Is hydrogen's tiny size an advantage or a disadvantage? Why hydrogen is a gas and water is a liquid?

The answer is their unique atomic structures make H2 a gas and H2O liquid. At ambient pressure (1 atm), hydrogen liquifies at a temperature of ?253 °C predominantly because of existence of repulsive forces in non-polar H2 molecule. Therefore, at ambient temperature H2 is a gas. The vapour pressure of water at room temperature is 23.8 mm Hg much smaller than atmospheric pressure of 760 mm, hence water is liquid at ambient temperature. The polar structure of H2O and its large intermolecular interactions make water liquid with much higher vapor pressure than H2 at ambient temperature.

Hydrogen

Hydrogen is a unique molecule

Hydrogen is a diatomic gas of two hydrogen atoms linked to each other. Hydrogen atom has one electron and one proton. It has no neutron. Its atomic number is 1 in the periodic table. It has the smallest size among all other elements in the periodic table. Its atomic structure and small size make it a unique molecule. This gives H2 some advantages and some disadvantages as well.

Advantages and disadvantages of H2 gas

Advantages

Because of small mass and large velocity, H2 has huge kinetic energy stored in its atoms, KE = ? mv^2.? This gives H2 very high specific heat. The specific heats of hydrogen at constant pressure and constant volume are 14.307 kJ/kg K and 10.183 kJ/kg K, respectively. One big advantage of small molecular size of H2 is it has the largest number of energy carrying molecules/gram than any other substances.? This is why H2 is an energy carrier and is an alternate to fossil fuels. When H2 is burnt, it releases this energy with almost no GHG generation.

To summarize, Hydrogen gas is renewable and readily available. It is a clean fuel and flexible energy source to support Zero-Carbon energy strategies. It is more powerful and energy efficient than fossil fuels with almost zero emissions. Hydrogen production is possible from a variety of resources, including water, biomass, and natural gas. Hydrogen can be thought of as a potential renewable energy source if renewable energy sources are used to create it.

Challenges: Cost is one of the big issues

Since there is no free electron in H2 molecule, H2 is a non polar molecule. Because of small size and close proximity of H atoms in H2 molecule, H2 has predominantly repulsive [ proton-proton repulsion] forces dominating in the molecule. This is the reason why the liquefaction of H2 is so difficult and energy intensive. Because of small mass, H2 has very high velocity, KE = ? mv^2 and that is one of the key disadvantages of H2. At atmospheric pressure and 20°C, the volumetric mass density of gaseous hydrogen is 0.083 kg/m3. As a result, storing gaseous hydrogen normally requires a large amount of space. Because it is so inefficient, hydrogen is essentially never transported or stored in gaseous form at atmospheric pressure. This is one of the key issues of H2’s cost in its path to replace fossil fuels. Hydrogen gas storage requires 700 bar of pressure, and liquid hydrogen storage requires cryogenic temperatures, making hydrogen gas storage an expensive process.

An important point to remember is that Although Hydrogen has the highest gravimetric energy density of all known substances (120-142 MJ/kg), it falls short to natural fuel sources when it comes to volumetric energy density (9 MJ/L).

Due to its small size, hydrogen can easily enter metal structures where it can cause a number of problems, including hydrogen embrittlement and corrosion. Hydrogen can disrupt the internal structure of metals, which weakens the substance and increases its susceptibility to failure or cracking under stress. In high-stress settings like pipelines, pressure vessels, and some industrial machinery, this phenomenon is more prevalent. Therefore, one way to avoid hydrogen-induced damage is to use expensive metal alloys that are less reactive to hydrogen, but this comes at a higher cost.

To summarize, it is the size of H2 gas that acts behind both its advantages and disadvantages. The key challenges are. high energy requirement in compressed hydrogen storage, due to low specific gravity, temperature and pressure requirements while storing hydrogen in solid form, safety concerns and cost.

Safety issues of H2

This is also related to small size of H2. The kinetics of H2 plays a critical role. H2 is extremely explosive and flammable, necessitating careful handling and safety precautions.

H2's ability to react with other elements, particularly oxygen, determines how flammable it is. Due to its high reactivity, hydrogen easily joins oxygen to form water, which releases a significant amount of energy in the process.

Hydrogen is a good reducing agent due to its high electro-positive character, which also increases its flammability and reactivity. In summary, H2's high reactivity, strong electro-positive character, and propensity to readily combine with oxygen to form water all contribute to its flammability.

Due to its extremely low thermal conductivity, hydrogen gas easily and quickly transfers heat. As a result, a hydrogen fire or explosion may spread quickly. H2, because of its small mass has very high velocity.

Water

Water is a unique molecule which can stay in three states of matter, as a solid, liquid or gas. Clouds, snow, and rain are all made of up of some form of water.

Water’s strength comes from its polar molecular structure. It is a triatomic molecule with one oxygen and two hydrogen atoms. With oxygen having negative charge and hydrogen having positive charge it has very high intermolecular forces called H-bonds. The polar structure of water gives water high heat storage capacity. When water is heated at its boiling point it produces saturated steam by transfer of its stored energy to steam called latent heat making it highly powerful source of thermal energy for heating.