Question

What intermolecular attractions will be formed when the following substances condense from the gas phase? (a) hexane \(\left(\mathrm{C}_{6} \mathrm{H}_{14}\right)\), (b) acetone \(\left(\mathrm{H}_{3} \mathrm{CCOCH}_{3}\right)\), (c) iodine \(\left(\mathrm{I}_{2}\right),\) and (d) argon

Short answer

The intermolecular attractions that will be formed are: (a) hexane \(\mathrm{C}_{6} \mathrm{H}_{14}\) with London dispersion forces, (b) acetone \(\mathrm{H}_{3} \mathrm{CCOCH}_{3}\) with dipole-dipole interactions and London dispersion forces, (c) iodine \(\mathrm{I}_{2}\) with London dispersion forces, and (d) argon with London dispersion forces.

Step-by-step solution

- Identify the Intermolecular Forces for Hexane

Hexane \(\mathrm{C}_{6} \mathrm{H}_{14}\) is a nonpolar molecule due to its symmetrical shape and the similar electronegativities of carbon and hydrogen. The primary intermolecular force in nonpolar molecules is London dispersion forces, which are the weakest type of intermolecular attraction.

- Identify the Intermolecular Forces for Acetone

Acetone \(\mathrm{H}_{3} \mathrm{CCOCH}_{3}\) has a polar carbonyl group (C=O), which can engage in dipole-dipole interactions. Additionally, although acetone does not have hydrogen bonding with itself, it can interact with other compounds that have O-H or N-H bonds via hydrogen bonding. London dispersion forces also play a role due to the molecule's nonpolar carbon chains.

- Identify the Intermolecular Forces for Iodine

Iodine (\(\mathrm{I}_{2}\)) is a nonpolar molecule and thus, the primary intermolecular force when it condenses is London dispersion forces. These forces are stronger in iodine compared to smaller nonpolar molecules because of its larger size and more electrons.

- Identify the Intermolecular Forces for Argon

Argon is a noble gas with atoms that are uncharged and nonpolar. The only intermolecular attraction that argon atoms can form upon condensation is London dispersion forces.

Key concepts

London Dispersion Forces

When considering how different substances behave in various states of matter, understanding London dispersion forces is essential. These are a type of intermolecular force that appear in all atomic and molecular interactions, although they're particularly significant in nonpolar molecules and noble gases, like hexane (C6H14) or argon.

These forces are generated by the fluctuating positions of electrons within molecules or atoms, which create a temporary dipole. Imagine electrons as a crowd of people in a room, constantly shifting from one side to another. Occasionally, more people crowd on one side than the other, creating an imbalance. Similarly, electrons can create momentary areas of positive and negative charge, attracting other nearby molecules or atoms momentarily. This fleeting attraction is the London dispersion force.

Interestingly, the term 'dispersion' hints at the probabilistic and temporary nature of these electron movements. While these forces are the weakest of all intermolecular attractions, their strength can increase significantly in molecules with larger electron clouds, like iodine (I2), making them a noteworthy factor in the condensation of gases into liquids or solids.

Dipole-Dipole Interactions

Acetone (H3CCOCH3), a common nail polish remover, is an excellent real-world example of a molecule involved in dipole-dipole interactions. This sort of force is principally at work in molecules that possess regions with a partial positive charge and regions with a partial negative charge – a quality known as 'polarity'.

The classic characteristic of a molecule hosting dipole-dipole interactions is the presence of polar bonds, which are found in molecules where atoms have varying degrees of electronegativity, or the ability to attract electrons in a bond. For acetone, the key player is the carbonyl group (C=O), whose oxygen atom is more electronegative than carbon, hoarding more electron density and thus setting off a dipole across the bond.

These molecular 'magnets' tend to align themselves with opposite charges, much like how the north and south poles of magnets snap together, resulting in a solid intermolecular attraction. It's the balance of attractive and repulsive forces between these permanent dipoles that dictates the physical properties and behaviors of polar substances.

Polar and Nonpolar Molecules

The distinction between polar and nonpolar molecules is a fundamental concept in chemistry, often dictating how different substances interact, dissolve, or react with one another. Polar molecules, like acetone, have an uneven distribution of electron density leading to a separation of charge within the molecule, famously summed up by the water (H2O) molecule with its partial positive and negative ends.

Nonpolar molecules, on the other hand, maintain a more uniform distribution of electron density and thus do not exhibit a noticeable charge difference across their structure. Substances such as hexane (C6H14) and iodine (I2) fit this profile. These molecules share electrons more or less equally and usually have symmetrical shapes that cancel out any potential dipoles. Due to the lack of strong polar bonds, the primary intermolecular forces in nonpolar molecules are London dispersion forces.

One's understanding of the polar vs. nonpolar dichotomy is crucial when predicting the solubility, boiling points, and melting points of substances, alongside the types of reactions they may engage in. It's the varying electron distribution, dictated by molecular geometry and atomic electronegativities, that leads to this critical divide in substance characteristics.