Problem 14 - Chemistry: The Central Science in SI Units, Global Edition

Question

(a) Are you more likely to see the density of a gas reported in \(\mathrm{g} / \mathrm{mL}, \mathrm{g} / \mathrm{L},\) or \(\mathrm{kg} / \mathrm{cm}^{3} ?(\mathbf{b})\) Which units are appropriate for expressing atmospheric pressures, \(\mathrm{N}, \mathrm{Pa}, \mathrm{atm}, \mathrm{kg} / \mathrm{m}^{2} ?\) (c) Which is most likely to be a gas at room temperature and ordinary atmospheric pressure, \(\mathrm{F}_{2}, \mathrm{Br}_{2}, \mathrm{~K}_{2} \mathrm{O} .\)

Step-by-Step Solution

Answer

(a) The density of a gas is more likely to be reported in \(g/L\). (b) The appropriate units for expressing atmospheric pressure are \(Pa\) and \(atm\). (c) Among \(F_2\), \(Br_2\), and \(K_2O\), fluorine (\(F_2\)) is most likely to be a gas at room temperature and ordinary atmospheric pressure.

Step 1: (Part a: Determine the most common units for gas density)

The density of a gas is usually reported in units that take up the lightness and the relatively large volume occupied by gases. The most common units for gas density are grams per liter (g/L) since these units represent the mass of a gas in a large volume. Therefore, the density of a gas is more likely to be reported in g/L.

Step 2: (Part b: Identify the units for atmospheric pressure)

Atmospheric pressure can be expressed in several units, including Newtons (N), Pascals (Pa), atmospheres (atm), and kg/m². However, some units are more appropriate and commonly used to express atmospheric pressure. Pascals (Pa) and atmospheres (atm) are the most common units to express atmospheric pressure since they directly relate to the force exerted by the atmosphere on a given area. Therefore, Pa and atm are the appropriate units for expressing atmospheric pressure.

Step 3: (Part c: Predict which substance is a gas at room temperature and atmospheric pressure)

To predict which substance will be a gas at room temperature and atmospheric pressure (F2, Br2, or K2O), we should consider the intermolecular forces and the molecular weight of each substance. 1. F2 (Fluorine) is a diatomic gas, and Halogens (group 17 elements) have weak London dispersion forces between their molecules. F2 has the lowest molecular weight among the halogens, making it more likely to be a gas at room temperature. 2. Br2 (Bromine) is also a halogen, but it is a liquid at room temperature due to its heavier molecular weight, which results in stronger London dispersion forces compared to F2. 3. K2O (Potassium Oxide) is an ionic compound composed of potassium and oxygen ions. Ionic compounds have strong electrostatic forces between the ions, which result in high melting and boiling points. Therefore, K2O is most likely to be a solid at room temperature. Taking these factors into consideration, F2 (fluorine) is the most likely to be a gas at room temperature and ordinary atmospheric pressure.

Key Concepts

Atmospheric Pressure

Atmospheric pressure is an essential concept in understanding how gases behave on Earth. It is the pressure exerted by the weight of the atmosphere on the surface of the Earth. This force is caused by air molecules colliding with surfaces, and it varies depending on altitude and weather conditions.

Atmospheric pressure can be measured in various units:

• Pascals (Pa): The derived SI unit for pressure, it measures force per unit area and is widely used in scientific contexts.
• Atmospheres (atm): This is a convenient unit as it simplifies many calculations involving pressure by treating 1 atm as standard atmospheric pressure at sea level.
• Newton per square meter (N/m²): Essentially equivalent to 1 Pascal, it is a way to express force (Newtons) distributed over an area (m²).

Understanding these units helps us comprehend how pressure influences weather patterns and the behavior of gases in our daily environment.

Intermolecular Forces

Intermolecular forces are the forces of attraction and repulsion between molecules. They play a crucial role in determining the physical states (solid, liquid, gas) of substances and their transition between these states.

There are several types of intermolecular forces, including:

• London Dispersion Forces: These are the weakest intermolecular forces, arising from temporary dipoles created when electrons in molecules shift positions slightly. This type of force is present in all molecular substances but is the dominant force in non-polar compounds.
• Dipole-Dipole Interactions: These occur between polar molecules where positive and negative charges attract each other. They are generally stronger than London dispersion forces.
• Hydrogen Bonds: A particularly strong type of dipole interaction occurring when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine.

These intermolecular forces explain why fluorine ( F_2 ) is a gas under normal conditions, as its atoms experience weak London dispersion forces that allow for more freedom of movement.

Diatomic Gases

Diatomic gases are molecules composed of two atoms. Some common examples of diatomic gases include nitrogen ( N_2 ), oxygen ( O_2 ), and the halogens like fluorine ( F_2 ) and chlorine ( Cl_2 ). These gases have distinct properties due to their molecular structure.

Characteristics of diatomic gases:

• They are often found as gases at room temperature due to the weak intermolecular forces between their molecules, particularly for those like fluorine and oxygen.
• They play a crucial role in atmospheric chemistry and are involved in various chemical reactions, such as combustion and respiration.
• Their simple molecular structure makes them ideal for studying basic principles of gas behavior, like the ideal gas law.

Understanding diatomic gases is fundamental in fields such as chemistry and environmental science, providing insight into both natural and industrial processes.