Question

List these substances in order of decreasing boiling point: $$ \mathrm{CO}_{2}, \mathrm{Ne}, \mathrm{CH}_{3} \mathrm{OH}, \mathrm{KF} $$

Short answer

KF > CH3OH > CO2 > Ne.

Step-by-step solution

Understand intermolecular forces involved

Identify the types of intermolecular forces present in each substance. Carbon dioxide (CO2) has London dispersion forces, Neon (Ne) has London dispersion forces, Methanol (CH3OH) has hydrogen bonding and London dispersion forces, and Potassium Fluoride (KF) is an ionic compound which has ionic bonds.

Rank by strength of intermolecular forces

Understand that the strength of intermolecular forces affects the boiling points of the substances: Ionic bonds > Hydrogen bonds > London dispersion forces. Substances with stronger intermolecular forces will have higher boiling points.

Organize substances in decreasing order of boiling points

Based on the strength of intermolecular forces determined, order the substances from highest to lowest boiling points. KF has the highest due to ionic bonding, followed by CH3OH with hydrogen bonding, CO2, and then Ne, both with London dispersion forces, but Ne being a monoatomic noble gas has very weak forces.

Key concepts

London Dispersion Forces

When exploring intermolecular forces, London dispersion forces are the weakest type but are ubiquitous in nature. These forces occur due to the temporary dipoles that happen when the electron distribution around atoms or molecules is momentarily uneven. This leads to an induced dipole in neighboring particles, resulting in a weak attraction.

All molecules experience these forces, including noble gases like Neon (Ne) and nonpolar molecules such as Carbon Dioxide (CO2). Despite being weak individually, London dispersion forces can become significant when many are present - for example, in large or heavy atoms and molecules with greater surface areas.

Hydrogen Bonding

Diving into the world of hydrogen bonding, it's vital to recognize this as a uniquely strong type of dipole-dipole interaction. Hydrogen bonds occur when hydrogen is covalently bonded to a highly electronegative atom, such as oxygen, nitrogen, or fluorine.

In substances like Methanol (CH3OH), hydrogen atoms bonded to the oxygen create an area of positive charge, which can attract neighboring negative regions on other molecules. These interactions are significantly stronger than London dispersion forces, giving rise to higher boiling points. For example, water's unusually high boiling point for its molar mass is due to hydrogen bonding.

Ionic Bonds

The strength seen in ionic bonds is due to the electrostatic attraction between positively and negatively charged ions. These bonds are typically formed between metals and non-metals, like Potassium (K) and Fluoride (F) in Potassium Fluoride (KF).

Ionic compounds have considerably high melting and boiling points because of the strong attraction between the oppositely charged ions. The energy required to break these bonds is much greater than that needed for hydrogen bonds or London dispersion forces, which is why substances like KF have high boiling points.

Boiling Point Comparison

Understanding boiling point trends is crucial when comparing substances. Boiling occurs when the vapor pressure of a liquid equals the surrounding pressure, and it requires enough energy to overcome intermolecular forces holding the molecules in the liquid state.

Substances with ionic bonds like KF generally have the highest boiling points, due to the strong interactions between ions. Those with hydrogen bonding, such as CH3OH, also exhibit high boiling points, but less so than ionic compounds. Finally, molecules with only London dispersion forces, like CO2 and Ne, will boil at lower temperatures. The more electrons in the molecules or atoms, the stronger the London forces; thus, CO2 will have a higher boiling point than Ne.