Classwork Series and Exercises {Chemistry – SS2}: Hydrogen

Week 5

Topic: Hydrogen

Introduction

Hydrogen was first isolated and shown to be a discrete element by Henry Cavendish in 1766.  Before that, Robert Boyle and Paracelsus both used reactions of iron and acids to produce hydrogen gas.  Antoine Lavoisier gave hydrogen its name because it produced water when ignited in air.  Hydrogen comes from Greek meaning “water producer” (“hydro” =water and “gennao”=to make).

Hydrogen is one of the most important elements in the world. It is all around us. It is a component of water (H₂O), fats, petroleum, table sugar (C₆H₁₂O₆), ammonia (NH₃), and hydrogen peroxide (H₂O₂)—things essential to life, as we know it.

Atomic hydrogen, having atomic number 1 and an atomic weight 1.008 amu, is the simplest element.  At ordinary temperatures and pressures it is a gas, composed of diatomic molecules, H₂ and is only very slightly soluble in water.  In this form, hydrogen is a colourless, odourless, and tasteless gas. The sun and other stars are composed largely of hydrogen. Naturally occurring hydrogen is mostly composed of ¹H atoms and a very small percentage of deuterium. Hydrogen containing compounds can both be ionic and covalent in nature.  Examples include acids, bases, and all organic compounds.  In organic compounds, hydrogen is covalently bonded.

Isotopes

Three Hydrogen Isotopes

• Protium (¹H) is the most common isotope, consisting of 99.98% of naturally occurring hydrogen. It is a nucleus containing a single proton.
• Deuterium (²H) is another an isotope containing a proton and neutron, consisting of only .0156% of the naturally occurring hydrogen.  Commonly indicated with symbol D, D20 is called heavy water, which has a higher density, melting point, and boiling point than regular water.  Replacing protium with deuterium (called deuteration) has important implications for the rate of reaction called kinetic-isotope effect.
• Tritium (³H) is a radioactive isotope with a 12.3-year half-life, which is continuously formed in the upper atmosphere due to cosmic rays.  It is can also be made in a lab from Lithium-6 in a nuclear reactor. Tritium is also used in hydrogen bombs.

Laboratory Preparation of Hydrogen

• Preparation of hydrogen gas from water

Water is the most common and probably most important compound of hydrogen.  Hydrogen is liberated when a direct current of electricity is passed through water containing a small amount of an electrically conducting substance such as sulfuric acid, H₂SO₄.  The chemical change that occurs is summarized in a chemical equation below:

2 H₂O(l)  + electrical energy  ——->  2 H₂(g)  + O₂(g)

• Preparation of hydrogen gas from active metals

Several of the most active metals-such as lithium, sodium, potassium, rubidium, cesium, magnesium, and calcium-will react with water to produce hydrogen gas.  The net chemical change is summarized in reaction below using the reaction of sodium metal with water as an example.  This reaction produces hydrogen gas and sodium hydroxide.

 2Na(s)  +2 H₂O(l) ——-> 2 NaOH(aq)  +  H₂(g)

• Preparation of hydrogen gas from metals with acids

This is the most convenient laboratory method of producing hydrogen.  Several of the metals most commonly used are shown in the reactions below.  In each of these reactions, the metal reacts with an acid to produce a salt and hydrogen gas.  The salts produced in the reactions listed below are magnesium chloride, magnesium sulfate, and zinc phosphate, respectively.

 Mg(s)  +  2 HCl(aq)  ——>  MgCl₂(aq)  +  H₂(g)

Mg(s)  +  H₂SO₄(aq)  ——>  MgSO₄(aq)  +  H₂(aq)

3Zn(s)  + 2 H₃PO₄(aq)  ——>  Zn₃(PO₄)₂(aq)  + 3H₂(g)

Physical properties of Hydrogen

Hydrogen is the lightest element known. It is 14.4 less dense than air

It is neutral to moist litmus paper i.e. it has no action on either red or blue litmus paper

Hydrogen is a colourless and odourless gas when pure

It is slightly soluble in water, two part dissolving in 100 parts of water by volume

Chemical Properties of Hydrogen

• Hydrogen wanting to give up its single electron causes it to act like an alkali metal:

H(g)→ H⁺(g) + e^–

• A half-filled valence shell with one e- also causes hydrogen to act like a halogen because it wants to gain Noble gas configuration by adding an e^–:

H(g) + e^– → H^–(g)

• Reactions with Active Metals: Hydrogen accepts e- from an active metal to form ionic hydrides like LiH. By forming an ion with -1 charge, the hydrogen behaves like a halogen.

2M(s)+H₂(g) → 2MH(s)  M= group 1 metals

Example: 2K(s)+H₂(g) → 2KH(s)  2K(s)+Cl₂(g) → 2KCl(s)

M(s)+H₂(g) → MH₂(s)  M= group 2 metals

Example: Ca(s)+H₂(g) → CaH₂(s)  Ca(s)+Cl₂(g) → CaCl₂(s)

• Reactions with Non-metals: Unlike metals forming ionic bonds with nonmetals, hydrogen forms polar covalent bonds. Despite being electropositive like the active metals that form ionic bonds with nonmetals, hydrogen is much less electropositive than the active metals, and forms covalent bonds.

Hydrogen + Halogen → Hydrogen Haliden(H₂(g)+ Cl₂(g) → HCl(g))

Hydrogen + Oxygen → Water (H₂(g)+O₂(g) → H₂O(g))

• Hydrogen is a good reducing agent e.g. it reduces copper (II) oxide to copper

CuO(s) + H₂(g) ——-> Cu(s) + H₂O(l)

• Reaction with Sulphur: if hydrogen is bubbled through molten sulphur, hydrogen sulphide which has a characteristics rotten egg smell is formed

H₂(g) + N₂(g) ——-> H₂S(g)

• Hydrogen also reacts with nitrogen directly to produce ammonia gas. This involves high temperature

3H₂(g) + N₂(g) ——-> 2NH₃(g)

The Test for Hydrogen Gas,  H_2(g)

Note the following characteristics when testing for hydrogen:

1. Hydrogen gas has no colour or smell.
2. Hydrogen gas has no effect on moist litmus paper or moist universal indicator paper – it is neutral.
3. Hydrogen gas burns with a characteristic ‘pop’.

 Specific Test for Hydrogen Gas

The test for hydrogen is a method used to test the presence of hydrogen. This is achieved by inserting a glowing splint into the reaction test tube. If it burns with a ‘pop’ sound, then hydrogen is present. The ‘pop’ is the sound of a small explosion, hydrogen is highly flammable

Uses

1. Hydrogen was formerly used for filling balloons, airships, and other lighter-than-air craft, because it has a low density.
2. Hydrogen is used in the Haber process for the fixation of atmospheric nitrogen, in the production of methanol, and in hydrogenation of fats and oils.
3. It is also important in low-temperature research. It can be liquefied under pressure and cooled; when the pressure is released, rapid evaporation takes place and some of the hydrogen solidifies.
4. Hydrogen is used in manufacture of ammonia by Haber process

Hydrogen Peroxide

Hydrogen peroxide is a chemical compound of hydrogen and oxygen with the formula H₂O₂. Pure, anhydrous hydrogen peroxide is a colourless, syrupy liquid with a specific gravity of 1.44. It blisters the skin and has a metallic taste. The liquid solidifies at –0.41 °C. Concentrated solutions are unstable, and the pure liquid can explode violently if heated to a temperature above 100 °C. It is soluble in water in all proportions, and the usual commercial forms are a 3 % and a 30 % aqueous solutions. To retard the decomposition of the peroxide into water and oxygen, organic substances, such as acetanilide, are added to the solutions, and they are kept in dark bottles at low temperature.

Preparation Methods of Hydrogen Peroxide

Hydrogen peroxide is manufactured in large amounts by the electrolysis of aqueous solutions of sulfuric acid (or of potassium bisulfate or ammonium bisulfate):

H₂SO₄ ⇄ H⁺ + HSO₄^–                                                        

On cathode                                           On anode

2H⁺ + 2e^– → H₂                             2HSO₄^– – 2e^– → H₂S₂O₈

2H₂SO₄ → H₂S₂O₈ + 2H⁺ + 2e^–;

H₂S₂O₈ +2H₂O→ 2H₂SO₄ + H₂O₂.

It is prepared also by the reaction of acid with other peroxides, such as those of sodium and barium:

Na₂O₂ + H₂SO₄ → Na₂SO₄ + H₂O₂;

BaO₂ + H₂SO₄ → BaSO₄¯ + H₂O₂ .

Chemical Properties of Hydrogen Peroxide

• Acid-base properties

Hydrogen peroxide is a week acid. In an aqueous solutions it ionizes forming hydronium-ion and peroxid-ion:

H₂O₂ + H₂O ⇄ H₃O⁺ + O₂^2–

• Oxidation-reduction properties

Hydrogen peroxide acts as both an oxidizing and a reducing agent.

In acidic solution it is an oxidizing agent:

2KI + H₂O₂ + H₂SO₄ → I₂ + K₂SO₄ + 2H₂O

However both in basic and in a neutral solutions hydrogen peroxide can be an oxidizing agent:

Cr₂(SO₄)₃ + 3H₂O₂ + 10NaOH → 2Na₂CrO₄ + 3Na₂SO₄ + 8H₂SO₄

PbS + 4H₂O₂ → PbSO₄ + 4H₂O

At the presence of oxidizing agent it exhibits reduction properties in acidic, basic and neutral solutions:

Cl₂ + H₂O₂ → 2HCl  + O₂­;

Ag₂O + H₂O₂ → H₂O + O₂­ + 2Ag;

2KMnO₄ + 5H₂O₂ + 3H₂SO₄ → 2MnSO₄ + 5O₂ + K₂SO₄ + 8H₂O

• Decomposition of hydrogen peroxide

Light, heating and the heavy metals hardly accelerate the process of hydrogen peroxide decomposition:

Uses Of Hydrogen

1. Its oxidizing properties are used in the bleaching of substances, such as hair, ivory, feathers, and delicate fabrics, which would be destroyed by other agents.
2. It is used also medicinally, in the form of a 3 % aqueous solution, as an antiseptic and throat wash.
3. Hydrogen peroxide is used in restoring the original colors on paintings that have darkened through the conversion of the white lead used in the paintings to lead sulfide.
4. The hydrogen peroxide oxidizes the black lead sulfide to white lead sulfate.
5. It is used also as a source of oxygen in the fuel mixture for many rockets and torpedoes.