Hydrogen and Helium: The nearest ideal gases

The basic answer is that they are quite small in size when compared to other gases. They have little intermolecular attraction since they are so tiny. They behave almost like ideal gases because of their modest intermolecular attractions, which distinguishes them from other real gases.

The most noticeable distinction between H2 / He and other real gases is that these are the only two gases that deviate from the properties of other real gases at all pressures and temperatures, 273K.

Three gases, hydrogen, helium, and neon, have no JT effect and do not cool when expanded. Because of the lack of intermolecular interactions, the compressibility Z of H2 increases, even more, when it is expanded. ?Its volume increases even more than that of an ideal gas. More translational kinetic energy is required for this gas expansion, which is provided by potential energy. This kinetic energy pushes molecules apart, releasing KE into the gas and heating up H2. Thus on expansion, these gases become hot. This explanation is not easily found on the internet. ?

Some of the properties of H2 and He are compared to those of other gases in the table below. We'd try to examine every feature of H2 and He and explain why they're so different.

Chemistry of H2 and He

Hydrogen

Hydrogen is the chemical element with the symbol H and atomic number 1. Its atomic number is one. It has one proton and one electron. Hydrogen is the lightest element. At standard conditions, hydrogen is a gas of diatomic molecules having the formula H2 with the molecular mass of 2. As an atom H is about 14 times smaller than nitrogen.?

Helium

Helium sits next to hydrogen in the periodic table. Its atomic number is 2. It has 2 protons and 2 electrons. Its atomic mass is 4. It has two neutrons. It is a monoatomic gas. Since it has a completely filled valency shell, He is an inert gas.

Thermodynamic properties of H2 and He

Because they have the least amount of excluded volume (making their molar volume close to that of an ideal gas) and the weakest intermolecular attractions, hydrogen and helium are the closest to ideal gases.

This may be explained as the electron clouds of smaller atoms and molecules are smaller and less polarizable, they have the weakest London dispersion forces. Gas molecules become less perfect as they become larger. Dipole-dipole interaction may develop when dispersion pressures increase.

They are, however, not perfectly ideal gas.

Helium is the real gas that behaves the most like an ideal gas. This is because, unlike most gases, helium exists as a single atom, resulting in the lowest conceivable van der Waals dispersion forces. Another aspect is that helium, like other noble gases, has an outer electron shell that is totally filled. As a result, it has a low proclivity for forming bonds with other atoms.

Hydrogen gas is an ideal gas with more than one atom. A hydrogen molecule, like a helium atom, has two electrons and small intermolecular interactions. Two atoms share the same electrical charge.

Two outstanding properties of H2 gas

The first one is related to hydrogen gas's high energy storage capacity and the second one is related to hydrogen's ability to do mechanical work.

[1] It has the highest specific heat among all gases seven times more than water 14.3 vs 2.1 j/g/k. The reason is because of its smaller mass at 2 g/mol vs 18 g/mol for water each mole contains 6.02 x 10^23 molecules with each molecule having its own energy holding capacity. This gives hydrogen's huge energy holding capacity.

[2] Hydrogen has a huge specific volume, volume/unit mass. The reason is hydrogen because its smaller mass has the highest velocity among all gases. That means 1 unit mass of H2 spreads in a much larger volume than any other gas. That is how it has a very high specific volume.

A specific volume of a gas is a measure of its compressibility when the gas expands and does mechanical work. H2 and He have highest the gas constant.

He has the same properties only the numbers are different.

What does it mean when a real gas behaves as an ideal gas?

Let us first look at compressibility factor Z for gases.

Hydrogen is the only exception in this image. At all pressure, H2 has z factor of more than 1. Gas compressibility factor Z is the ratio of the gas volume at a given temperature and pressure to the volume the gas would occupy if it were an ideal gas at the same temperature and pressure. He follows the same behavior. Z > 1 indicates their low intermolecular attractions and rather repulsion between molecules at high pressure.?This is the most important point to note. Because of their intermolecular repulsion at high pressure, they do not cool by JT effect. ?Rather the temperature shoots up when you compress these gases above ambient temperature. This is the reason you need to cool H2 and He before you can liquify them.

The compressibility Z of H2 further increases when H2 is expanded because of lack of intermolecular attractions Its volume becomes even larger than ideal gas. This gas expansion needs more translational kinetic energy which is supplied by the potential energy. This kinetic energy sends molecules farther apart adding KE to the gas that heats up H2.

Credit: Google