Question

Why is \(\mathrm{N}_{2}\) a gas at room temperature? Explain why lowering the temperature allows for liquid \(\mathrm{N}_{2}\) to form.

Short answer

Nitrogen (\(\mathrm{N}_{2}\)) is a gas at room temperature due to its weak intermolecular forces, specifically London dispersion forces, which are insufficient to hold the molecules together in a solid or liquid state. When the temperature is lowered, the kinetic energy of nitrogen molecules decreases, and the London dispersion forces can hold them closer together. At approximately -195.8°C (-320.4°F), these forces are strong enough to form liquid nitrogen through a process called condensation.

Step-by-step solution

Understanding Intermolecular Forces

Intermolecular forces are the forces of attraction (or repulsion) between particles (atoms or molecules) within a substance. The strength of these forces determines the phase of a substance at a given temperature. Generally, stronger intermolecular forces result in a more solid-like state, while weaker forces result in a gaseous state.

Nitrogen (N2) at Room Temperature

Nitrogen molecules (\(\mathrm{N}_{2}\)) consist of two nitrogen atoms held together by a strong triple covalent bond. However, the intermolecular forces between these molecules are relatively weak. This is because nitrogen, being a non-polar molecule, only experiences London dispersion forces, which are the weakest type of intermolecular force. At room temperature, these weak intermolecular forces are insufficient to hold the nitrogen molecules together in a solid or liquid state, and thus nitrogen exists as a gas.

Lowering the Temperature

When the temperature of a substance is lowered, its kinetic energy (the energy possessed by particles due to their motion) also decreases. As the kinetic energy decreases, the particles move more slowly, and the attractive forces between them become more significant in comparison.

Formation of Liquid Nitrogen

As the temperature of nitrogen gas is reduced, its kinetic energy decreases, and the particles slow down enough that the weak London dispersion forces between nitrogen molecules can hold them closer together. When the temperature reaches a low enough point (approximately -195.8°C or -320.4°F), the nitrogen molecules are sufficiently close to form liquid nitrogen. This process is known as condensation. Overall, the reason nitrogen exists as a gas at room temperature is due to its weak intermolecular forces, specifically London dispersion forces. However, by lowering the temperature, the kinetic energy of the molecules decreases, allowing these weak forces to become more significant and enabling the formation of liquid nitrogen.

Key concepts

Nitrogen (N2)

Nitrogen is an elemental diatomic molecule, which means it consists of two nitrogen atoms bonded together. These nitrogen atoms are linked by an exceptionally strong triple covalent bond. This strong bond makes the nitrogen molecule stable and non-reactive under standard conditions. However, despite this robust internal bond, nitrogen, as a whole, exhibits weak intermolecular forces among its molecules.

At room temperature, nitrogen exists as a gas. This is because the attractive forces between separate nitrogen molecules are not strong enough to hold the molecules together in a liquid or solid form. You're probably wondering what kind of force is at play here. It's primarily the London dispersion forces, which are the weakest type of intermolecular force.

London Dispersion Forces

London dispersion forces, sometimes simply called dispersion forces, are a type of van der Waals force. They arise due to temporary fluctuations in the electron distribution within a molecule, leading to transient dipoles. These forces are termed 'instantaneous' or 'induced' because they occur in an instantaneous moment.

For nitrogen, which is a non-polar molecule, London dispersion forces are the sole intermolecular forces at play. Here's how they work: occasionally, the electron cloud of a nitrogen molecule shifts slightly, creating a temporary dipole. This transient dipole can induce a dipole in neighboring molecules, leading to weak attractions between them.

These forces are

• present in all molecules, polar or non-polar,
• stronger in larger molecules due to more electrons, and
• always weaker than other types of intermolecular forces like hydrogen bonds or dipole-dipole interactions

Hence, nitrogen's existence as a gas at room temperature is due to these very weak forces not being able to hold the molecules in a more condensed state.

Phase Changes

Phase changes occur when a substance changes from one state of matter to another, say from gas to liquid. This change involves a shift in the energy of particles and their arrangement, influenced largely by temperature and pressure conditions. In the case of nitrogen, lowering the temperature is key to understanding its phase change.

As temperature drops, the kinetic energy of the nitrogen molecules decreases. Kinetic energy is the energy of motion, and when molecules move slower, it's easier for them to be attracted by the already weak London dispersion forces. This results in these molecules coming closer together, ultimately forming a liquid phase when a critical temperature is reached. For nitrogen, this occurs around -195.8°C (-320.4°F).

• Condensation is the process of gas turning into liquid,
• It occurs when intermolecular forces, even if weak, outweigh the kinetic energy pushing molecules apart,
• For nitrogen, a significant drop in temperature fosters the alignment of these weak forces

Therefore, understanding phase changes in nitrogen emphasizes the interplay of energy, temperature, and intermolecular forces.