Question

What is one reason the noble gases are \(preferable\) to air (which is mostly nitrogen and oxygen) as an insulating material? (a) Noble gases are monatomic, so no rotational modes contribute to their molar heat capacity; (b) noble gases are monatomic, so they have lower molecular masses than do nitrogen and oxygen; (c) molecular radii in noble gases are much larger than those of gases that consist of diatomic molecules; (d) because noble gases are monatomic, they have many more degrees of freedom than do diatomic molecules, and their molar heat capacity is reduced by the number of degrees of freedom.

Short answer

Option A is correct: Monatomic nature limits heat capacity modes.

Step-by-step solution

Understanding the Problem

The problem asks why noble gases are preferable for insulation over air, which consists of diatomic gases (mainly nitrogen and oxygen). We need to evaluate the given options to determine the reason based on physical properties of gases.

Analyzing Options

Review the options provided: (a) Noble gases are monatomic, so no rotational modes contribute to their molar heat capacity. (b) Noble gases are monatomic, so they have lower molecular masses than nitrogen and oxygen. (c) Molecular radii in noble gases are much larger than those of diatomic molecules. (d) Noble gases are monatomic, offering more degrees of freedom, reducing molar heat capacity.

Evaluating Option A

Monatomic gases, like noble gases, do not have rotational modes, meaning they only have translational modes that contribute to their heat capacity. This results in a lower specific heat capacity than diatomic gases. This property can be advantageous for insulation since it can potentially limit heat transfer by conduction.

Evaluating Option B

Monatomic gases do not necessarily have lower molecular masses than diatomic gases. For example, Argon (a noble gas) has a higher atomic mass than nitrogen and oxygen. Thus, this option does not provide a reason related to insulation performance.

Evaluating Option C

The molecular radius of gases does influence how they pack or interact; however, noble gases do not generally have significantly larger radii than diatomic molecules like nitrogen or oxygen. This doesn't directly contribute to their insulating ability in this context.

Evaluating Option D

Monatomic gases, like noble gases, have fewer degrees of freedom (only translational), which means their heat capacity is not higher due to additional modes. Yet, the explanation given doesn't turn into a benefit for insulation in terms of reduced heat capacity.

Conclusion

Option A is the correct reason: noble gases, being monatomic, do not have rotational modes, contributing to lower molar heat capacity, which can be beneficial when used as an insulating material because it facilitates less energy being stored and transferred through heat. Their inertness and density also contribute to their effectiveness.

Key concepts

Monatomic Gases

Monatomic gases, such as noble gases, consist of single atoms rather than molecules made up of two or more atoms. This atomic structure is quite different compared to diatomic gases, which typically form molecules with two atoms. Since monatomic gases have only translational modes of energy, this affects their thermal properties significantly.

• Monatomic gases like helium, neon, and argon have lower molar heat capacities.
• They lack vibrational and rotational energy modes, which are present in diatomic gases.
• This leads to their energy being stored and transferred less effectively compared to diatomic gases.

For insulation purposes, the reduced energy transfer capability of monatomic gases makes them more effective at limiting heat transfer, which is why they are often preferred over gases like nitrogen or oxygen.

Insulating Materials

Insulating materials aim to reduce heat transfer between objects or environments. Noble gases are used as insulators because their physical properties are particularly suited to this role. Their inertness and low thermal conductivity make them ideal choices.

• Noble gases do not easily form chemical bonds, providing stability in thermal applications.
• They minimize heat conduction and are excellent for insulation in double-paned windows.
• Their density aids in limiting convective heat transfer.

These properties collectively make noble gases favorable insulating materials in both industrial and residential applications, providing better energy efficiency.

Molar Heat Capacity

Molar heat capacity is an important concept when considering the insulating capabilities of a material. It represents the amount of heat energy required to raise the temperature of one mole of a substance by one degree Celsius. For noble gases:

• Their molar heat capacity is lower than that of diatomic gases, due to their lack of rotational and vibrational modes.
• Only translational motions contribute to their heat capacity.
• This property is critical for insulation, as it means noble gases store less energy, reducing heat transfer.

In essence, the low molar heat capacity of monatomic gases like noble gases contributes to their effectiveness in reducing heat flow, thus enhancing their use as insulators.

Diatomic Gases

Diatomic gases consist of molecules made up of two atoms. Nitrogen and oxygen, the primary components of air, are examples. Their structure means they have more complex modes of energy storage compared to monatomic gases.

• They exhibit rotational and vibrational energy modes.
• This results in a higher molar heat capacity since more energy modes can be excited.
• Their higher level of energy storage implies they can transfer more heat energy compared to monatomic gases.

Due to their complex molecular structure, diatomic gases like nitrogen and oxygen are less efficient as insulating materials compared to noble gases. They transfer more energy through heat due to their higher heat capacity, making them less desirable for insulation purposes.