Preparation and Properties of Dihydrogen

Introduction

Hydrogen is the lightest and simplest element in the periodic table. It is the most abundant element in the universe. In its atomic form, it consists of only one proton and one electron. However, in its elemental form, it exists as a diatomic (H₂) molecule and is called dihydrogen.

In this article, we are looking briefly into what dihydrogen is, how the preparation of dihydrogen is done, and lastly all the properties of dihydrogen. So, if you are one of those students looking for learning dihydrogen, then you are in the right place. Keep reading, and you will get everything that you need for CBSC class 11.

What is Dihydrogen?

So, as previously discussed, in its elemental form, hydrogen exists as a diatomic (H₂) molecule and is called dihydrogen. It is the most abundant element in the universe. It occupies 70% of the total mass of the universe. Furthermore, it is the principal element in the solar atmosphere. Although it is abundant in a universe, it can’t be found in a free state.

Dihydrogen is a diatomic molecule. It is the product of an exothermic reaction between H-atoms.

H(g) + H(g) ⟶ H₂ (g)

Preparation of Dihydrogen

There are numerous methods for preparing dihydrogen from metal and metal hydrides. Let’s see how laboratory preparation of dihydrogen and commercial production of dihydrogen is done.

Laboratory Preparation of Dihydrogen

In the laboratory, dihydrogen is usually prepared by the:

1. The reaction of reactive metals with dilute acids.
2. The reaction of atmospheric metals with alkalies.

1. Reaction of granulated zinc with dilute hydrochloric acid.

The metals, which are reactive and occupy a higher position in the electrochemical series than hydrogen, evolve hydrogen when treated with acids.

Zn + 2HCl ⟶ ZnCl2 + H₂ ↑

Mg + 2HCl ⟶ MgCl2 + H₂ ↑

2. Reaction of Zn, Al, Sn, Pb, Si (atmospheric metals) with boiling NaOH or KOH to evolve hydrogen. 

The reaction of zinc with aqueous alkali produces dihydrogen along with sodium zincate.

Zn + 2NaOH ⟶ Na2ZnO2 + H₂ ↑

2Al + 2NaOH +2H₂O ⟶ 2NaAlO2 + 3H₂ ↑

Uyeno’s Method

Very pure hydrogen is obtained rapidly by the action of caustic potash (KOH) on scrap aluminum. This method is used for military purposes.

2Al + 2KOH + 2H₂O ⟶ 2KAlO2 + 3H₂ ↑

Commercial Production of Dihydrogen

Here are commonly used processes for the preparation of dihydrogen.

1. Water is a bad conductor of electricity. It can be made a conductor by either the addition of an acid or an alkali. The electrolysis of acidified water using platinum electrodes evolves hydrogen.

2H₂O (l) ⟶ 2H₂ (g) + O2 (g)

  2. High purity (>99.95%) dihydrogen is obtained by electrolysis of warm aqueous barium hydroxide solution between nickel electrodes.

  3. In the manufacturing process of sodium hydroxide and chlorine by electrolysis of brine solution, hydrogen can be obtained as a byproduct.

2Na+ (aq) + 2Cl- (aq) + 2H₂O (l) ⟶ Cl2 (g) + H₂ (g) + 2Na+ (aq) + 2OH- (aq)

  4. The reaction of steam on hydrocarbons/ coke at high temperatures gives hydrogen. (catalyst needed)

CH4 (g) + H₂O (g) ⟶ CO (g) + 3H₂ (g)↑ (Here we used a nickel catalyst & temp is 1270K). 

The mixture of CO and H₂ is used for the synthesis of methanol and many other hydrocarbons. This mixture is also called a ‘synthesis gas’ or syngas. The process of producing syngas from coal is called ‘coal gasification.

C (s) + H₂O (g) ⟶ CO (g) + H₂ (g) (Reaction done at temp. 1270K)

  5. Bosch Process (water gas reaction):

A large quantity of commercial dihydrogen is obtained using this method. Water-gas (syngas) are produced first by passing steam over hot coke at 1270 Kelvin.

C (s) + H₂O (g) ⟶ CO (g) + H₂ (g)

Now, this water gas is mixed with twice its volume of steam and passed over a mixture of ferric oxide and chromium oxide heated to 500 °C. This mixture acts as a catalyst and helps convert carbon monoxide to carbon dioxide.

CO + H₂ + H₂O ⟶ CO2 + 2H₂

CO₂ dissolves in water under pressure. The mixture of CO₂ and H₂ is passed through the water under a pressure of 25-30 atmospheres. CO₂ dissolves completely in water. Traces of CO present in hydrogen are removed by passing the gas through an ammoniacal cuprous chloride solution under a pressure of 200 atmospheres.

The hydrogen manufactured by this method is utilized for the synthesis of ammonia. Here, CO acts as a poison for the catalyst. To make hydrogen completely free from CO, it is passed over a nickel catalyst where any amount of CO left is converted to methane.

CO + 3H₂ ⟶ CH4 + H₂O

Properties of Dihydrogen

So, we saw how we could prepare dihydrogen at laboratory scale and commercial scale. Now, let’s dive into the properties of dihydrogen. Generally, properties can be divided into two categories:

1. Physical Properties
2. Chemical Properties

Physical Properties of Dihydrogen

1. It is a colorless, odorless, tasteless, and combustible gas.
2. It is only slightly soluble in water, about 2 volumes in 100 volumes of water at 0 °C.
3. It is the lightest of all elements. Density -0.08987 gL-1.
4. It is diatomic.
5. It has a low melting and boiling points.
   1. Boiling point: -252.5 °C (20.5 K)
   2. Melting point: -259.0 °C (14.0 K)

Chemical Properties of Dihydrogen

The chemical properties of dihydrogen are illustrated below.

• Reaction with Halogens

Dihydrogen reacts with halogens. X, to give hydrogen halides, HX.

H₂(g) + X₂ (g) → 2HX(g) (X = F, Cl, Br, I)

While the reaction with fluorine occurs even in the dark, with iodine it requires a catalyst.

• Reaction with Dioxygen

It reacts with dioxygen to form water. The reaction is highly exothermic.

2H₂(g) + O₂(g) → 2H₂O (l) ΔH = -285.9 kJ mol-1

• Reaction with Dinitrogen

On reaction with dinitrogen Dihydrogen forms ammonia.

3H₂ (g) + N₂ (g) ⟶ 2NH3 (g) ΔH-92.6 kJ mol-1

This is the method for the manufacture of ammonia by the Haber process.

• Reactions with Metals

With many metals, it combines at high temperatures to yield the corresponding hydrides.

H₂(g) + 2M(g) → 2MH(s) ….where M is an alkali metal 

• Reactions with metal ions and metal oxides

It reduces some metal ions in an aqueous solution and oxides of metals (less active than iron) into corresponding metals.

H₂(g) + Pd² (aq) → Pd(s) + 2H+ (aq) 

yH₂ (g) + MxOy (s) → Mx(s) + yH₂O(l)

• Reactions with Organic Compounds

Dihydrogen reacts with many organic compounds in the presence of catalysts to give useful hydrogenated products of commercial importance. 

For example, The hydrogenation of vegetable oils using nickel catalyst gives edible fats.

Conclusion

So, we saw what dihydrogen is, the preparation methods of dihydrogen, and the properties of dihydrogen. Hope you got a clear understanding of Dihydroge