Problem 110 - Chemistry Matter and Change

Question

Jodine Solid iodine that is left at room temperature subli- mates from a solid to a gas. But when heated quickly, a different process takes place, as described here. "About 1 g of iodine crystals is placed in a sealed glass ampoule and gently heated on a hot plate. A layer of purple gas is formed at the bottom, and the iodine liquefies. If one tilts the tube, this liquid flows along the wall as a narrow stream and solidifies very quickly". $$\begin{array}{l}{\text { Why does solid iodine sublime readily? Use your }} \\\ {\text { knowledge of intermolecular forces to explain. }}\end{array}$$

Step-by-Step Solution

Answer

Solid iodine sublimates readily due to weak intermolecular forces that are easily overcome by thermal energy at room temperature.

Step 1: Understanding Sublimation

Sublimation is the process where a substance transitions directly from a solid to a gas without passing through the liquid state. This occurs under specific conditions of temperature and pressure, especially when the molecules have sufficient energy to overcome intermolecular forces and enter the gas phase directly.

Step 2: Exploring Iodine's Intermolecular Forces

Solid iodine is composed of I₂ molecules that are held together in a solid form by weak intermolecular forces known as Van der Waals forces (specifically, London dispersion forces). These forces are considerably weaker compared to covalent or ionic bonds.

Step 3: Sublimation of Iodine at Room Temperature

At room temperature, the thermal energy is sufficient to overcome the weak intermolecular forces holding the iodine molecules in its solid form, allowing the iodine to sublime readily into a gas without a liquid intermediate.

Step 4: Comparing with Heating Process

In the provided scenario, when iodine is heated in a sealed ampoule, the added energy allows the iodine to first form a purple gas and then liquefy as the kinetic energy of the molecules is increased beyond the sublimation point, temporarily favoring a liquid phase before transitioning back to solid upon cooling.

Key Concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles (atoms, molecules, or ions). These are much weaker than the intramolecular forces which hold a compound together, such as covalent or ionic bonds. In the case of solid iodine, these intermolecular forces are what keep the I₂ molecules together in a solid state at room temperature.

In general, there are several types of intermolecular forces:

• Dipole-Dipole Forces: Occur between molecules that have permanent dipoles.
• Hydrogen Bonds: A specific type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine.
• London Dispersion Forces: These are temporary, induced dipole-induced dipole attractions and are the only type of intermolecular force present in nonpolar molecules.

London dispersion forces, which are also known as Van der Waals forces, are the forces at play in solid iodine. These forces are relatively weak, making it possible for iodine to sublime readily as the molecules can easily overcome these attractions at room temperature by gaining kinetic energy from ambient thermal energy.

Van der Waals Forces

Van der Waals forces encompass a variety of weak attractions that contribute to the stability of molecules within a solid. There are three main types: dipole-dipole interactions, dipole-induced dipole interactions, and London dispersion forces. In iodine, London dispersion forces are predominant. These forces act due to temporary fluctuations in electron distribution, which induce small dipoles even in nonpolar molecules.

In solid iodine, each I₂ molecule influences the adjacent molecules to create a transient flicker of attraction as electrons constantly move about, leading to temporary imbalances that produce weak attractions. These transient attractions are easily overcome by thermal energy, which explains iodine's sublimation at room temperature.

Unlike ionic or covalent bonds, which require significant energy to break, Van der Waals forces hold substances more softly, allowing changes in state to happen more readily. This is why substances like iodine, dry ice, and naphthalene, all held together by Van der Waals forces, can switch directly from a solid to a gaseous state, skipping the liquid state.

Iodine Transition Steps

When iodine crystals are subjected to different temperature conditions, they can undergo several transitions, depending on the energy provided. Here's a simple breakdown of the transition steps iodine undergoes:

• In Normal Conditions: Solid iodine directly sublimes to a gas at room temperature due to the weak London dispersion forces holding the molecules together. The ambient temperature provides enough energy for the molecules to overcome these forces.
• During Gentle Heating: As the iodine is gently heated in a sealed glass ampoule, the added thermal energy increases the kinetic energy of the molecules. Initially, this causes the solid iodine to sublime, forming a purple gas at the bottom of the ampoule.
• Further Heating: Continued heating provides sufficient energy for the iodine to enter a liquid state temporarily. This happens as the thermal energy exceeds the sublimation energy threshold, melting the iodine.
• Cooling Off: When the liquid iodine moves along the glass wall and cools, it quickly solidifies. This rapid transformation occurs because of the loss of kinetic energy as heat is transferred away, causing the iodine to reassemble into a solid.

These transition steps demonstrate iodine's behavior when subjected to gradual or rapid changes in energy, highlighting the role of intermolecular forces and Van der Waals attractions in its various phase transitions.