region of positive charge and a region of negative charge. These forces arise from the electrostatic attraction between the positive end of one polar molecule and the negative end of another.
Methanol is a polar molecule due to its -OH group, which results in a separation of charges. The dipole-dipole forces within methanol must also be disrupted for iodine to dissolve. Although not as strong as hydrogen bonds, these forces are still significant.

• They play a vital role in determining the boiling and melting points of substances.
• They contribute to methanol's solvency and ability to dissolve various solutes, like iodine.

In a methanol solution containing iodine, the non-polar \(\mathrm{I}_{2}\) interacts with methanol by inducing temporary dipoles, resulting in dipole-induced dipole interactions.