Problem 62 - Introductory Chemistry Atoms First

Question

Which of the following would most likely be a gas at room temperature? Justify your choice. (a) \(\mathrm{NaCl}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{2}\) (c) Na metal (d) \(\mathrm{CH}_{3} \mathrm{~F}\)

Step-by-Step Solution

Answer

The most likely substance to be a gas at room temperature is \(\mathrm{C}_{2} \mathrm{H}_{2}\) due to its weak Vander Waals dispersion forces. Other substances such as \(\mathrm{NaCl}\), Na metal, and \(\mathrm{CH}_{3} \mathrm{~F}\) have stronger types of bonding and intermolecular forces, resulting in higher melting and boiling points which make them less likely to exist as gases at room temperature.

Step 1: Identify bonding type and intermolecular forces in each substance

(a) \(\mathrm{NaCl}\): Ionic bonding - very strong electrostatic attraction between positively charged metal cations (Na+) and negatively charged nonmetal anions (Cl-). (b) \(\mathrm{C}_{2} \mathrm{H}_{2}\): Covalent bonding - consists of carbon-hydrogen bonds, primarily experiencing Vander Waals dispersion forces which are very weak. (c) Na metal: Metallic bonding - consists of a sea of delocalized electrons surrounding closely packed metal cations. (d) \(\mathrm{CH}_{3} \mathrm{~F}\): Covalent bonding - consists of carbon-hydrogen and carbon-fluorine bonds, experiencing Vander Waals dispersion forces as well as dipole-dipole interactions due to the polar nature of the carbon-fluorine bond.

Step 2: Compare the forces present in each substance

(a) \(\mathrm{NaCl}\): Ionic bonding is very strong, resulting in a high melting and boiling point. This substance is unlikely to be a gas at room temperature. (b) \(\mathrm{C}_{2} \mathrm{H}_{2}\): Vander Waals dispersion forces are very weak, which often result in low melting and boiling points. This substance has a high possibility of being a gas at room temperature. (c) Na metal: Metallic bonding is strong and results in high melting and boiling points for metals. This substance is unlikely to be a gas at room temperature. (d) \(\mathrm{CH}_{3} \mathrm{~F}\): The presence of dipole-dipole interactions due to the polar nature of the carbon-fluorine bond makes the intermolecular forces stronger than in \(\mathrm{C}_{2} \mathrm{H}_{2}\). However, these forces are still weaker compared to ionic or metallic bonding. This substance could be a gas at room temperature, but less likely than \(\mathrm{C}_{2} \mathrm{H}_{2}\).

Step 3: Choose the most likely substance to be a gas at room temperature

Considering the type of bonding and the intermolecular forces present in each of the given substances, \(\mathrm{C}_{2} \mathrm{H}_{2}\), with its weak Vander Waals dispersion forces, is the most likely substance to be a gas at room temperature.

Key Concepts

Intermolecular Forces

Intermolecular forces are interactions that occur between molecules. They are essential in determining the physical state, such as solid, liquid, or gas. These forces govern how molecules attract or repel each other, affecting their boiling and melting points.

There are various types of intermolecular forces:

• Dipole-Dipole Interactions: These occur between molecules that have permanent dipole moments, meaning they have areas of partial positive and negative charge. These forces influence molecules with polar covalent bonds, like in water.
• Hydrogen Bonding: A specific type of dipole-dipole interaction, stronger than others, occurring when hydrogen is bound to N, O, or F.
• Vander Waals Forces: These are the weakest intermolecular forces, arising from temporary fluctuations in electron distribution. They include dispersion forces, especially relevant for nonpolar substances.

Understanding these interactions helps in predicting the physical properties of substances at certain temperatures, like room temperature.

Covalent Bonding

Covalent bonding is a type of chemical bond that involves the sharing of electron pairs between atoms. This bonding usually occurs between nonmetals. The atoms involved in covalent bonds share electrons to achieve a full valence shell, aiming for stability akin to noble gases.

Key characteristics of covalent bonds include:

• Low Melting and Boiling Points: Substances with covalent bonds often have lower melting and boiling points compared to ionic compounds, making many of these substances gases at room temperature.
• Directional Bonds: The sharing of electrons is directional, meaning that the atoms are bound more tightly in certain directions.
• Discrete Molecules: Unlike ionic compounds, covalent substances often exist as distinct molecules.

For example, in methane ( \( \mathrm{CH}_4 \)), carbon shares electrons with hydrogen atoms, resulting in a stable molecule. Understanding covalent bonding is crucial in grasping why some substances are more likely to be gases, like \( \mathrm{C}_{2} \mathrm{H}_{2} \), due to their weak intermolecular forces.

Ionic Bonding

Ionic bonding is a chemical bond wherein one atom donates an electron to another, leading to the formation of positively and negatively charged ions. These ions attract each other through electrostatic forces, creating the bond. Ionic bonds typically form between metals and nonmetals.

The properties of ionic bonds include:

• High Melting and Boiling Points: Due to strong attractions between ions, ionic compounds like \( \mathrm{NaCl} \) are usually solid at room temperature.
• Conductivity: These compounds can conduct electricity when melted or dissolved in water, as the ions are free to move.
• Form Crystal Lattices: Ionic compounds often form large, repeating structures known as crystal lattices.

Ionic bonding results in very stable compounds that do not tend to be gases at room temperature due to their strong intermolecular forces.

Vander Waals Dispersion Forces

Vander Waals dispersion forces, also known as London dispersion forces, are the weakest type of intermolecular forces. They arise from temporary shifts in the electron cloud around molecules, creating a momentary dipole.

Some important points about Vander Waals dispersion forces are:

• Universal: These forces are present in all molecules, although they are usually overshadowed by stronger interactions like hydrogen bonds or dipole-dipole forces.
• Nonpolar Molecules: Substances with no permanent dipole, such as gases like \( \mathrm{C}_{2} \mathrm{H}_{2} \), rely predominantly on these weak forces.
• Impact on Physical States: Substances with predominant dispersion forces tend to have low boiling and melting points, and are often gases at room temperature.

Understanding Vander Waals dispersion forces is key in explaining why certain nonpolar substances, like some hydrocarbons, are gaseous at everyday temperatures.