Question

(a) With respect to absorption of radiant energy, what distinguishes a greenhouse gas from a nongreenhouse gas? (b) ${\mathrm{CH}}_4$ is a greenhouse gas, but ${\mathrm{N}}_2$ is not. How might the molecular structure of ${\mathrm{CH}}_4$ explain why it is a greenhouse gas?

Short answer

(a) Greenhouse gases can effectively absorb and emit infrared radiation due to their molecular structure, which allows them to vibrate in ways that can absorb the energy of infrared radiation. Non-greenhouse gases do not absorb or emit infrared radiation effectively due to their limited vibrational modes. (b) The tetrahedral molecular structure of methane (${\mathrm{CH}}_4$) enables a variety of vibrational modes that match the energies of infrared radiation, making it a greenhouse gas. In contrast, the linear and symmetric structure of nitrogen (${\mathrm{N}}_2$) limits its capacity for vibrations that can absorb infrared radiation, making it a non-greenhouse gas.

Step-by-step solution

Define Greenhouse Gas and Non-Greenhouse Gas

Greenhouse gases are those gases in the Earth's atmosphere that can absorb and emit infrared radiation, thus, trapping and holding heat. Non-greenhouse gases do not effectively absorb or emit infrared radiation, thus, they do not play a significant role in the trapping and holding of heat.

Discuss the reason behind absorption of radiant energy by greenhouse gases.

Greenhouse gases can effectively absorb and emit infrared radiation due to their molecular structure that allows them to vibrate in a way that can absorb the energy of infrared radiation. These vibrations can occur in multiple ways, such as stretching or bending of bonds between the atoms within the molecule.

Examine the molecular structure of CH4 (methane).

Methane (CH4) is a tetrahedral molecule with one carbon atom at the center and four hydrogen atoms surrounding it. The bonds between the carbon and hydrogen atoms can vibrate in various modes, like the symmetric stretch, the asymmetric stretch, and bending vibrations. These vibrations enable methane to effectively absorb and emit infrared radiation.

Examine the molecular structure of N2 (nitrogen).

Nitrogen (N2) is a diatomic molecule, consisting of two nitrogen atoms bonded together by a triple bond. Due to its linear and symmetric structure, nitrogen has very limited ways of vibrating, which are not compatible with the absorption of infrared radiation.

Relate the molecular structures to their greenhouse properties.

The molecular structure of CH4 allows it to have a variety of vibrational modes that match the energies of infrared radiation, making it an effective greenhouse gas. In contrast, the linear and symmetric structure of N2 limits its capacity for vibrations that can absorb infrared radiation, making it a non-greenhouse gas. In conclusion, the molecular structure plays a significant role in determining whether a gas can effectively absorb and emit infrared radiation, classifying it as a greenhouse gas or a non-greenhouse gas. Methane's tetrahedral structure enables it to absorb infrared radiation, while nitrogen's diatomic structure does not.

Key concepts

Infrared Radiation

Infrared radiation is a type of electromagnetic radiation that is particularly important in the Earth's energy balance. It is emitted by the Earth as it absorbs heat from the Sun. This energy is later emitted back as infrared radiation.
This form of radiation is invisible to the human eye but is felt as heat. Many greenhouse gases have the ability to absorb and emit infrared radiation, contributing to the greenhouse effect.

• Infrared radiation occurs at wavelengths longer than visible light.
• This radiation is primarily responsible for the thermal energy exchange in our atmosphere.
• When infrared radiation is absorbed by molecules in the air, it can cause them to vibrate more vigorously, which is a key aspect in the function of greenhouse gases.

Molecular Vibrations

Molecular vibrations refer to the movements within a molecule as its bonds stretch, compress, or bend. These vibrations enable certain molecules to interact with infrared radiation. When the energy of the infrared matches the natural frequency of these vibrational modes, the molecule can absorb the radiation.
Here’s how it works:

• Stretching vibrations occur when the bonds between atoms lengthen and shorten.
• Bending vibrations involve changes in the angle between bonds.
• Molecules like methane have several vibrational modes that align well with the energy levels of infrared radiation, making them effective greenhouse gases.

The ability of molecules to absorb and emit this radiation determines whether or not they play a role as greenhouse gases.

Methane (CH4)

Methane (CH4) is a potent greenhouse gas with a molecular structure that makes it highly efficient at absorbing infrared radiation. It consists of one carbon atom surrounded tetrahedrally by four hydrogen atoms. This structure allows for a variety of vibrational modes.

• The symmetric and asymmetric stretches involve the carbon-hydrogen bonds lengthening and contracting synchronously or asynchronously.
• Bending vibrations occur when the angle between these bonds changes.

Due to these vibrational capabilities, methane is very effective at trapping heat in the atmosphere, contributing significantly to global warming, despite being present in smaller quantities compared to other greenhouse gases like carbon dioxide (CO2).

Nitrogen (N2)

Nitrogen (N2) is a non-greenhouse gas because it makes up a large portion of the Earth's atmosphere but does not significantly absorb infrared radiation. N2 consists of two nitrogen atoms connected by a triple bond, forming a simple linear molecule.

• The symmetry of the N2 molecule restricts its ability to vibrate in a manner that would allow for the absorption of infrared radiation.
• Its vibrational modes do not correspond to the energies of infrared radiation, making it ineffective as a greenhouse gas.

Unlike methane, nitrogen's inability to absorb and emit infrared radiation ensures that it does not contribute to the greenhouse effect, even though it is the most abundant gas in our atmosphere.