Problem 41 - Chemistry An Atoms First Approach

Question

In each of the following groups of substances, pick the one that has the given property. Justify your answer. a. highest boiling point: HBr, \(\mathrm{Kr},\) or \(\mathrm{Cl}_{2}\) b. highest freezing point: \(\mathrm{H}_{2} \mathrm{O}, \mathrm{NaCl},\) or \(\mathrm{HF}\) c. lowest vapor pressure at \(25^{\circ} \mathrm{C}: \mathrm{Cl}_{2}, \mathrm{Br}_{2},\) or \(\mathrm{I}_{2}\) d. lowest freezing point: \(\mathrm{N}_{2}, \mathrm{CO},\) or \(\mathrm{CO}_{2}\) e. lowest boiling point: \(\mathrm{CH}_{4}, \mathrm{CH}_{3} \mathrm{CH}_{3},\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) f. highest boiling point: HF, HCl, or HBr g.(EQUATION CAN'T COPY)

Step-by-Step Solution

Answer

a. HBr has the highest boiling point. b. NaCl has the highest freezing point. c. I2 has the lowest vapor pressure at 25°C. d. N2 has the lowest freezing point. e. CH4 has the lowest boiling point. f. HF has the highest boiling point. g. Unable to answer due to missing equation.

Step 1: Analyze molecule types

Analyze the given molecules: HBr: polar diatomic molecule with LDF and dipole-dipole forces Kr: noble gas with nonpolar atoms, only has LDF forces Cl2: nonpolar diatomic molecule with only LDF forces

Step 2: Rank boiling points

Since HBr is a polar molecule, it has both LDF and dipole-dipole forces, making it have the highest boiling point. Kr and Cl2 only have LDF forces, and since Kr has a larger atomic size than Cl2, it has stronger LDF forces and a higher boiling point than Cl2. Answer: HBr has the highest boiling point. b. Highest freezing point: H2O, NaCl, or HF

Step 1: Analyze molecule types

H2O: polar molecule with hydrogen bonding (strongest force) NaCl: ionic compound, has strong electrostatic attractions between ions HF: polar molecule with hydrogen bonding (strongest force)

Step 2: Rank freezing points

Since NaCl is an ionic compound, it has stronger forces than both H2O and HF and has the highest freezing point. Between H2O and HF, the hydrogen bonding in H2O is stronger due to the more electronegative oxygen atom, resulting in a higher freezing point. Answer: NaCl has the highest freezing point. c. Lowest vapor pressure at 25°C: Cl2, Br2, or I2

Step 1: Analyze molecule types

Cl2, Br2, and I2 are all nonpolar diatomic molecules with only LDF forces.

Step 2: Rank vapor pressures

Since they only have LDF forces, the magnitude of the forces depends on the molecular size/weight. Cl2 has the smallest molecular weight, followed by Br2 and I2. As the molecular weight increases, the vapor pressure decreases, so Cl2 has the highest vapor pressure, while I2 has the lowest vapor pressure. Answer: I2 has the lowest vapor pressure at 25°C. d. Lowest freezing point: N2, CO, or CO2

Step 1: Analyze molecule types

N2: nonpolar diatomic molecule with only LDF forces CO: polar molecule with dipole-dipole and LDF forces CO2: nonpolar, linear molecule with only LDF forces

Step 2: Rank freezing points

Since CO is polar and has the strongest forces among the given molecules, it has the highest freezing point. Comparing N2 and CO2 with only LDF forces, we see that CO2 has a greater molecular weight than N2, resulting in stronger LDF forces. N2 has weaker LDF forces and a lower freezing point. Answer: N2 has the lowest freezing point. e. Lowest boiling point: CH4, CH3CH3, or CH3CH2CH3

Step 1: Analyze molecule types

CH4, CH3CH3, and CH3CH2CH3 are all nonpolar hydrocarbons with only LDF forces. Molecular weight increases as the number of carbons increases.

Step 2: Rank boiling points

As molecular weight increases, boiling point increases due to stronger LDF forces. CH4 has the lowest molecular weight among them, so it has the lowest boiling point. Answer: CH4 has the lowest boiling point. f. highest boiling point: HF, HCl, or HBr

Step 1: Analyze molecule types

HF, HCl, and HBr are all polar diatomic molecules and can form hydrogen bonds.

Step 2: Rank boiling points

Hydrogen bonding strength is determined by the difference in electronegativity between the bonded atoms. Fluorine is the most electronegative element, which means HF has the strongest hydrogen bonding, followed by HCl and HBr. The strongest hydrogen bonding results in the highest boiling point. Answer: HF has the highest boiling point. For g, unfortunately, the equation could not be copied.

Key Concepts

Intermolecular Forces

Intermolecular forces (IMFs) are the attractions between molecules that dictate various physical properties, including boiling and freezing points. Understanding IMFs is essential in predicting how a substance will behave in different conditions.

There are several types of IMFs, each with varying strengths:

• London Dispersion Forces (LDF): Present in all molecules, they arise from temporary dipoles created when electrons are unevenly distributed around a nucleus. These are the weakest bonds and are more significant in larger, heavier atoms or molecules.
• Dipole-Dipole Interactions: Occur between polar molecules due to the attraction of opposite charges on adjacent molecules. These are stronger than LDFs but weaker than hydrogen bonds.
• Hydrogen Bonds: A type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative elements like oxygen, nitrogen, or fluorine.
• Ionic Bonds: Present in ionic compounds, where there is a complete transfer of electrons from one atom to another, creating cations and anions. This type of interaction is generally stronger than covalent IMFs.

Knowing these forces helps explain why substances like sodium chloride (NaCl) have high melting and boiling points due to the strong ionic bonds, while smaller molecules like methane (CH4) have lower melting and boiling points because of weaker LDFs.

Vapor Pressure

Vapor pressure is a measure of the tendency of particles to escape from a liquid or a solid to form a gas. At a given temperature, every substance has a characteristic vapor pressure that is a reflection of its molecules' inherent energy.

Several factors affect vapor pressure:

• Temperature: As temperature increases, vapor pressure increases because molecules have more kinetic energy and can escape the surface of the liquid more readily.
• Intermolecular Forces: Stronger intermolecular forces within the liquid mean that more energy is required for a molecule to escape into the gas phase. Therefore, substances with strong IMFs have lower vapor pressures.

For instance, at 25°C, iodine (I2) has a lower vapor pressure compared to chlorine (Cl2) because iodine's larger molecular size leads to stronger London Dispersion Forces, making it harder for iodine molecules to evaporate.

Hydrogen Bonding

Hydrogen bonding is one of the strongest types of intermolecular forces and a special case of dipole-dipole interactions. It occurs when a hydrogen atom is covalently bonded to a highly electronegative atom—oxygen, nitrogen, or fluorine—and is also attracted to another electronegative atom nearby.

The unique strength of hydrogen bonds imparts special properties to substances. For example, water's high boiling point relative to its molecular weight can be attributed to the strong hydrogen bonds between water molecules. In comparing hydrogen fluoride (HF), hydrogen chloride (HCl), and hydrogen bromide (HBr), HF has the highest boiling point due to the very strong hydrogen bond that forms due to fluorine's high electronegativity.

Understanding hydrogen bonding is crucial for explaining the properties of water and biological molecules like DNA, where hydrogen bonds play a central role in the structure and stability of the double helix.