Hydrogen

In the modern periodic table, Hydrogen is the first element and also it has only one valence electron in its 1s subshell. Therefore hydrogen exhibits properties of both the alkali metals and the halogens. Hence the position of Hydrogen is not fixed in the periodic table it is placed on the top of Alkali metals and also on the top of the halogens. If hydrogen is considered to be a halogen it requires only one electron to acquire the stable noble gas configuration of helium and if it is considered to be the Alkali metal then hydrogen can lose the only valence electron to form hydrogen ion. The dual nature of hydrogen can be justified by following points:

1. As stated earlier, hydrogen like other alkali metal lose the only electron to form unipositive ion.

3. When hydrogen is present in compound state like HCl it exhibits +1 oxidation state like other alkali metals.

4. Hydrogen has strong affinity towards electronegative elements and combine with them to form compounds.

Resemblance with halogens:

1. Like other halogen elements hydrogen requires only one electron to complete its octet and to achieve stable noble gas configuration.

2. Hydrogen exists as diatomic molecule and also form many covalent bonds to form covalent compounds.

Though hydrogen exhibits properties of both alkali metals and halogens, it does not resemble any of the groups completely. For instance hydrogen does not exhibit any metallic characteristics like other alkali metals and it is also less reactive as compared to halogens. Hence, hydrogen is best placed separately in the periodic table.

Hydrogen has three isotopes:

Protium = ₁H¹

Deutrium = ₁H²

Tritium = ₁H³

Therefore the mass ratio of these isotopes are as follows:

Thus Mass Ratio is 1:2:3

Hydrogen cannot exists as monoatomic molecule because in monoatomic state hydrogen atom is unstable. Hydrogen atom contains only one electron and to achieve the stable noble gas configuration as that of Helium two hydrogen atoms share one electron with each other to form a diatomic molecule and hence hydrogen exists as a diatomic molecule in Free State or under normal conditions.

The process of producing ‘syn-gas’ from coal is called ‘coal gasification’. The mixture of CO and H₂O is called SYNGAS.

The production of dihydrogen can be increased by reacting CO of syngas mixtures with steam in the presence of iron chromate as catalyst.

Electrolysis of water is the best way to generate hydrogen and when an electric current is passed through water, hydrogen is obtained at cathode and oxygen is obtained at anode. Pure water is a non-electrolyte and hence salt is added to water to increase its conductivity. Platinum is used as the electrode material because if any other material is used at anode, then the anode would be consumed during the electrolysis process that is the oxygen obtained at anode would react with anode metal to form some compound.

The reactions taking place at the cathode and anode is as given below.

Cathode (reduction): 2H₂O (l) + 2e⁻→ H₂ (g) + 2OH⁻

Anode (oxidation): 4OH⁻→ O₂ (g) + 2H₂O (l) + 4e⁻

If we combine both the reactions, the overall reaction can be represented as given below.

Net reaction: 2H₂O (l) → 2H₂ (g) + O₂ (g)

(i) mM + H₂O

(ii) CH₃OH

(iii) 3CO + 7H₂

(iv) Na₂ZnO₂ + H₂

Since hydrogen exists in nature as a diatomic molecule, it is highly inert in nature as this configuration of hydrogen is highly stable. Hydrogen in its diatomic form achieves the stable noble gas configuration as that of Helium. The bond dissociation enthalpy of hydrogen bond is 435-88 kj/mol making the dihydrogen molecule is quite stable.

Even when hydrogen atom is heated up to very high temperatures hydrogen bonds does not dissociate and making it unreactive in nature under normal conditions.

1. Electron deficient hydrides are referred to those hydrides which contains deficit of electrons after the hydrogen bonding has been formed that is the octet is partially filled after the bond formation.Examples of such hydrides include the hydrides of Group 13 elements. These hydrides cannot be represented by simple Lewis structures and also they have tendency to accept electrons. 2.Electron precise hydride is referred to those class of hydrides in which the octet is completely filled. For example- CH₄
3. Electron rich hydride is referred to those class of hydrides which contains more electrons then actually needed to from the octet. Such hydride has the tendency to lose the extra electrons to form positive radicals. For example- NH₃

1. Electron deficient hydrides are referred to those hydrides which contains deficit of electrons after the hydrogen bonding has been formed that is the octet is partially filled after the bond formation.2. Examples of such hydrides include the hydrides of Group 13 elements.3. These hydrides cannot be represented by simple Lewis structures and also they have tendency to accept electrons.

4. Since these hydrides have a tendency to accept electrons. Hence, they act as Lewis acids.

For carbon hydrides of type C_nH_2n+2, the following hydrides are possible for

n=1, CH₄

n=2, C₂H₆

The hydride acts as Lewis acid only when the hydride is deficit of electrons and then only it will have the tendency to accept electrons.

Taking C₂H₆ as an example,

the total number of electrons are 14 and the total covalent bonds are seven. Hence, the bonds are regular 2e^- -2 centred bonds.

Hence, hydride C₂H₆ has sufficient electrons to be represented by a conventional Lewis structure. Therefore, Ethane is an electron-precise hydride, having all atoms with complete octets. Thus, it can neither donate nor accept electrons to act as a Lewis acid or Lewis base.

1. The hydrides in which the ratio of the metal and hydrogen is found to be fractional are called non-stoichiometric hydrides. It is also observed that this fractional ratio is not fixed but varies with the temperature and pressure.

2. Non-stoichiometric hydrides is generally formed by d and f block elements as in these hydrides, the hydrogen atoms occupy some of the holes in the lattice of the metal and also all the holes are not occupied.

3. Alkali Metal react with hydrogen to form an ionic compound that is alkali metal form ionic bonds with hydrogen and alkali metal donate electron to hydrogen atom and hydrogen thus exists as electronegative element in the hydride.

1. Metallic hydrides prove as a strong source of hydrogen as hydrogen is adsorbed on the surface of these metals and when these hydride is dissociated it gives away the trapped hydrogen.

2. Metals like Platinum, Palladium etc has the capability to trap very large quantity of hydrogen and this trapped hydrogen can be used later as source of hydrogen for energy purposes or reaction purposes.

The main use of atomic hydrogen and oxy-hydrogen flame or torch is for welding and cutting purposes.

Atomic hydrogen atoms which are produced by dissociation with the help of an electric arc are allowed to recombine on the surface to be cut/welded to generate temperature of 4000 K.

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1. Hydrogen fluoride molecule has only straight chain of bonding whereas on the other hand water molecule which consists of two hydrogen and one oxygen atom forms a huge ring like structure.2. The hydrogen bonding in ammonia is limited because nitrogen atom possess only one lone pair of electrons. Hence, H₂O has the strongest hydrogen bonded structure.

3. The strength of hydrogen bonding also depend upon the electronegativity of the element and it also depend upon the number of hydrogen atoms which are available for bonding. Among nitrogen, fluorine, and oxygen, the increasing order of their electro negativities are N < O < F.

4. Hence, the expected order of the extent of hydrogen bonding is H₂O > HF > NH₃. But, the actual order is H₂O > HF > NH₃.

5. Though the electronegativity of fluorine is greater as compared to that of oxygen, the strength of bonding in water molecule is greater as compared to the bonding in HF molecule because the HF molecule has shortage of hydrogen atoms. Hence water has more number of hydrogen atoms and thus forms a ring like structure and HF only forms a planar structure.

6. In case of ammonia, the extent of hydrogen bonding is limited because nitrogen has only one lone pair. Therefore, it cannot satisfy all hydrogens.

1. Saline hydrides basically refer to the metal hydrides like NaH, LiH etc and such hydrides when come in contact with water they react violently with water to form base and liberate hydrogen gas.

2. The generalized reaction is as follows: MH + H₂O → MOH + H₂

3. The reaction is very violent and has the tendency to catch fire.

4. CO₂ is heavier than dioxygen and the mains reason for its use in fire extinguisher is its ability to form a blanket over the fire which limits the supply of oxygen and hence the fire extinguishes.

5. In the present case also carbon dioxide is heavier than hydrogen and it can be used to extingiush the fire.

The property of electrical conductivity is dependent upon the nature of the molecule. Ionic compounds conduct whereas the covalent compounds do not conduct.

BeH₂ is a covalent compound and hence do not exhibit conductivity and on the other hand CaH₂ is an ionic hydride because of which it conducts in molten or aqueous state. TiH₂ is totally metallic in nature and hence it conducts under normal operating conditions.

Hence, the increasing order of electrical conductance is as follows:

BeH₂ < CaH₂ < TiH_2.

Ionic character of compound depend upon the electronegativity of the atoms present in the compound. Higher the difference between electronegativity of atoms, smaller is the ionic character.

On moving down the group, the electronegativity decreases and hence, the ionic character of their hydrides will increase as shown below.

LiH < NaH < CsH

The factors on which the bond dissociation enthalpy depend upon is the strength of the bond and the nature of attractive and repulsive forces present in the molecule.

The bonding in D-D bond is strongly attracted by the nucleus of the atoms and thereby the increasing the strength of the bond which in turn increases the bond dissociation enthalpy of the bond. The D-D bond is stronger than the H-H bond because the molecular weight of D is greater as compared to H.

In case of the F-F bond the bond dissociation enthalpy would be minimum because the bond in F-F experiences a strong repulsion from the lone pair of electrons thereby weakening of the bond.

Therefore, the increasing order of bond dissociation enthalpy is as follows:

F-F < H-H < D-D

The reducing property of hydrides is dependent upon the nature of hydride molecule that is whether the hydride is ionic or covalent in nature.

Ionic hydrides exhibit strong reducing nature as they have the tendency to lose electrons whereas covalent hydrides do not show reducing property.

Both, MgH₂ and H₂O are covalent hydrides. H₂O is less reducing than MgH₂ since the bond dissociation energy of H₂O is higher than MgH₂.

Hence, the increasing order of the reducing property is

H₂O < MgH₂ < NaH

1. In the water vapour, the molecule of water has a bent shape with a bond angle of approximately 104.5° and the bond length of OH bond is 95.7 pm. The structure can be shown as:

2. In gas as well as the solid state hydrogen peroxide possess a non-planar structure and the dihedral angle in the gas and solid state is 111.5° and 90.2° respectively. The structure can be shown as:

1. Auto-protolysis or self-ionization reaction of water molecule is defined as that chemical reaction in which two molecules of water react with each other to form hydroxyl ion and hydronium ion.

2. The reaction involved can be represented as:

3. The auto-protolysis reaction of water molecule exhibit the amphoteric nature of water molecule that is the molecule of water can acts as an acid as well as a base.

4. The acid-base reaction can be written as:

1. The reaction of fluorine with water can be represented as given below:

2Fe_2(g) + 2H₂O_(l)→ 4H⁺_(aq) + O_2(g)

2. The above reaction is an example of redox reaction in which water is being oxidized and fluorine is being reduced.

3. The oxidation numbers of various species can be represented as:

4. Fluorine is reduced from zero to (–1) oxidation state. A decrease in oxidation state indicates the reduction of fluorine.

5. Water is oxidized from (- 2) to zero oxidation state. An increase in oxidation state indicates oxidation of water.

(i) In the given reaction H₂O₂ is oxidizing agent and hence the reaction is a redox reaction.

(ii) In the given reaction H₂O₂ is reducing agent and hence the reaction is a redox reaction.

(iii) The reactions in which a compound reacts with water to produce other compounds are called hydrolysis reactions. So the given reaction is hydrolysis.

(iv) The reactions in which a compound reacts with water to produce other compounds are called hydrolysis reactions. So the given reaction represents hydrolysis of AlCl₃.

(v) The reactions in which a compound reacts with water to produce other compounds are called hydrolysis reactions. The given reaction represents hydrolysis of Ca₃ N₂.

1. The solid state of the water molecule or rather the crystalline form of water molecule.

2. The arrangement of atoms in ice molecule is dependent upon the temperature at which the water crystallizes into ice. The structure of ice hexagonal form if water crystallizes at normal pressure and the structure of ice is cubic in nature if water crystallizes at very low temperatures.

3. The three-dimensional structure of ice is represented as:

4. The structure is highly ordered and has hydrogen bonding. Each oxygen atom is surrounded tetrahedral by four other oxygen atoms at a distance of 276 pm. The structure also contains wide holes that can hold molecules of appropriate sizes interstitially.

1. When salts [generally carbonates, bicarbonates, chlorides etc] of alkaline earth metals get dissolved in water then such water is termed as hard water.

2. Temporary hardness of water is due to the presence of soluble salts of magnesium and calcium in the form of hydrogen carbonates (MHCO₃, where M = Mg, Ca) in water.

3. Permanent hardness of water is because of the presence of insoluble salts of calcium and magnesium in the form of chlorides in water.

Permanent hardness of water can be removed by treating hard water with synthetic resins which involve the exchange of cations (e.g., Na⁺, Ca⁺², Mg⁺² etc) and anions (e.g., Cl-, SO₄^2-, HCO₃^- etc) present in water by H⁺ and OH^- ions respectively.

Synthetic resins are of two types:

1) Cation exchange resins:

Cation exchange resins are large organic molecules that contain the –SO₃H group. The resin is firstly changed to RNa (from RSO3H) by treating it with NaCl. This resin then exchanges sodium ions with calcium and magnesium ions, thereby making the water soft.

2RNa + M²⁺_(aq)→ R₂M_(s) + 2Na⁺_(aq)

There are cation exchange resins in H⁺ form. The resins exchange hydrogen ions for sodium, calcium, magnesium ions.

2) Anion exchange resins

Anion exchange resins exchange hydroxyl ions for anions like chloride, bicarbonate, and sulphate present in water.

During the hardness treatment process, water is first passed through the cation exchange process. The water obtained after this process is free from mineral cations and is acidic in nature.

This acidic water is further passed through the anion exchange process where hydroxyl ions neutralize the H⁺ ions and de-ionize the water obtained.

The amphoteric nature of water can be described on the basis of the following reactions:

1) Reaction withH₂S

The reaction takes place as:

In the forward reaction, water molecule accepts a proton from hydrogen sulphide and hence water acts as a Lewis base.

2) Reaction withNH₃

The reaction takes place as:

In the forward reaction, water donates its proton to ammonia molecule and hence in this case water acts as a Lewis acid.

3) Self-ionization of water

The auto-protolysis reaction of water molecule exhibit the amphoteric nature of water molecule that is the molecule of water can acts as an acid as well as a base.

The acid-base reaction can be written as:

Thus the above mentioned reactions depict the amphoteric nature of water molecule.

Hydrogen peroxide, H₂O₂ can act as an oxidizing as well as a reducing agent in both acidic and alkaline media.

Reactions involving oxidizing actions are:

(i) PbS + H₂O₂→ PbSO₄ + H₂O

(ii) 2F⁺² + H₂O₂→ 2F⁺³ + 2OH^-

Reactions involving reduction actions are:

(i) MnO₄^- + H₂O₂→ 2Mn⁺² + 8H₂O + 5O₂

(ii) HOCl + H₂O₂→ H3O⁺ + Cl^- + O₂

1. Demineralized water is that water which do not contain any dissolved cations, anions and salts in it. Demineralized Water can be obtained by passing the water through cation and anionic exchangers.

2. During the hardness treatment process, water is first passed through the cation exchange process. The water obtained after this process is free from mineral cations and is acidic in nature.

3. This acidic water is further passed through the anion exchange process where hydroxyl ions neutralize the H⁺ ions and de-ionize the water obtained.

Water is the most important and vital element of life. The water which is suitable for human consumption should contain some dissolved ions and minerals which are important for life. Demineralized or distilled water is not at all useful for drinking purposes and to make it fit for human consumption certain minerals need to be dissolved in water.

1. Water is essential for all forms of life. It constitutes around 65% of the human body and 95% of plants. Water plays an important role in the biosphere owing to its high specific heat, thermal conductivity, surface tension, dipole moment, and dielectric constant. Water is the only source of life to all the living organisms on earth and also to all the marine animals.

2. The high heat of vaporization and heat of capacity of water helps in moderating the climate and body temperature of all living beings. Water is the only source of life to all the living organisms on earth and also to all the marine animals.

3. It acts as a carrier of various nutrients required by plants and animals for various metabolic reactions.

A high value of dielectric constants (78.39 C²/Nm²) and dipole moment and water is also called as universal solvent as it has the ability to dissolve almost every compound in it.

Water is able to dissolve most ionic and covalent compounds. Ionic compounds dissolve in and dissolve in water.

Water can hydrolyse metallic and non-metallic oxides, hydrides, carbides, phosphides, nitrides and various other salts. During hydrolysis, H⁺ and OH^- ions of water interact with the reacting molecule.

Some reactions are:

(i) CaO + H₂O → Ca (OH) ₂

(ii) AlCl₃ + H₂O → Al (OH) ₃ + 3HCl

Heavy Water or D₂O is used as coolant in the nuclear reactors and it has the ability to slow down the rate of a chemical reaction. With consideration of this property of heavy water, it is clearly understood that it is not fit for human consumption as it can also slow the metabolic reactions taking place in the body. Hence heavy water is not fir for human consumption.

Hydrolysis is defined as a chemical reaction in which hydrogen and hydroxide ions (H⁺ and OH^- ions) of water molecule react with a compound to form products. For example:

CaO + H₂O → Ca (OH) ₂

Hydration is defined as the addition of one or more water molecules to ions or molecules to form hydrated compounds. For example:

CuSO₄ + 5H₂O → CuSO₄.5H₂O

Saline hydrides basically refer to the metal hydrides like NaH, LiH etc and such hydrides when come in contact with water they react violently with water to form base and liberate hydrogen gas and these hydrides are ionic in nature. The reaction of saline hydrides with water can be represented as:

AH_(s) + H₂O_(l)→ AOH_(aq) + H_2(g)

(where, A = Na, Ca,……)

When added to an organic solvent, they react with water present in it. Hydrogen escapes into the atmosphere leaving behind the metallic hydroxide. The dry organic solvent distils over.

The elements of atomic numbers 15, 19, 23, and 44 are nitrogen, potassium, vanadium, and ruthenium respectively.

1) Hydride of nitrogen:

Hydride of nitrogen (NH₃) is a covalent molecule. This hydride molecule contains excess of electrons and hence it is electron rich hydride molecule.

2) Hydride of potassium:

This hydride is ionic in nature because of the strong electropositive nature of potassium ion and hence it is crystalline in nature.

3) Hydrides of Vanadium and Ruthenium:

d and f-block elements form non-stoichiometric hydrides and vanadium and ruthenium also belong to the d-block of the periodic table. Hydrides of vanadium and ruthenium are therefore, metallic in nature having a deficiency of hydrogen.

4) Behaviourof hydrides towards water:

Potassium hydride reacts violently with water and the reaction is given as below:

KH + H₂O → KOH + H₂

Ammonia (NH₃) behaves as a Lewis base and the reaction of ammonia with water is given as below:

NH₃ + H₂O ß→ OH^- + NH₄⁺

Vanadium and ruthenium hydride do not react with water.

Hence, the increasing order of reactivity of the hydrides is:

(V, Ru) H < NH₃< KH.

Potassium chloride is the salt of strong acid [HCl] and strong base [KOH] and hence it is neutral in nature and thus when dissolved in water it dissociate to give potassium and chloride ions. The reaction is given as below:

KCl → K⁺ + Cl^-

In acidified and alkaline water, the ions do not react and remain as such.

Aluminium chloride is formed due to the reaction of a strong acid [HCl] with a weak base [Al (OH)₃]. Hence, it undergoes hydrolysis in normal water.

AlCl₃ + 3H₂O → Al (OH)₃ + 3H⁺ + 3Cl^-

In acidified water, H⁺ ions react with Al (OH)₃ forming water and giving Al³⁺ ions. Hence, in acidified water, AlCl₃ will exist as Al³⁺ and Cl^-ions.

In alkaline water, the following reaction takes place:

H₂O₂ or hydrogen peroxide acts as a strong oxidizing and reducing agent both in acidic and basic media. When added to a cloth, it breaks the chemical bonds of the chromophores (colour producing agents). Hydrogen Peroxide oxidises coloured vegetable and organic materials into colourless materials. Hence, the visible light is not absorbed and the cloth gets whitened.

(i) Hydrogen economy:

It refers to the techniques of efficient usage of dihydrogen. It also involves the transportation and storage of hydrogen in the form of liquid or gas.

Hydrogen has a very high calorific value as compared to petrol and also it is the most eco-friendly fuel as it burns to give water vapour and hence it can be used in fuel cells to produce electricity. Hydrogen economy is about the transmission of this energy in the form of dihydrogen.

(ii) Hydrogenation:

The process of addition of hydrogen to a given reactant in the presence of catalyst is termed as hydrogen. Hydrogenation is also used to obtain alkanes from alkenes and alkynes. This process is used to reduce a compound in the presence of a suitable catalyst. For example, hydrogenation of vegetable oil using nickel as a catalyst gives edible fats such as vanaspati, ghee etc.

(iii) Syngas

The mixture of carbon monoxide and hydrogen is termed as syngas. Since the mixture of the two gases is used for the synthesis of methanol, it is called syngas, synthesis gas, or water gas.

Syngas is produced on the action of steam with hydrocarbons or coke at a high temperature in the presence of a catalyst.

(iv) Water shift reaction:

It is a reaction of carbon monoxide of syngas mixture with steam in the presence of a catalyst. This reaction is used to increase the yield of dihydrogen obtained from the coal gasification reaction.

(v) Fuel cells:

Fuel cells are devices for producing electricity from fuel in the presence of an electrolyte. Hydrogen is considered to be one of the best fuel for fuel cells as it is eco-friendly and also gives more energy per unit mass of fuel as compared to other substitutes like petrol or diesel etc.