Question

Describe the intermolecular forces that must be overcome to convert each of the following from a liquid or solid to a gas: (a) \(\mathrm{I}_{2}\), (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\), (c) \(\mathrm{H}_{2}\) Se.

Short answer

To convert the given substances to a gas, the following intermolecular forces must be overcome: (a) I2: London dispersion forces. (b) CH3CH2OH: London dispersion forces, dipole-dipole interactions, and hydrogen bonding. (c) H2Se: London dispersion forces, dipole-dipole interactions, and hydrogen bonding.

Step-by-step solution

Identify Intermolecular Forces in I2

Iodine (I2) is a non-polar molecule. The only significant intermolecular forces present in such molecules are London dispersion forces (LDF), also known as van der Waals forces.

Identify Intermolecular Forces in CH3CH2OH

Ethanol (CH3CH2OH) is a polar molecule with hydrogen bonding capability due to the presence of an -OH group. Therefore, there are three types of intermolecular forces present in ethanol: 1. London dispersion forces 2. Dipole-dipole interactions 3. Hydrogen bonding

Identify Intermolecular Forces in H2Se

Hydrogen Selenide (H2Se) is a polar molecule with hydrogen bonding capability, as it has hydrogen bonded to a highly electronegative atom (selenium). The intermolecular forces present in this molecule are as follows: 1. London dispersion forces 2. Dipole-dipole interactions 3. Hydrogen bonding

Determine Forces to be Overcome for Each Substance

(a) I2: London dispersion forces are the only intermolecular forces present in I2, so they must be overcome to convert it into a gas. (b) CH3CH2OH: To convert ethanol into a gas, all three types of intermolecular forces (London dispersion forces, dipole-dipole interactions, and hydrogen bonding) must be overcome. (c) H2Se: In the case of hydrogen selenide, all three types of intermolecular forces (London dispersion forces, dipole-dipole interactions, and hydrogen bonding) must also be overcome to convert it into a gas.

Key concepts

London Dispersion Forces

London dispersion forces, also known as van der Waals forces, are one of the weakest types of intermolecular forces. They occur when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles.
These temporary dipoles are due to the electrons in an atom or molecule being distributed unevenly at any given moment. Unlike other intermolecular forces, London dispersion forces are present in all molecules, whether they are polar or nonpolar.

• Key characteristic: Weakest and most general intermolecular force.
• When present: All molecules experience these forces due to electron clouds.
• Importance: Although weak individually, they play a significant role in larger, more massive molecules like Iodine (I extsubscript{2}).

Understanding these forces is crucial, as they affect properties like boiling and melting points. The larger the electron cloud, the stronger the London dispersion forces can be, as seen in larger, nonpolar molecules like I extsubscript{2}, where these forces are primarily responsible for holding the molecules together.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules that have permanent dipoles. These forces arise because of the positive end of one polar molecule experiencing an attraction to the negative end of another polar molecule.
This intermolecular force is stronger than London dispersion forces due to the nature of permanent dipoles. They require closer proximity and specific orientation to exert their attractive force.

• Key characteristic: Stronger than London dispersion due to polarity involved.
• Interaction type: Exists between permanent dipoles in polar molecules.
• Importance: Affects the boiling and melting points in polar substances like CH extsubscript{3}CH extsubscript{2}OH (ethanol).

Ethanol, being a polar molecule, utilizes these interactions along with hydrogen bonding to determine its physical properties. The presence of these interactions signifies significant energy requirements to overcome them during phase changes.

Hydrogen Bonding

Hydrogen bonding is a unique and especially strong type of dipole-dipole interaction. It occurs when hydrogen is covalently bonded to one of the highly electronegative atoms: nitrogen, oxygen, or fluorine.
Once bonded, the hydrogen takes on a partial positive charge, allowing it to strongly attract electronegative atoms from neighboring molecules. This force is much stronger than typical dipole-dipole interactions.

• Key characteristic: Strong and highly directional force.
• Occurrence: Involves hydrogen and electronegative atoms like N, O, F.
• Importance: Plays a critical role in properties like high boiling points in water and ethanol.

For hydrogen selenide (H extsubscript{2}Se), while it contains hydrogen, its hydrogen bonding is less significant due to selenium's lower electronegativity compared to oxygen. Nonetheless, hydrogen bonding adds to the strength of the intermolecular forces that must be overcome to transition from liquid to gas.