Problem 18 - Chemistry

Question

What kind of attractive forces must be overcome in order to (a) melt ice, (b) boil molecular bromine, (c) melt solid iodine, and (d) dissociate \(\mathrm{F}_{2}\) into F atoms?

Step-by-Step Solution

Answer

(a) To melt ice, hydrogen bonds must be overcome. (b) To boil molecular bromine, dispersion forces must be overcome. (c) To melt solid iodine, dispersion forces must be overcome. (d) To dissociate \(\mathrm{F}_{2}\) into F atoms, the covalent bond within the \(\mathrm{F}_{2}\) molecule must be broken.

Step 1: Identify the Force for Ice Melting

Ice is a molecular compound made up of water molecules which are polar and capable of forming hydrogen bonds. To melt ice, it is necessary to overcome the hydrogen bonds that hold the water molecules in the solid structure.

Step 2: Identify the Force for Boiling Molecular Bromine

Bromine is a diatomic molecule with only dispersion forces as its intermolecular force. Dispersion forces (London dispersion forces) are present in all molecules, irrespective of polarity. To convert bromine from its liquid to gaseous state, the dispersion forces must be overcome.

Step 3: Identify the Force for Melting Solid Iodine

Iodine, like bromine, is a diatomic molecule and only dispersion forces are present. The dispersion forces need to be overcome to melt solid iodine into liquid iodine.

Step 4: Identify the Force for Dissociating \(\mathrm{F}_{2}\) into F Atoms

Fluorine is also a diatomic molecule with dispersion forces as its intermolecular force. In this case, the dispersion forces between F2 molecules aren't overcome. Instead, the covalent bond within the F2 molecule, a much stronger force, needs to be broken to dissociate it into separate F atoms.

Key Concepts

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs only in molecules where hydrogen is covalently bonded to highly electronegative atoms such as oxygen, nitrogen, or fluorine. Water, being a prime example with its H-O bonds, showcases hydrogen bonding between the slightly positive hydrogen atoms and the lone pairs of oxygen atoms on neighboring water molecules.

This intermolecular force is significantly stronger than regular dipole-dipole interactions due to the high polarity of the involved atoms. As a consequence, hydrogen bonds have a profound impact on the physical properties of water, including its high boiling point and its abnormal expansion upon freezing. To convert ice (solid water) into liquid, one must input sufficient energy to break these hydrogen bonds, facilitating the phase change.

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are weak intermolecular forces that arise from temporary fluctuations in the electron distribution within molecules or atoms. These momentary dipoles induce corresponding dipoles in neighboring particles, leading to an attraction.

Dispersion forces are the weakest intermolecular force but are omnipresent since all particles have electrons that can form instantaneous dipoles. Molecules such as bromine (Br2) and iodine (I2) rely on dispersion forces as their primary intermolecular force. To evoke a phase change from solid to liquid for iodine or liquid to gas for bromine, for example, one would need to supply enough energy to surpass these dispersion forces.

Phase Changes

Phase changes refer to the transitions between solid, liquid, and gas states of matter, each having distinctive characteristics. For instance, during melting, a substance transitions from solid to liquid by absorbing heat, which disrupts its organized structure.

Boiling, or vaporization, is the transition from liquid to gas, requiring the overcoming of forces that hold the liquid molecules closely together. Sublimation is another phase change where a substance goes directly from solid to gas, bypassing the liquid phase. The type of intermolecular forces present in a substance greatly influences the amount of energy needed to induce these phase changes.

Covalent Bonds

Covalent bonds are strong chemical bonds where atoms share pairs of electrons to achieve a more stable electron configuration. They are different from intermolecular forces in that they are intramolecular, meaning they occur within a molecule to hold atoms together, forming the molecule's structure.

The strength of a covalent bond mainly depends on the bond length and the electronegativity difference between the bonded atoms. In the compound F2, for example, the covalent bond between the two fluorine atoms is much stronger than any intermolecular force, meaning that breaking the bond to separate the atoms requires a significant amount of energy.