Problem 60 - Chemistry The Molecular Science

Question

What types of forces must be overcome in each change? (a) sublimation of solid \(\mathrm{C}_{10} \mathrm{H}_{8}\) (b) melting of propane, \(\mathrm{C}_{3} \mathrm{H}_{8}\) (c) decomposition of water into \(\mathrm{H}_{2}\) and \(\mathrm{O}_{2}\) (d) evaporation of liquid \(\mathrm{PCl}_{3}\) (e) unzipping the DNA double helix during replication

Step-by-Step Solution

Answer

(a) Van der Waals forces, (b) London dispersion forces, (c) Covalent bonds, (d) Dipole-dipole interactions, (e) Hydrogen bonds.

Step 1: Sublimation of Solid Naphthalene

For part (a), naphthalene ( aphthaleneC_{10}H_{8} aphthalene) sublimates when it changes from a solid directly to a gas. During this process, the intermolecular forces that must be overcome are van der Waals forces, which include London dispersion forces, as naphthalene is non-polar.

Step 2: Melting of Propane

For part (b), the melting of propane ( propaneC_{3}H_{8} propane) involves changing it from a solid to a liquid. The intermolecular forces overcome during this process are predominantly London dispersion forces due to propane being a nonpolar molecule.

Step 3: Decomposition of Water

For part (c), the decomposition of water involves breaking apart the water molecule into hydrogen and oxygen gases ( H_2 and O_2 ). This process requires breaking intramolecular covalent bonds within the water molecule.

Step 4: Evaporation of Phosphorus Trichloride

For part (d), the evaporation of liquid phosphorus trichloride ( PCl_3 ) involves changing from a liquid to a gas. The intermolecular forces overcome include dipole-dipole interactions as PCl_3 is a polar molecule.

Step 5: Unzipping the DNA Double Helix

For part (e), unzipping the DNA double helix requires overcoming hydrogen bonds between the paired nitrogenous bases. These hydrogen bonds hold the two complementary strands of DNA together.

Key Concepts

Sublimation

Sublimation is an intriguing process in which a substance transitions directly from a solid to a gas, bypassing the liquid phase entirely. This phenomenon is driven by the energy that tackles the intermolecular forces keeping the solid intact. In the case of naphthalene, a non-polar compound, it is primarily the van der Waals forces, such as London dispersion forces, that need to be overcome. These dispersion forces are weak attractions that arise due to temporary dipoles forming between molecules. During sublimation, enough energy is supplied to break these attractions, allowing the solid molecules to disperse as gas.

• Sublimation occurs under specific conditions of temperature and pressure.
• Common substances that sublimate include dry ice and iodine.

Understanding sublimation enhances our comprehension of phase transitions and energy requirements for overcoming intermolecular attractions in a solid.

DNA Replication

DNA replication is a fundamental biological process where a double-stranded DNA molecule is copied to produce two identical DNA molecules. This process is critical for cell division and the maintenance of genetic information. During the replication process, the DNA double helix must "unzip" to allow each strand to serve as a template for creating a new complementary strand.

The unzipping involves breaking hydrogen bonds between the paired nitrogenous bases: adenine pairs with thymine, and cytosine pairs with guanine through hydrogen bonding. These hydrogen bonds are relatively weak but numerous, making them strong enough to hold the double helix together yet flexible enough to allow separation during replication.

• Replication occurs during the S phase of the cell cycle.
• Enzymes like helicase and DNA polymerase play crucial roles in this process.

By breaking these hydrogen bonds, replication enables life continuity by ensuring genetic material is accurately reproduced.

Covalent Bonds

Covalent bonds are the backbone of molecular structure and stability, formed when two atoms share one or more pairs of electrons. This type of bond is fundamental in molecules such as water ( H_2O ). In water, each hydrogen atom shares an electron with the oxygen atom, resulting in a covalent bond that holds the molecule together.

These bonds are intramolecular, meaning they exist within molecules, providing strength and stability. Breaking covalent bonds—as in the decomposition of water into hydrogen and oxygen gases—requires significant energy because these bonds are strong and require overcoming the electron-sharing mechanism.

• Covalent bonds can be single, double, or triple, based on the number of shared electron pairs.
• The strength and length of a covalent bond depend on the participating atoms.

Understanding covalent bonds is crucial for grasping chemical reactions and molecular interactions.

Hydrogen Bonds

Hydrogen bonds play a crucial role in biology and chemistry, acting as special attractions between molecules or within different parts of a single large molecule. These are not true bonds but rather strong intermolecular forces occurring when hydrogen is covalently bonded to a highly electronegative element, such as oxygen, nitrogen, or fluorine.

For instance, in DNA, hydrogen bonds hold the two strands of the double helix in place by connecting complementary base pairs. Despite being individually weak, these bonds are collectively significant, enabling the DNA structure to be stable yet flexible enough to unzip during replication.

• Hydrogen bonds influence water's unique properties, like its high boiling point and surface tension.
• They are vital for the three-dimensional structures of proteins and nucleic acids.

Understanding hydrogen bonds offers insights into many natural phenomena and chemical processes.