Question

Describe in detail the microscopic processes that take place when a solid melts and when a liquid boils. What kind of forces must be overcome? Are any chemical bonds broken during these processes?

Short answer

During the melting of a solid, the rigid structure is disrupted as molecules gain kinetic energy and overcome the attractive forces between them, such as London dispersion forces, dipole-dipole interactions, and hydrogen bonds. No chemical bonds are broken in this process. Boiling occurs when liquid molecules gain enough kinetic energy to overcome the intermolecular forces between them, leading to the formation and rising of vapor bubbles. Again, no chemical bonds are broken, only the intermolecular forces are overcome, allowing the molecules to separate and transition into the gas phase.

Step-by-step solution

1. Melting of a Solid

Melting occurs when a solid changes into a liquid upon heating. At the molecular level, the solid's rigid structure is disrupted, and the molecules gain enough kinetic energy to move more freely and break away from their fixed positions. The main forces that need to be overcome during this process are the attractive forces that hold the molecules together in the solid. #a. Intermolecular Forces# In most solids, the molecules are held together by one or more types of intermolecular forces, such as London dispersion forces, dipole-dipole interactions, and hydrogen bonds. To initiate melting, heat is supplied to the solid, which increases the kinetic energy of the molecules, helping them overcome these intermolecular forces and move more freely as they transition to the liquid phase. #b. Chemical Bonds# No chemical bonds are broken during the melting process. Only the intermolecular forces holding the molecules together in the solid lattice are weakened, allowing the molecules to move more freely and transition into the liquid phase.

2. Boiling of a Liquid

Boiling occurs when a liquid changes into a gas upon heating. During boiling, the liquid molecules gain enough kinetic energy to overcome the attractive forces between them and become widely separated, forming the gas phase. #a. Intermolecular Forces# Similar to the melting process, during boiling, the main forces to be overcome are the intermolecular forces between the molecules in the liquid, such as van der Waals forces, dipole-dipole interactions, and hydrogen bonds. As the liquid is heated, the molecules gain kinetic energy, eventually having enough energy to overcome these forces, allowing them to break free from the liquid phase and enter the gas phase. #b. Formation of Bubbles: As the liquid is heated, vapor bubbles form within the liquid and eventually rise to the surface, where they escape as gas. During the boiling process, the vapor pressure inside these bubbles is equal to or higher than the pressure exerted by the surrounding liquid and atmospheric pressure, allowing the bubbles to form and rise. #b. Chemical Bonds# Similar to the melting process, no chemical bonds are broken during boiling. The molecules remain unchanged, and only the intermolecular forces holding them together in the liquid phase are overcome, allowing them to separate and transition into the gas phase.

Key concepts

Melting Process

The melting process is an intriguing transformation wherein a solid changes into a liquid. This occurs when heat is applied to the solid, causing the molecules to absorb energy and vibrate more vigorously. As the temperature increases, the molecules gain enough kinetic energy to overcome the intermolecular forces binding them in a fixed, rigid structure. The intermolecular forces, such as London dispersion forces and hydrogen bonds, are weakened during this process.

It is crucial to note that during melting, only the forces between molecules are altered. The actual chemical bonds within a molecule remain intact, meaning the chemical identity of the substance does not change. Melting simply allows the molecules to move more freely, transitioning from a fixed, orderly arrangement in the solid phase to a more disorderly and fluid structure in the liquid phase.

Boiling Process

Boiling is another fascinating phase change, where a liquid turns into a gas upon heating. When a liquid boils, its molecules gain enough energy to overcome the strong attractive forces between them. These forces, known as intermolecular forces, include van der Waals forces, dipole-dipole interactions, and hydrogen bonds. As the molecules get energized, they begin to separate, moving from a closely packed liquid phase to a widely spaced gas phase.

Boiling is characterized by the formation of vapor bubbles within the liquid. These bubbles rise to the surface and escape as gas. Inside these bubbles, the vapor pressure must equal or exceed the surrounding liquid's pressure for the bubble to rise. In terms of molecular activity, just like with melting, boiling does not lead to the breaking of chemical bonds within the molecules. The molecules themselves remain unharmed, while their arrangement relative to each other changes drastically, allowing for this phase change.

Intermolecular Forces

Intermolecular forces play a critical role in both melting and boiling processes. These forces are the attractions between molecules, not within them. They determine how easily a substance can change phases under different conditions.

For phase changes like melting and boiling, the key intermolecular forces include:

• London Dispersion Forces: Present in all molecules, especially significant in non-polar molecules due to temporary dipoles.
• Dipole-Dipole Interactions: Occur in polar molecules where the positive end of one molecule is attracted to the negative end of another.
• Hydrogen Bonds: Strong interactions occurring in molecules where hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine.

Intermolecular forces must be overcome for molecules to escape into the liquid or gas phase. The stronger these forces are, the more energy (in the form of heat) is required for phase change. However, even as these forces weaken, the molecules themselves remain chemically unchanged.