Introduces Hydrogen

Hydrogen

Hydrogen is the lightest element known. It possesses the following atomic characters.

Atomic weight: 1.008

Valency: 1

Molecular weight: 2.016

Atomic number: 1

Electronic Configuration: 1s¹

Isotopes: Protium¹₁H, Deuterium²₁H, Tritium³₁H

The word hydrogen has been taken from two Greeks words (hydro = Water and gene = Produce i.e., water producer). Hydrogen was firstly discovered by Henry Cavendish in 1766. Then it was named hydrogen by Lavoisier, the father of modern chemistry. Lavoisier was the first chemist to explain the redox reactions which occur during burning.

Position of Hydrogen in the periodic table

The position of hydrogen in the periodic table is anomalous. Hydrogen resembles both the alkali metals of the group I-A and halogens of group VII-A. This dual behavior of hydrogen can be explained with the help of its peculiar electronic arrangement.

Resemblances with Group IA (Alkali Metals)

1. Electronic Configuration: It has same valence shell electronic configuration as that of alkali metals. For example:
   Hydrogen =1s¹
   Lithium = 2s¹
   Sodium = 3s¹
2. Electronic character: Hydrogen can form H⁺ ion losingthe valence electron like alkali metals. Alkali metals also form monovalent cations.
   $$H \longrightarrow {H^++e} $$
   $$Li\longrightarrow {Li^++e}$$
   $$Na\longrightarrow{Na^++e} $$
3. Valency and Oxidation state:Hydrogen is univalent element having +1 oxidation state. Alkali metals also are monovalent and possess +1 oxidation state.
4. Reducing character: Like alkali metals, hydrogen acts as a reducing agent. For example;
   $$ CuO + H_2 \longrightarrow {Cu+H_2O}$$
   $$B_2O_3+6K\longrightarrow {2B+3K_2O}$$
5. Both, hydrogen and alkali metals, are liberated at the cathode during electrolysis.

Differences from Alkali Metals

1. Atomicity:Hydrogen is a diatomic molecule (a molecule made up of two atoms) where as alkali metals are monatomic.
2. Non-metallic character:Hydrogen is a non-metal but all members of alkali metals are typical metals.
3. Nature of compounds: Hydrogen generally forms covalent compounds while alkali metals form ionic compounds. For example:
   HCl, NaBr, Na₂O are ionic compounds.
4. Nature of oxides:Oxides of alkali metals are basic in nature but oxide of hydrogen (H₂O) is neutral in nature.

Resemblances with Group VIIA (Halogens)

1. Non-metallic character: Hydrogen is a nonmetal and reactive in nature. Halogens are also reactive non-metals.
2. Electronic configuration: Hydrogen requires one electronic to occupy inert electronic configuration as halogens.
   H (1s¹) + 1 electron = 1s² (similar to He configuration)
   F (1s² 2s² 2p⁵) + 1 electron = 1s² 2s² 2p⁶ (similar to Ne configuration)
3. Oxidation state and valency: Hydrogen shows -1 oxidation state in their compounds like metal hydrides (Na⁺ H^-) and so do halogens in halides like NaCl.
4. Atomicity: Halogen molecules are also diatomic molecule as hydrogen molecules. (H₂, F₂, Cl₂, Br₂, I₂)
5. Nature of chemical bond: Hydrogen forms the covalent bond with other non-metals. For example; CH₄, H₂. Similarly, halogens form the covalent bond with other non-metals. For example; HCl, HBr, Cl₂, NCl₃.

Methods of Preparation of Hydrogen (H₂)

The main sources of hydrogen are water, acids, and alkalis.

1. From acid: Active metals like zinc, magnesium, iron etc. which have higher oxidation potential than that of hydrogen can displace hydrogen from dilute mineral acids like HCl, H₂SO₄etc.
   $$Zn + H_2SO_4\longrightarrow{ZnSO_4+H_2↑}$$
   $$Fe+2HCl\longrightarrow{FeCl_2+H_2↑}$$
   $$Mg+2HCl\longrightarrow{MgCl_2+H_2↑}$$
2. From water:There are various methods of preparing hydrogen from water.
   a) Action with metals:
   i) With very active metals: Alkali metals react with water to produce hydrogen.
   $$2Na+2H_2O\longrightarrow{2NaOH+H_2}$$
   $$2K+2H_2O\longrightarrow{2KOH+H_2}$$
   $$2Li+2H_2O\longrightarrow{2LiOH+H_2}$$
   The reaction takes place vigorously producing heat which catches fire.In order to slow down the reaction amalgams of these metals are used.
   b) Action with calcium:Calcium reacts in presence of heat with water.
   $$Ca+2H_2O\xrightarrow\Delta{Ca (OH)_2 +H_2↑}$$
   c) Action with Zn, Mg etc:
   $$Zn+H_2O\xrightarrow\Delta{ZnO+H_2↑}$$
   $$Mg+H_2O\xrightarrow\Delta{MgO+H_2↑}$$
   d) Action with Fe, Ni:Fe or Ni can react with steam.
   $$3Fe+4H_2O(steam)\longrightarrow{Fe_3O_4+4H_2}$$(ferroso-ferric oxide)
   e) Electrolysis of water:When electricity is passed through water adding little amount of salt, acid or base, dihyrogen is produced. It is collected at cathode and oxygen at anode.
   $$2H_2O\rightleftharpoons{H^++OH^-}$$
   At cathode: $$2H^++2e^-\longrightarrow{H_2}$$
   At anode: $$4OH^--4e^-\longrightarrow{2H_2O+O_2}$$
   Thus obtained hydrogen is highly Pure.
3. From alkalis:Certain metals like zinc, tin, aluminum etc. react with NaOH or KOH to give H₂ gas.
   $$Zn+2NaOH\longrightarrow{Na_2ZnO_2+H_2↑}$$ (sod. zincate)
   $$Sn+2NaOH\longrightarrow{Na_2SnO_2+H_2↑}$$ (sod. stannite)
   $$2Al+2NaOH+2H_2O\longrightarrow{2NaAlO_2+3H_2}$$ (sod. meta aluminate)

Industrial Preparation of Hydrogen Gas

The industrial preparation of hydrogen is carried out by Bosch-method.

Working Principle

The equimolar mixture of carbon monoxide and hydrogen is called water gas. Water gas is prepared by passing steam over red-hot coke at 897°C.

$$C+H_2O\longrightarrow{CO+H_2}$$

Thus obtained water gas is further passed over heated catalytic mixture of Fe₂O₃ (ferric oxide) and Cr₂O₃ (chromium oxide) at 500°C to get CO₂ and H₂.

The resulting mixture is passed over distilled water compressing about 25 atmospheric pressure. CO₂ gets dissolved in water leaving free hydrogen.

$$CO+H_2+H_2O \;\xrightarrow{500°C\; Fe_2O_3/Cr_2O_3}\;{CO_2+2H_2↑}$$

$$CO_2 H_2 \xrightarrow{water/25\;atoms}{H_2↑}$$

Laboratory Preparation of Hydrogen

Principle

Granulated zinc (impure zinc) is treated with dilute sulphuric or hydrochloric acid to get hydrogen. The metals which have higher oxidation potential than that of hydrogen are used for the preparation of H₂ gas.
$$Zn\longrightarrow{ZnSO_4+H_2↑}$$

In fact, it is a redox reaction. Pure zinc is not used for the preparation of H₂ gas because the chemical reaction takes place very slowly. The presence of impurities increases the rate of reaction due to the formation of electrochemical couples. Instead of dilute sulphuric acid, concentrated sulphuric acid can't be used because of its oxidizing character.

With conc. H₂SO₄, the following reaction takes place and the evolution of SO₂ gas occurs along with H₂ gas.

$$Zn +2H_2SO_4\longrightarrow{ZnSO_4+H_2O+SO_2}$$

Nitric acid also has similar oxidizing behavior and hence can not be applied for the preparation of H₂ gas. The gasses like NO, NO₂ evolved by the reduction of nitric acid contaminate H₂ gas. Thus prepared H₂ gas is collected by downward displacement of water.

Process

Dry and clean Woulfe's bottle fitted with thistle funnel and delivery tube is taken. Before fitting the instrument, granulated zinc is kept in the bottle. The airtightness of the apparatus is checked. Dilute H₂SO₄ is poured dropwise through the thistle funnel little at a time. Thus produced H₂ gas is collected over water in a gas jar by downward displacement of water.

Test

When a burning candle is introduced to the gas jar of H₂ gas, it produces pop sound.

Properties of Hydrogen

Physical Properties

1. It is colorless, orderless and tasteless.
2. It is insoluble in water.
3. It can be liquefied at high pressure and low temperature.
4. It is the lightest gas having very low density. One liter of H2 at NTP weighs 0.0982g.
5. It is neutral to litmus.

Chemical Properties

The bond dissociation energy of hydrogen molecule is 436 KJ mol^-1 and atomic size is very small. Due to these reasons, it is less reactive in nature in nature. However, it reacts with metals, non-metals and other compounds under suitable conditions.

1. Hydrogen is a stable molecule.
2. Action with metals:
   
   a) With reactive metals like: Na, K, Ca etc. respective hydrides are formed.
   $$Na +H_2 \xrightarrow\Delta{2NaH}$$(Sod. hydride)
   $$Ca+H_2\xrightarrow\Delta{CaH_2}$$(calcium hydride)
   
   b) With less reactive metals:Finely divided metals like palladium (Pd), platinum (Pt), nickel (Ni), cobalt (Co) etc. absorb hydrogen to form interstitial hydrides. This absorption is a surface phenomenon and not a chemical process. The absorption of a gas by metals is known as occlusion. Thus formed hydrides are known as non-stoichiometric hydrides.
   
   
3. Action with non-metals:Hydrogen reacts with the number of non-metals under the specific condition.
   a) With oxygen:When hydrogen is heated with oxygen water is produced.
   $$2H_2+O_2\xrightarrow{800°C}{2H_2O}$$
   
   b)With halogens:Hydrogen reacts with halogens to produce hydrogen halides. Reactivity of hydrogen decreases from fluorine to iodine.
   $$H_2+F_2\xrightarrow{dark}{2HF}$$
   $$H_2+Cl_2\xrightarrow{sunlight}{2HCl}$$
   $$H_2+Br_2\xrightarrow{673K}{2HBr}$$
   $$H_2+I_2\xrightarrow{713K/Pt}{2HI}$$
   
   c) With nitrogen:When nitrogen is treated with hydrogen about 773K and in the presence of catalyst Fe & Mo as a promoter under a pressure of about 200 atmospheric pressure, ammonia gas is formed. The reaction is reversible and exothermic.
   $$N_2+3H_2\rightleftharpoons{2NH_3+\;Heat}$$
   
   d) With sulphur:Sulphur when heated with hydrogen, produces hydrogen sulphide (H₂S).
   $$S_8+8H_2\xrightarrow{700K}{8H_2S}$$
   
   e) With carbon:Hydrogen reacts with carbon on heating to produce hydrocarbons.
   $$C+2H_2\xrightarrow{1373K}{CH_4}$$
   $$2C+3H_2\xrightarrow{1773K}{C_2H_6}$$
   $$2C+H_2\xrightarrow{2775K}{C_2H_2}$$
   
   
4. With other compounds:
   a) With metal oxides:Hydrogen is a reducing agent. So, it reduces metal oxides into pure metals.
   $$CuO+H_2\xrightarrow\Delta{Cu+H_2O}$$
   $$MgO+H_2\xrightarrow\Delta{Mg+H_2O}$$
   $$Fe_3O_4+4H_2\xrightarrow\Delta{3Fe+4H_2O}$$
   
   b)With carbon monoxide:Carbon monoxide reacts with hydrogen on heating about 537K and in the presence of catalyst ZnO and Cr₂O₃ atmos. pressure to get methyl alcohol (methanol).
   $$CO+H_2\xrightarrow{573K\;ZnO,Cr_2O_3(200\;atom)}{CH_3OH}$$
   
   c) With unsaturated hydrocarbon:Unsaturated organic compounds like ethene (CH2 = CH2) reacts with hydrogen to produce saturated organic compounds like ethane. This reaction takes place in the presence of Ni, Pb or Pt powder on heating at about 473K.
   $$CH_2=CH_2+H_2\xrightarrow{Ni\; or\;Pd\;or\;Pt}{vegetable\;ghee\;(fat)}$$

Types of Hydrogen

Nascent Hydrogen

The word 'nascent' means 'newly born'. The hydrogen obtained during the chemical reaction in the beginning which is in association with the substance to be reduced is called nascent hydrogen. In this state, it contains high energy and is very reactive. Symbolically, nascent hydrogen is represented with the atomic symbol of hydrogen, H.

The nascent hydrogen is more reactive and hence more powerful reducing agent than molecular hydrogen. The following illustrations support this fact.

1. Acidified potassium permanganate solution remains unaffected by hydrogen gas (molecular hydrogen). But when Zinc granules are placed in acidified KMnO₄ solution, the pink color of KMnO₄ is discharged. In the later case, nascent hydrogen is produced from the source Zn and H₂SO₄ and it readily reduces KMnO₄ into Mn⁺⁺ salt.
   $$a)\; KMnO_4 +H_2SO_4+H_2\longrightarrow{No\;action}$$
   $$b)\;Zn+H_2SO_4\longrightarrow{ZnSO_4+2H}$$
   $$c)\; 2KMnO_4+2H_2SO_4+10H\longrightarrow{K_2SO_4+2MnSO_4+8H_2O}$$
   
   
2. Acidified potassium dichromate solution remains unaffected by hydrogen gas. But when granulated zinc is added into K₂Cr₂O₇ solution, the orange colored dichromated is reduced to green Cr₂(SO₄)₃.
   $$a)\; K_2Cr_2O_7+H_2SO_4+H_2\longrightarrow{No\;action}$$
   $$b)\;Zn+H_2SO_4\longrightarrow{ZnSO_4+2H}$$
   $$c)\;K_2Cr_2O_7+4H_2SO_4+6H\longrightarrow{K_2SO_4+Cr_2(SO_4)_3+7H_2O}$$
   
   
3. H₂ gas dosen't produce any effects on ferric chloride solution. When granulated zinc and sulphuric acid are introduced into FeCl₃ solution, the yellow colour of latter changes to light green. This means Fe⁺⁺⁺ ion is reduced to Fe⁺⁺ ion by nascent hydrogen.
   $$a)\;FeCl_3+H_2\longrightarrow{No\;action}$$
   $$b)\;Zn+H_2SO_4\longrightarrow{ZnSO_4+2H}$$
   $$c)\;FeCl_3+H\longrightarrow{FeCl_2+HCl}$$
   
   
   

Theories of Nascent Hydrogen

1. Atomic hypothesis:Previously it was thought that in the case of ordinary hydrogen, during reduction a certain amount of energy has to be supplied for breaking the molecules into atoms while the nascent hydrogen being in atomic state attacks directly. This theory, however, fails to explain why nascent hydrogen from different sources is not equally effective. For e.g. the nascent hydrogen from Zn and dilute H₂SO₄ readily reduces chlorates into chlorides but that from NaHg and water does not.
2. Energy hypothesis: A more recent explanation is based on the association of energy as a result of the transfer of electrons from metals of low electronegativity to H⁺ ions in order to change its valency from +1 to zero.
   $$Zn\longrightarrow{Zn^{++}+2e}$$
   For conversion into the free state, the hydrogen ion found in solution accepts these electrons.
   $$H^++e\longrightarrow{H}$$
   Here, nascent hydrogen is associated with energy liberated and thus, it's reactive. Since the energy liberated in the transfer of electrons is different for different reactions, the different reducing capacity of nascent hydrogen from different sources in a reaction can well be understood.
3. Internal pressure theory:According to this theory, hydrogen at the moment of its formation is in the form of tiny bubbles with very high internal pressure, which is the cause of great activity of nascent hydrogen.
   
   

Atomic Hydrogen

When the molecules of hydrogen are dissociated by providing the necessary energy, these get dissociated and atomic hydrogen (H) is produced. When a stream of H2 gas under pressure is passed through an electric arc struck between tungsten rods, atomic hydrogen is produced.
$$H_2\rightleftharpoons{2H\;-\;104KCal}$$

Nascent hydrogen is produced in the reaction mixture in solution in the presence of oxidizing agents. But atomic hydrogen is produced at a high temperature of about 3000°C by dissociating H2 molecule. It can't be produced in solution at ordinary temperature. Nascent hydrogen can never be isolated but atomic hydrogen can be. Atomic hydrogen is much stronger reducing agent than nascent hydrogen. For example, it can reduce certain metallic compounds AgAl, CuO, CdS to the respective metal while nascent hydrogen can not.

Ortho and Para Hydrogen

Bonhoeffer and Harteck (1929) have shown from various physical measurements that ordinary hydrogen is an equilibrium mixture of two forms of hydrogen: ortho and para, which slightly differ in their physical properties.

A hydrogen molecule consists of two hydrogens atoms, each having one proton in the nucleus with an electron moving around it. Both the electron and proton possess the property of spin about an axis. The two electrons in the hydrogen molecule always have an opposite spin but the two protons may have their spins either in the same sense or in opposite senses. When the proton spins are in the same sense, the molecular hydrogen is called para hydrogen.

The ortho form is more stable than para form. At room temperature about 75% of ortho hydrogen is present. At low temperature, there will be a greater proportion of para hydrogen. It's possible to determine their proportion because the two forms differ in certain physical properties like specific heat, thermal conductivity etc. On heating above 800°C, para hydrogen fully converts into ortho hydrogen. When atomic hydrogen is made collided with para hydrogen, ortho hydrogen is formed.

Isotopes of Hydrogen

Isotopes are different forms of an atom of the same element which have a same atomic number but different mass numbers. The mass number differs due to the difference in a number of neutrons. It has been established by means of the mass spectrograph that natural hydrogen consists of three isotopes: protium or ordinary hydrogen, deuterium or heavy hydrogen and tritium or radioactive hydrogen.

Deuterium exists as about 1 part in 4500 parts of hydrogen. Ordinary water contains about 1 part of heavy water in60,000 parts and by electrolysis of heavy water heavy hydrogen is obtained at the cathode. The chemical properties of heavy hydrogen are identical with those of ordinary hydrogen but the physical properties of these two differ since these two isotopes of each other.

Use of Different Isotopes of Hydrogen

1. Hydrogen is used:
   a) In Haber's synthesis of ammonia.
   b) In hydrogenation of oil and manufacture of vanaspati.
   c) To produce oxyhydrogen flame used for welding.
   d) To produce reducing atmosphere.
   e) As a fuel in liquefied form.
   f) In filling ballons.
   
   
2. Deuterium is used:
   a) In the transformation of elements because high-speed neutrons are found to be much more efficient projectiles in bombarding an atom than high-speed protons.
   b) Deuterium compounds are used as tracers in the study of the reaction mechanism.
   
   
3. Tritium is used:
   a) As an artificial radioactive tracerin chemistry, medicine and biology.
   b) In nuclear fusion reactions to produce a huge amount of energy which is used in the hydrogen bomb.

Bibliography

Tiwari , Rishi and Mohan Bikram Gewali. Principles of Chemistry. second edition . Kathmandu: Buddha Academic Publisher and Distributors Pvt. Ltd, 2009.