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Hydrides

Hydrogen forms binary hydrides with many electropositive elements including metals and non-metals. It also forms ternary hydrides with two metals. E.g., LiBH₄ and LiAlH₄. The hydrides are classified as ionic, covalent and metallic hydrides according to the nature of bonding. Hydrides formed with elements having lower electronegativity than hydrogen are often ionic, whereas with elements having higher electronegativity than hydrogen form covalent hydrides.

Ionic (Saline) Hydrides:

These are hydrides composed of an electropositive metal, generally, an alkali or alkaline-earth metal, except beryllium and magnesium, formed by transfer of electrons from metal to hydrogen atoms. They can be prepared by the reaction of elements at about 400°C. These are salt-like, high-melting, white crystalline solids having hydride ions (H^–) and metal cations (Mⁿ⁺).

2 Li + H₂ → 2 LiH
2 Ca + 2H₂ → 2 CaH₂

Covalent (Molecular) Hydrides:

They are compounds in which hydrogen is attached to another element by sharing of electrons. The most common examples of covalent hydrides of non-metals are methane, ammonia, water and hydrogen chloride. Covalent hydrides are further divided into three categories, viz., electron precise (CH₄, C₂H₆, SiH₄, GeH₄), electrondeficient (B₂H₆) and electron-rich hydrides (NH₃, H₂O). Since most of the covalent hydrides consist of discrete, small molecules that have relatively weak intermolecular forces, they are generally gases or volatile liquids.

Metallic (Interstitial) Hydrides:

Metallic hydrides are usually obtained by hydrogenation of metals and alloys in which hydrogen occupies the interstitial sites (voids). Hence, they are called interstitial hydrides; the hydrides show properties similar to parent metals and hence they are also known as metallic hydrides.

Most of the hydrides are non-stoichiometric with variable composition (TiH_1.5-1.8 and PdH_0.6-0.8), some are relatively light, inexpensive and thermally unstable which make them useful for hydrogen storage applications. Electropositive metals and some other metals form hydrides with the stoichiometry MH or sometimes MH₂ (M = Ti, Zr, Hf, V, Zn).

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Hydrogen Peroxide

Hydrogen peroxide (H₂O₂) is one of the most important peroxides. It can be prepared by treating metal peroxide with dilute acid.

BaO₂ + H₂SO₄ → BaSO₄ + H₂O₂
Na₂O₂ + H₂SO₄ → Na₂SO₄ + H₂O₂

On an industrial scale, hydrogen peroxide is now prepared exclusively by autoxidation of 2-alkyl anthraquinol.

Physical Properties:

Pure hydrogen peroxide is almost a colorless liquid (pale blue), less volatile and more viscous than water.

A 30 % solution of hydrogen peroxide is marketed as ‘100-volume’ hydrogen peroxide indicating that at S.T.P., 100 ml of oxygen is liberated by 1 ml of this solution on heating.

Chemical Properties:

Hydrogen peroxide is highly unstable and the aqueous solution spontaneously disproportionates to give oxygen and water. The reaction is, however, slow but is explosive when catalyzed by metal. If it is stored in glass container, it dissolves the alkali metals from the glass, which catalyzes the disproportionation reaction. For this reason, H₂O₂ solutions are stored in plastic bottles.

H₂O₂ → H₂O + ½O₂

Hydrogen peroxide can act both as an oxidizing agent and a reducing agent. Oxidation is usually performed in acidic medium while the reduction reactions are performed in basic medium.

In Acidic Conditions:

H₂O₂ + 2 H⁺ + 2e^– → 2 H₂O (E° = + 1.77 V)

For Example

2FeSO₄ + H₂SO₄ + H₂O₂ → Fe₂(SO₄)₃ + 2H₂O

In Basic Conditions:

HO₂^– + OH^– → 2H₂O

For Example,

2KMnO₄(aq) + 3 H₂O₂(aq)
↓
2MnO₂ + 2KOH + 2H₂O + 3O₂(g)

Uses of Hydrogen Peroxide:

The oxidizing ability of hydrogen peroxide and the harmless nature of its products, i.e., water and oxygen, lead to its many applications. It is used in water treatment to oxidize pollutants, as a mild antiseptic, and as bleach in textile, paper and hair-care industry.

Hydrogen peroxide is used to restore the white colour of the old paintings which was lost due to the reaction of hydrogen sulphide in air with the white pigment Pb₃(OH)₂(CO₃)₂ to form black colored lead sulphide. Hydrogen peroxide oxidises black coloured lead sulphide to white coloured lead sulphate, there by restoring the colour.

PbS + 4H₂O₂ → PbSO₄ + 4 H₂O

Structure of Hydrogen Peroxide:

Both in gas-phase and solid-phase, the molecule adopts a skew conformation due to repulsive interaction of the OH bonds with lone-pairs of electrons on each oxygen atom. Indeed, it is the smallest molecule known to show hindered rotation about a single bond.

H₂O₂ has a non-planar structure. The molecular dimensions in the gas phase and solid phase differ as shown in figure 4.5. Structurally, H₂O₂ is represented by the dihydroxyl formula in which the two OH groups do not lie in the same plane.

One way of explaining the shape of hydrogen peroxide is that the hydrogen atoms would lie on the pages of a partly opened book, and the oxygen atoms along the spine. In the solid phase of molecule, the dihedral angle reduces to 90.2° due to hydrogen bonding and the O-O-H angle expands from 94.8° to 101.9°.

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Heavy Water of Hydrolysis

Heavy water (D₂O) is the oxide of heavy hydrogen. One part of heavy water is present in 5000 parts of ordinary water. It is mainly obtained as the product of electrolysis of water, as D₂O does not undergo electrolysis as easily as H₂O.

D₂O is a colorless, odorless and tasteless liquid. However, there is a marked difference between physical properties of water and heavy water as shown in Table 4.2.

Table: Properties of water, heavy water and super heavy water.

2 NaOH + D₂O → 2NaOD + HOD
HCl + D₂O → DCl + HOD
NH₄Cl + 4D₂O → ND₄Cl + 4HOD

These exchange reactions are useful in determining the number of ionic hydrogens present in a given compound. For example, when D₂O is treated with of hypo-phosphorus acid only one hydrogen atom is exchanged with deuterium. It indicates that, it is a monobasic acid.

H₃PO₂ + D₂O → H₂DPO₂ + HDO

It is also used to prepare some deuterium compounds:

Al₄C₃ + 12D₂O → 4Al(OD)₃ + 3CD₄
CaC₂ + 2 D₂O → Ca(OD)₂ + C₂D₂
Mg₃N₂ + 6D₂O → 3Mg(OD)₂ + 2ND₃
Ca₃P₂ + 6D₂O → 3Ca(OD)₂ + 2PD₃

Uses of Heavy Water:

1. Heavy water is widely used as moderator in nuclear reactors as it can lower the energies of fast neutrons
2. It is commonly used as a tracer to study organic reaction mechanisms and mechanism of metabolic reactions
3. It is also used as a coolant in nuclear reactors as it absorbs the heat generated

The heavy water produced is used as a moderator of neutrons in nuclear power plants. In the laboratory heavy water is employed as an isotopic tracer in studies of chemical and biochemical processes.

Heavy water is a form of water with a unique atomic structure and properties coveted for the production of nuclear power and weapons. Like ordinary water-H₂O-each molecule of heavy water contains two hydrogen atoms and one oxygen atom. The difference, though, lies in the hydrogen atoms.

The hydrogen is then liquefied and distilled to separate the two components, then the deuterium is reacted with oxygen to form heavy water. Producing heavy water requires advanced infrastructure, and heavy water is actively produced in Argentina, Canada, India, and Norway.

The heavy water is not manufactured, but rather it is extracted from the quantity that is found naturally in lake water. The water is separated through a series of towers, using hydrogen sulphide as an agent.

It was accepted by Norsk Hydro, and production began in 1935. The technology is straightforward. Heavy water (D₂O) is separated from normal water by electrolysis, because the difference in mass between the two hydrogen isotopes translates into a slight difference in the speed at which the reaction proceeds.

Heavy water is indeed heavier than normal water (which contains a tiny amount of heavy water molecules naturally), and heavy-water ice will sink in normal water.

Since the chemical properties of the heavier hydrogen-nucleus-with-a-neutron are slightly different, heavy water starts to gum up all manner of body parts. Eventually, if you drank enough purified heavy water-more than 20 gallons, at least a quarter heavy-you’d die.

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Compounds of Hydrogen

Water

Water is one of the most abundant compounds of hydrogen and our earth’s surface contains approximately 70 % of ocean which is the major source of water. However, sea water contains many dissolved salts hence it can not be used directly. Water is essential for all living things and our body contains about 65% water.

Physical Properties:

Water is a colourless and volatile liquid. The peculiar properties of water in the condensed phases are due to the presence of inter molecular hydrogen bonding between water molecules. Hydrogen bonding is responsible for the high melting and boiling points of water. Some of the physical parameters of water are listed in Table 4.2.

‘Unless otherwise stated, all data are at 298 K.

Chemical Properties:

Water reacts with metals, non-metals and other compounds differently. The most reactive metals are the alkali metals. They decompose water even in cold with the evolution of hydrogen leaving an alkali solution.

2Na + 2 H₂O → 2 NaOH + H₂

The group 2 metals (except beryllium) react in a similar way but less violently. The hydroxides are less soluble than those of Group 1.

Ba + 2H₂O → Ba(OH)₂+ H₂

Some transition metals react with hot water or steam to form the corresponding oxides. For example, steam passed over red hot iron results in the formation of iron oxide with the release of hydrogen.

3Fe + 4H₂O → Fe₃O₄ + 4H₂

Lead and copper decompose water only at a white heat. Silver, gold, mercury and platinum do not have any effect on water. In the elemental form, the non-metals such as carbon, sulphur and phosphorus normally do not react with water. However, as we have seen earlier, carbon will react with steam when it is red (or white) hot to give water gas.

On the other hand, the halogens react with water to give an acidic solution. For example, chlorine forms hydrochloric acid and hypo chlorous acid. It is responsible for the antibacterial action of chlorine water, and for its use as bleach.

Cl₂ + H₂O → HCl + HOCl

Fluorine reacts dif erently to liberate oxygen from water.

2F₂ + 2 H₂O → 4HF + O₂

In a similar way, compounds of nonmetals react with water to give acidic or alkaline solutions. For example, solutions of carbonates are slightly alkaline.

CO₃^2- + H₂O → HCO₃^– + OH^–

Water is an amphoteric oxide. It has the ability to accept as well as donate protons and hence it can act as an acid or a base. For example, in the reaction with HCl it accepts proton where as in the reaction with weak base ammonia it donates proton.

NH₃ + H₂O → NH₄⁺ + OH^–
HCl + H₂O → H₃O⁺ + Cl^–

Water dissolves ionic compounds. In addition, it also hydrolyses some covalent compounds.

SiCl₄ + 2 H₂O → SiO₂ + 4 HCl
P₄O₁₀ + 6 H₂O → 4 H₃PO₄

Many salts crystallized from aqueous solutions form hydrated crystals. The water in the hydrated salts may form co-ordinate bond or just present in interstitial positions of crystals.

Examples:

[Cr(H₂O)₆]Cl₃ – All six water molecules form co-ordinate bond

BaCl₂. 2H₂O – Both the water molecules are present in interstitial positions.

CuSO₄.5H₂O – In this compound four water molecules form co-ordinate bonds while the fifth water molecule, present outside the co-ordination, can form intermolecular hydrogen bond with another molecule. [Cu(H₂O)₄)]SO₄.2H₂O

Hard and Soft Water:

Hard water contains high amounts of mineral ions. The most common ions found in hard water are the soluble metal cations such as magnesium & calcium, though iron, aluminium, and manganese may also be found in certain areas.

Presence of these metal salts in the form of bicarbonate, chloride and sulphate in water makes water ‘hard’. When hard water is boiled carbonates of magnesium and calcium present in it gets precipitated. On the other hand, water free from soluble salts of calcium and magnesium is called sof water. The hardness of water is of two types, viz., temporary hardness and permanent hardness.

Temporary Hardness and its Removal:

Temporary hardness is primarily due to the presence of soluble bicarbonates of magnesium and calcium. This can be removed by boiling the hard water followed by filtration. Upon boiling, these salts decompose into insoluble carbonate which leads to their precipitation. The magnesium carbonate thus formed further hydrolysed to give insoluble magnesium hydroxide.

Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂
Mg(HCO₃)₂ → MgCO₃ + H₂O + CO₂
MgCO₃ + H₂O → Mg(OH)₂ + CO₂

The resulting precipitates can be removed by filtration.

Alternatively, we can use Clark’s method in which, calculated amount of lime is added to hard water containing the magnesium and calcium, and the resulting carbonates and hydroxides can be filtered – off

Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃ + 2H₂O
Mg (HCO₃)₂ + 2 Ca(OH)₂
↓
2CaCO₃ + Mg(OH)₂ +2 H₂O

Permanent Hardness:

Permanent hardness of water is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in it. It can be removed by adding washing soda, which reacts with these metal (M = Ca or Mg) chlorides and sulphates in hard water to form insoluble carbonates.

MCl₂ + Na₂CO₃ → MCO₃ + 2 NaCl
MSO₄ + Na₂CO₃ → MCO₃ + Na₂SO₄

In another way to soften the hard water is by using a process called ionexchange. That is, hardness can be removed by passing through an ion-exchange bed like zeolites or column containing ionexchange resin. Zeolites are hydrated sodium alumino-silicates with a general formula,
Na₂O∙Al₂O₃.xSiO₂.yH₂O (x = 2 to 10, y = 2 to 6).

Zeolites have porous structure in which the monovalent sodium ions are loosely held and can be exchanged with hardness producing metal ions (M = Ca²⁺ or Mg²⁺) in water. The complex structure can conveniently be represented as Na₂ – Z with sodium as exchangeable cations.

Na₂ – Z + M²⁺ → M-Z + 2 Na²⁺

When exhausted, the materials can be regenerated by treating with aqueous sodium chloride. The metal ions (Ca²⁺ and Mg²⁺) caught in the zeolite (or resin) are released and they get replenished with sodium ions.

M-Z + 2NaCl → Na₂-Z + MCl₂

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Uses of Hydrogen

1. Over 90 % hydrogen produced in industry is used for synthetic applications. One such process is Haber process which is used to synthesis ammonia in large scales. Ammonia is used for the manufacture of chemicals such as nitric acid, fertilizers and explosives.

2. It can be used to manufacture the industrial solvent, methanol from carbon monoxide using copper as catalyst.

3. Unsaturated fatty oils can be converted into saturated fats called Vanaspati (margarine) by the reduction reaction with Pt/H₂.

4. In metallurgy, hydrogen can be used to reduce many metal oxides to metals at high temperatures.

CuO + H₂ → Cu + H₂O
WO₃ + 3H₂ → W + 3H₂O

5. Atomic hydrogen and oxy-hydrogen torches are used for cutting and welding.

6. Liquid hydrogen is used as a rocket fuel.

7. Hydrogen is also used in fuel cells for generating electrical energy. The reversible uptake of hydrogen in metals is also attractive for rechargeable metal hydride battery.

1. Hydrogen is used in the synthesis of ammonia and the manufacture of nitrogenous fertilizers.
2. Hydrogenation of unsaturated vegetable oils for manufacturing vanaspati fat.
3. It is used in the manufacture of many organic compounds, for example, methanol.

It is also used to make epoxyethane (ethylene oxide), used as antifreeze and to make polyester, and chloroethene, the precursor to PVC. Oxygen gas is used for oxy-acetylene welding and cutting of metals. A growing use is in the treatment of sewage and of effluent from industry.

Hydrogen use today is dominated by industry, namely: oil refining, ammonia production, methanol production and steel production. Virtually all of this hydrogen is supplied using fossil fuels, so there is significant potential for emissions reductions from clean hydrogen.

Use of hydrogen. Nearly all of the hydrogen consumed in the United States is used by industry for refining petroleum, treating metals, producing fertilizer, and processing foods.

• Rocket fuel is a major use of hydrogen for energy.
• Hydrogen fuel cells produce electricity.
• Hydrogen use in vehicles.
• The refueling challenge.

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Physical Properties of Hydrogen

Physical Properties:

Hydrogen is a colorless, odorless, tasteless, lightest and highly flammable gas. It is a non-polar diatomic molecule. It can be liquefied under low temperature and high pressure. Hydrogen is a good reducing agent. Various physical constants of hydrogen molecule are listed in Table 4.1.

Chemical Properties:

Hydrogen reacts with oxygen to give water. This is an explosive reaction and releases lot of energy. This is used in fuel cells to generate electricity.

2H₂ + O₂ → 2 H₂O

Similarly, hydrogen also reacts with halogens to give corresponding halides. Reaction with fluorine takes place even in dark with explosive violence while with chlorine at room temperature under light. It combines with bromine on heating and reaction with iodine is a photochemical reaction.

H₂ + X₂ → 2 HX (X = F, Cl, Br & I)

In the above reactions the hydrogen has an oxidation state of +1. It also has a tendency to react with reactive metals such as lithium, sodium and calcium to give corresponding hydrides in which the oxidation state of hydrogen is -1.

2 Li + H₂ → 2 LiH
2 Na + H₂ → 2 NaH

These hydrides are used as reducing agents in synthetic organic chemistry. It is used to prepare other important hydrides such as lithium aluminium hydride and sodium boro hydride.

4 LiH + AlCl₃ → Li[AlH₄] + 3 LiCl
4 NaH + B(OCH₃) → Na[BH₄] + 3 CH₃ONa

Hydrogen itself acts as a reducing agent. In the presence of finely divided nickel, it adds to the unsaturated organic compounds to form saturated compounds.

Chemical properties of Deuterium

Like hydrogen, deuterium also reacts with oxygen to form deuterium oxide called heavy water. It also reacts with halogen to give corresponding halides.

2 D₂ + O₂ → 2 D₂O
D₂ + X₂ → 2 DX
(X = F, Cl, Br & I)

Deuterium Exchange Reactions:

Deuterium can replace reversibly hydrogen in compounds either partially or completely depending upon the reaction conditions. These reactions occur in the presence of deuterium or heavy water.

CH₄ + 2 D₂ → CD₄ + 2 H₂
2 NH₃ + 3D₂ → 2 ND₃ + 3 H₂

Properties of Tritium

It is a β-emitter with a half-life period of 12.3 years.

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Preparation of Hydrogen

High purity hydrogen (>99.9 %) is obtained by the electrolysis of water containing traces of acid or alkali or the electrolysis of aqueous solution of sodium hydroxide or potassium hydroxide using a nickel anode and iron cathode. However, this process is not economical for large-scale production.

At Anode:

2 OH^– → H₂O + ½ O₂ + 2e^–

At Cathode:

2 H₂O + 2 e^– → 2 OH^– + H₂

Overall Reaction:

H₂O → H₂ + ½ O₂

Laboratory Preparation

Hydrogen is conveniently prepared in laboratory by the reaction of metals, such as zinc, iron, tin with dilute acid.

Industrial Production

In the large-scale, hydrogen is produced by steam-reforming of hydrocarbons. In this method hydrocarbon such as methane is mixed with steam and passed over nickel catalyst in the range 800-900 °C and 35 atm pressures.

In an another process, steam is passed over a red-hot coke to produce carbon monoxide and hydrogen. The mixture of gases produced in this way is known as water gas (CO+H₂). This is also called syngas (Synthetic gas) as it is used in the synthesis of organic compounds such as methanol and simple hydrocarbons.

Conversion of Carbon Monoxide in Water gas to Carbon Dioxide:

The carbon monoxide of the water gas can be converted to carbon dioxide by mixing the gas mixture with more steam at 400°C and passed over a shift converter containing iron/copper catalyst. This reaction is called as water-gas shift reaction.

CO + H₂ → CO₂ + H₂

The CO₂ formed in the above process is absorbed in a solution of potassium carbonate.

CO₂ + K₂CO₃ + H₂O → 2KHCO₃

Preparation of Deuterium:

Electrolysis of Heavy Water:

Normal water contains 1.6 × 10^-4 percentage of heavy water. The dissociation of protium water (H₂O) is more than heavy water (D₂O). Therefore, when water is electrolysed, hydrogen is liberated much faster than D₂. The electrolysis is continued until the resulting solution becomes enriched in heavy water. Further electrolysis of the heavy water gives deuterium.

Preparation of Tritium:

As explained earlier the tritium is present only in trace amounts. So it can be artificially prepared by bombarding lithium with slow neutrons in a nuclear fission reactor. The nuclear transmutation reaction for this process is as follows.

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Position in Periodic Table

The hydrogen has the electronic configuration of 1s¹ which resembles with ns¹ general valence shell configuration of alkali metals and shows similarity with them as follows:

1. It forms unipositive ion (H⁺) like alkali metals (Na⁺, K⁺, Cs⁺)
2. It forms halides (HX), oxides (H₂O), peroxides (H₂O₂) and sulphides (H₂S) like alkali metals (NaX, Na₂O, Na₂O₂, Na₂S)
3. It also acts as a reducing agent.

However, unlike alkali metals which have ionization energy ranging from 377 to 520 kJ mol^-1, the hydrogen has 1, 314 KJ mol^-1 which is much higher than alkali metals. Like the formation of halides (X^–) from halogens, hydrogen also has a tendency to gain one electron to form hydride ion (H^–) whose electronic configuration is similar to the noble gas, helium. However, the electron affinity of hydrogen is much less than that of halogen atoms. Hence, the tendency of hydrogen to form hydride ion is low compared to that of halogens to form the halide ions as evident from the following reactions:

½ H₂ + e^– → H^– ΔH = + 36 kcal mol^-1
½ Br₂ + e^– → Br^– ΔH = – 55 kcal mol^-1

Since, hydrogen has similarities with alkali metals as well as the halogens; it is difficult to f nd the right position in the periodic table. However, in most of its compounds hydrogen exists in +1 oxidation state. Therefore, it is reasonable to place the hydrogen in group 1 along with alkali metals as shown in the latest periodic table published by IUPAC.

Isotopes of Hydrogen

Hydrogen has three naturally occurring isotopes, viz., protium (₁H¹ or H), deuterium (₁H² or D) and tritium (₁H³ or T). Protium (₁H¹) is the predominant form (99.985 %) and it is the only isotope that does not contain a neutron.

Deuterium, also known as heavy hydrogen, constitutes about 0.015 %. The third isotope, tritium is a radioactive isotope of hydrogen which occurs only in traces (~1 atom per 10¹⁸ hydrogen atoms). Due to the existence of these isotopes naturally occurring hydrogen exists as H₂, HD, D₂, HT, T₂ and DT. The properties of these isotopes are shown in Table 4.1.

Ortho and Para-Hydrogen:

In the hydrogen atom, the nucleus has a spin. When molecular hydrogen is formed, the spins of two hydrogen nuclei can be in the same direction or in the opposite direction as shown in the figure. These two forms of hydrogen molecules are called ortho and para hydrogens respectively.

At room temperature, normal hydrogen consists of about 75% ortho-form and 25% paraform. As the ortho-form is more stable than para-form, the conversion of one isomer into the other is a slow process. However, the equilibrium shift in favour of para hydrogen when the temperature is lowered.

The para-form can be catalytically transformed into ortho-form using platinum or iron. Alternatively, it can also be converted by passing an electric discharge, heating above 800°C and mixing with paramagnetic molecules such as O₂, NO, NO₂ or with nascent/atomic hydrogen.

Ortho and para hydrogen are similar in chemical properties but differ in some of the physical properties. For example, the melting point of para hydrogen is 13.83 K while that of ortho hydrogen 13.95 K; boiling point of para hydrogen is 20.26 K while that of ortho hydrogen 20.39 K. Since the nuclear spins are in opposite directions the magnetic moment of para hydrogen is zero and ortho hydrogen has magnetic moment twice that of a proton.