Problem 16 - Chemistry The Central Science In Si Units

Question

(a) Which is generally stronger, intermolecular interactions or intramolecular interactions? (b) Which of these kinds of interactions are broken when a liquid is converted to a gas?

Step-by-Step Solution

Answer

(a) Intramolecular interactions are generally stronger than intermolecular interactions. (b) Intermolecular interactions are broken when a liquid is converted to a gas.

Step 1: Understand Intermolecular vs. Intramolecular Interactions

Intermolecular interactions occur between molecules, while intramolecular interactions occur within a molecule, such as covalent, ionic bonds, and metallic bonds. Intramolecular interactions are generally the stronger forces responsible for holding atoms together in a molecule.

Step 2: Compare Interaction Strengths

Evaluate the strengths of intramolecular forces, such as covalent bonds, which are typically hundreds of kJ/mol, against intermolecular forces, such as hydrogen bonds, dipole-dipole interactions, and dispersion forces, which are weaker, usually less than 40 kJ/mol. This comparison shows that intramolecular interactions are generally stronger than intermolecular ones.

Step 3: Identify the Phase Change Process

When a liquid is converted to a gas (vaporization), the molecules move from a condensed phase to a dispersed phase. This phase change primarily involves overcoming the intermolecular forces holding the liquid molecules close together.

Step 4: Determine Which Interactions Are Broken

During vaporization, the intermolecular forces are broken, as the process of converting a liquid to a gas requires overcoming these forces to allow the molecules to move independently. The intramolecular interactions within each molecule remain intact during this change.

Key Concepts

Phase Change

A phase change occurs when a substance transitions from one state of matter to another. These states are primarily solid, liquid, and gas. For instance, when ice melts into water or when water boils into steam, a phase change is taking place. During a phase change, the energy in the system either increases or decreases. This energy change is crucial in overcoming or releasing from interactions between molecules.

• When a substance absorbs enough energy, its molecules gain the ability to overcome intermolecular forces, thus transitioning to a more energetic phase.
• Conversely, releasing energy strengthens intermolecular forces, solidifying or condensing the substance into a lower energy phase.

This transformative process relies heavily on temperature and pressure. These conditions dictate whether the substance will remain in its current state or move to a new state. Phase changes underscore the dynamic nature of matter, illustrating how molecules interact and change under variable conditions.

Vaporization

Vaporization is a specific type of phase change where a liquid converts into a gas. This process requires energy input to overcome the intermolecular forces between the molecules in the liquid state.

• Two main forms of vaporization are boiling and evaporation. Boiling occurs at a specific temperature (boiling point) where the vapor pressure equals atmospheric pressure, while evaporation can occur at any temperature below the boiling point.
• During vaporization, molecules break free from the liquid's bonded state to move as a gas.

To achieve this transition, tremendous energy is required to break the bonds that hold the liquid molecules together. This energy is often referred to as the latent heat of vaporization. Despite the energy input, the individual intramolecular bonds within each molecule remain unaffected. This is why vaporization doesn’t alter the chemical composition of the substance, but rather changes its state.

Molecular Bonds

Molecular bonds are the forces that hold atoms together within a molecule. They include covalent, ionic, and metallic bonds. These bonds form the backbone of a molecule's structure.

• Covalent bonds occur when atoms share electrons, creating a strong bond that stabilizes the molecule.
• Ionic bonds form when one atom donates an electron to another atom, resulting in opposite charges that attract and hold the atoms together.
• Metallic bonds involve pooling of electrons across a lattice of metal atoms, contributing to properties like high conductivity and malleability.

These intramolecular forces are significantly stronger than the intermolecular forces. This strength difference is key when considering processes like phase changes, as phase changes typically target weaker intermolecular forces while leaving strong molecular bonds intact. Thus, the core structure of molecules remains stable.