Chapter: Hydrogen 1. Hydrogen is the lightest element known since it has an atomic mass of 1.0079 2.

Hydrogen resembles Group 1 elements because it has electronic configuration (1s 1) similar to alkali metals and can lose one electron to form unipositive ions 3. Hydrogen also resembles elements of group 17 ns2 np5 configuration (Halogens) because hydrogen just like halogens needs just one electron to acquire the configuration of the nearest noble gas i.e. Helium 4. Hydrogen has three isotopes - Protium, deuterium and tritium

5. In elemental form Hydrogen exists as a diatomic molecule H 2 and is called dihydrogen. Q. Justify the position of hydrogen in the periodic table on the basis of its electronic configuration. Resemblance with alkali metals: 1. Like alkali metals, hydrogen contains one valence electron in its valency shell. H : 1s1 Li : [He] 2s1 Hence, it can lose one electron to form a unipositive ion. 2. Like alkali metals, hydrogen combines with electronegative elements to form oxides, halides, and sulphides. Resemblance with halogens: 1. Both hydrogen and halogens require one electron to complete their octets. H : 1s1 F : 1s2 2s2 2p5 Cl : 1s2 2s2 2p6 3s2 3p5 Hence, hydrogen can gain one electron to form a uninegative ion. 2. Like halogens, it forms a diatomic molecule and several covalent compounds. 3. In fact, in terms of ionization enthalpy, hydrogen resembles more with halogens. 4. Like halogens, it forms a diatomic molecule, combines with elements to form hydrides and a large number of covalent compounds. Though hydrogen shows some similarity with both alkali metals and halogens, it differs from them on some grounds. Unlike alkali metals, hydrogen does not possess metallic characteristics. On the other hand, it possesses a high ionization enthalpy. Also, it is less reactive than halogens. Owing to these reasons, hydrogen cannot be placed with alkali metals (group I) or with halogens. In addition, it was also established that H+ ions cannot exist freely as they are extremely small. H+ ions are always associated with other atoms or molecules. Hence, hydrogen is best placed separately in the periodic table. Q. Explain the laboratory preparation of dihydrogen? (i) It is usually prepared by the reaction of granulated zinc with dilute hydrochloric acid. Zn + 2H+ Zn2+ + H2 (ii) It can also be prepared by the reaction of zinc with aqueous alkali. Zn + 2NaOH Na2ZnO2 + H2 Sodium zincate Q. Explain the commercial production of Dihydrogen. 1. Electrolysis of acidified water using platinum electrodes gives hydrogen.

2. It is obtained as a byproduct in the manufacture of sodium hydroxide and chlorine by the electrolysis of brine solution. 3. Reaction of steam on hydrocarbons or coke at high temperatures in the presence of catalyst yields hydrogen.

Water gas is name given to the mixture of carbon monoxide and hydrogen. Water gas is also called synthesis gas or syngas. The process of producing 'syngas' from coal is called 'coal gasification'

The production of dihydrogen can be increased by reacting carbon monoxide of syngas mixtures with steam in the presence of iron chromate as catalyst. This reaction is called as water gas shift reaction.

Q. Discuss the consequences of high enthalpy of HH bond in terms of chemical reactivity of dihydrogen. Molecular hydrogen is relatively inert at room temperature due to the high HH bond enthalpy. Since ionization enthalpy is very high, hydrogen does not possess metallic characteristics (lustre, ductility, etc.) like metals. Q. How is atomic hydrogen prepared?

Atomic hydrogen is produced at a high temperature by passing an electric arc or ultraviolet radiations through molecular hydrogen. Q. What are the different ways by which hydrogen undergo chemical reactions? Since its orbital is incomplete with 1s1 electronic configuration it accomplishes reactions by (i) loss of the only electron to give H+, (ii) gain of an electron to form H, and (iii) sharing electrons to form a single covalent bond. Q. explain the reaction of dihydrogen reaction with halogens: It reacts with halogen dioxygen, dinitrogen, metals, metal ions and metal oxides. 1. Reaction with halogen: Hydrogen react with halogens, X2 to give hydrogen halides, HX, 2. Reaction with dioxygen: It reacts with dioxygen to form water. The reaction is highly exothermic.

3. Reaction with dinitrogen: With dinitrogen it forms ammonia. This is the method for the manufacture of ammonia by the Haber process.

4. Reactions with metals: With many metals it combines at high a temperature to yield the corresponding hydrides. where M is an alkali metal 5. Reactions with metal ions and metal oxides: It reduces some metal ions in aqueous solution and oxides of metals (less active than iron) into corresponding metals.

H2 + CuO

CU + H2O

6. Reactions with organic compounds: It reacts with many organic compounds in the presence of catalysts to give useful hydrogenated products. For example: (i) Hydrogenation of vegetable oils using nickel as catalyst gives edible fats (margarine and vanaspati ghee) (ii) Hydroformylation of olefins yields aldehydes which further undergo reduction to give alcohols.

Q. Write the uses of dihydrogen. used in the synthesis of ammonia which is used in the manufacture of nitric acid and nitrogenous fertilizers. Dihydrogen is used in the manufacture of vanaspati fat by the hydrogenation of polyunsaturated vegetable oils like soyabean, cotton seeds etc. It is used in the manufacture of organic chemicals, particularly methanol. l It is widely used for the manufacture of metal hydrides. It is used for the preparation of hydrogen chloride, a highly useful chemical. In metallurgical processes, it is used to reduce heavy metal oxides to metals. Atomic hydrogen and oxy-hydrogen torches find use for cutting and welding purposes. Atomic hydrogen atoms (produced by dissociation of dihydrogen with the help of an electric arc) are allowed to recombine on the surface to be welded to generate the temperature of 4000 K. It is used as a rocket fuel in space research. Dihydrogen is used in fuel cells for generating electrical energy. Q. What are Hydrideds? How are they classified? They are binary compounds of hydrogen with elements other than noble gases. e.g., MgH2, B2H6. The hydrides are classified into three categories : (i) Ionic or saline or salt like hydrides (ii) Covalent or molecular hydrides (iii) Metallic or non-stoichiometric hydrides (i) Ionic or saline or salt like hydrides: These are stoichiometric compounds of dihydrogen formed with most of the s-block elements which are highly electropositive in character. Except LiH, BeH2 and MgH2- Covalent. BeH2 and MgH2 are polymeric in structure. The ionic hydrides are crystalline, non-volatile. And They do not conduct electricity in solid state. However, in the molten state or in aqueous state they conduct electricity and on electrolysis liberate dihydrogen gas at anode, which confirms the existence of H ion.

Saline hydrides react violently with water producing dihydrogen gas. Lithium hydride is rather unreactive at moderate temperatures with O2 or Cl2. It is, therefore, used in the synthesis of other useful hydrides, e.g.,

(ii) Covalent or molecular hydrides: These are the compounds of dihydrogen formed with most of the p-block elements. Examples are CH4, NH3, H2O and HF. Being covalent, they are volatile compounds.

Molecular hydrides are further classified according to the relative numbers of electrons and bonds in their Lewis structure into : (i) electron-deficient (ii) electron-precise (iii) electron-rich hydrides. (i) An electron-deficient hydrides: These are the binary compounds of hydrogen with group 13 elements which have too few electrons for writing its conventional Lewis structure. Diborane (B2H6) is an example. They act as Lewis acids i.e., electron acceptors. (ii) Electron-precise hydrides: These are the binary compounds of hydrogen with group 14 elements which have the required number of electrons to write their conventional Lewis structures. e.g,CH4. They have tetrahedral in geometry. (iii) Electron-rich hydrides: These are the binary compounds of hydrogen with elements of group 15 17. They have excess electrons which are present as lone pairs. (NH3 has 1- lone pair, H2O 2 and HF 3 lone pairs). They will behave as Lewis bases i.e., electron donors. The presence of lone pairs on highly electronegative atoms like N, O and F in hydrides results in hydrogen bond formation between the molecules. This leads to the association of molecules (iii) Metallic or non-stoichiometric hydrides(or) interstitial hydrides: These are the hydrides formed by many d-block and f-block elements. However, the metals of group 7, 8 and 9 do not form hydride. Even from group 6, only chromium forms CrH. Since the composition of metallic hydrides does not correspond to simple whole number ratio they are also called non-stoichiometric hydrides These hydrides conduct heat and electricity though not as efficiently as their parent metals do. They are almost always nonstoichiometric, being deficient in hydrogen. For example, LaH2.87, TiH1.51.8 etc. In such hydrides, the law of constant composition does not hold good. The property of absorption of hydrogen on transition metals is widely used in catalytic reduction / hydrogenation reactions for the preparation of large number of compounds. Some of the metals (e.g., Pd, Pt) can accommodate a very large volume of hydrogen and, therefore, can be used as its storage media. This property has high potential for hydrogen storage and as a source of energy. Water Physical properties of water: 1. It is a colourless and tasteless liquid. 2. Due to the presence of extensive hydrogen bonding between water molecules it has high freezing point, high boiling point, high heat of vaporisation and high heat of fusion in comparison to H2S and H2Se. 3. In comparison to other liquids, water has a higher specific heat, thermal conductivity, surface tension, dipole moment and dielectric constant, etc. 4. It is a solvent of great importance. Due to hydrogen bonding with polar molecules, even covalent compounds like alcohol and carbohydrates dissolve in water. Q. How dooes water help in moderation of the climate and body temperature of living beings? The high heat of vaporisation and heat capacity are responsible for moderation of the climate and body temperature of living beings. Q. Explain the structure of water molecule. In the gas phase water has a bent shape with a bond angle of 104.5.

In the liquid phase water molecules are associated together by hydrogen bonds. Q. Explain the structure of ice. (or) Density of ice is less than that of water (or) why does ice float in water. In ice water molecules has a highly ordered three dimensional hydrogen bonded structure. Each oxygen atom is tetrahedrally bonded with four other oxygen atoms; hence it has an open type structure with wide holes. Q. How does ice help the aquatic life survive? In winter season ice formed on the surface of a lake provides thermal insulation which ensures the survival of the aquatic life. Q. Water is amphoteric in nature. Explain. Because water has the ability to act as an acid as well as base.

Q. What is meant by Autoprotolysis or self ionization of water? What is its significance?

One water molecule acts as an acid by donating a proton to another water molecule which acts as a base. This is also called autoprotolyis of water. Auto-protolysis of water indicates its amphoteric nature i.e., its ability to act as an acid as well as a base.

(2) Redox Reactions Involving Water: Water can be easily reduced to dihydrogen by highly electropositive metals. Thus, it is a great source of dihydrogen. Water is oxidised to O2 during photosynthesis. 6CO2 (g) + 12H2O (l) C6H12O6 (aq) + 6H2O (l) + 6O2 (g) With fluorine also it is oxidised to O2. 2F2 (g) + 2H2O (l) 4HF (aq) + O2 (g) (3) Hydrolysis Reaction: Due to high dielectric constant, it has a very strong hydrating tendency. It dissolves many ionic compounds. However, certain covalent and some ionic compounds are hydrolysed in water.

Ca3N2 + 6 H2O 2 NH3 + 3 Ca (OH)2

(4) Hydrates Formation: From aqueous solutions many salts can be crystallised as hydrated salts. Such an association of water is of different types viz., (i) coordinated water e.g.,

(ii) Interstitial water e.g., 2 BaCl .2H2 O (iii) hydrogen-bonded water e.g.,

Q. How many hydrogen-bonded water molecule(s) are associated in CuSO4.5H2O? Only one water molecule, which is outside the brackets (coordination sphere), is hydrogen-bonded. The other four molecules of water are coordinated. Hard and soft water Water free from soluble salts of calcium and magnesium is called soft water. Water containing soluble salts of calcium and magnesium in form of hydrogen carbonate, chlorides and sulphates is called hard water Temporary hardness in water is due to soluble salts of hydrogen carbonates of magnesium and calcium Permanent hardness in water is due to soluble salts of chlorides and sulphates of calcium and magnesium

Methods to remove temporary hardness: 1. Boiling: During boiling soluble magnesium hydrogen carbonate is changed to magnesium hydroxide and calcium hydrogen carbonate on heating gets converted into calcium carbonate. Magnesium hydroxide and calcium carbonate being insoluble is filtered off.

2. Clarks method: Calculated amount of calcium hydroxide is added to a given amount of water. It precipitates out calcium as calcium carbonate and magnesium as magnesium hydroxide which can be filtered off.

Methods to remove permanent hardness: 1. Treatment with washing soda (sodium carbonate): Washing soda reacts with hard water forming insoluble metal carbonate which can be filtered and removed.

Q. What are the disadvantages of using hard water? 1. It is not fit for washing clothe:

Hard water forms scum/precipitate with soap. Soap containing sodium stearate (C17H35COONa) reacts with hard water to precipitate out Ca/Mg stearate.

2. It is harmful for boilers, because of the deposition of salts in the form of scale. This reduces efficiency of the boilers. ==================================================================== READER BACK QUESTIONS 9.9 What characteristics do you expect from an electron-deficient hydride with respect to its structure and chemical reactions? An electron-deficient hydride does not have sufficient electrons to form a regular bond. They exist as dimer. e.g., B2H6, Al2H6 etc. These hydrides cannot be represented by conventional Lewis structures. Since these hydrides are electron-deficient, they have a tendency to accept electrons. Hence, they act as Lewis acids. 9.10 Do you expect the carbon hydrides of the type (CnH2n + 2) to act as Lewis acid or base? Justify your answer. For carbon hydrides of type CnH2n + 2, the following hydrides are possible for

For a hydride to act as a Lewis acid i.e., electron accepting, it should be electron-deficient. Also, for it to act as a Lewis base i.e., electron donating, it should be electron-rich. 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 centered bonds.

Hence, hydride C2H6 has sufficient electrons to be represented by a conventional Lewis structure. Therefore, it 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. 9.11 What do you understand by the term non-stoichiometric hydrides? Do you expect this type of the hydrides to be formed by alkali metals? Justify your answer. Non-Stoichiometric hydrides are hydrogen-deficient compounds formed by the reaction of dihydrogen with d-block and f-block elements. These hydrides do not follow the law of constant composition. For example: LaH2.87, YbH2.55, TiH1.5 1.8 etc. Alkali metals form stoichiometric hydrides. These hydrides are ionic in nature. Hydride ions have comparable sizes (208 pm) with alkali metal ions. Hence, strong binding forces exist between the constituting metal and hydride ion. As a result, stoichiometric hydrides are formed. Alkali metals will not form non-stoichiometric hydrides. 9.12 How do you expect the metallic hydrides to be useful for hydrogen storage? Explain. Metallic hydrides are hydrogen deficient, i.e., they do not hold the law of constant composition. It has been established that in the hydrides of Ni, Pd, Ce, and Ac, hydrogen occupies the interstitial position in lattices allowing further absorption of hydrogen on these metals. Metals like Pd, Pt, etc. have the capacity

to accommodate a large volume of hydrogen. Therefore, they are used for the storage of hydrogen and serve as a source of energy. 9.13 How does the atomic hydrogen or oxy-hydrogen torch function for cutting and welding purposes ? Explain. Atomic hydrogen atoms are produced by the dissociation of dihydrogen with the help of an electric arc. This releases a huge amount of energy (435.88 kJ mol1). This energy can be used to generate a temperature of 4000 K, which is ideal for welding and cutting metals. Hence, atomic hydrogen or oxyhydrogen torches are used for these purposes. For this reason, atomic hydrogen is allowed to recombine on the surface to be welded to generate the desired temperature. 9.14 Among NH3, H2O and HF, which would you expect to have highest magnitude of hydrogen bonding and why? The extent of hydrogen bonding depends upon electronegativity and the number of hydrogen atoms available for bonding. Among nitrogen, fluorine, and oxygen, the increasing order of their electronegativities are N < O < F. Hence, the expected order of the extent of hydrogen bonding is HF > H 2O > NH3. But, the actual order is H2O > HF > NH3. Although fluorine is more electronegative than oxygen, the extent of hydrogen bonding is higher in water. There is a shortage of hydrogens in HF, whereas there are exactly the right numbers of hydrogens in water. As a result, only straight chain bonding takes place. On the other hand, oxygen forms a huge ring-like structure through its high ability of hydrogen bonding. In case of ammonia, the extent of hydrogen bonding is limited because nitrogen has only one lone pair. Therefore, it cannot satisfy all hydrogens. 9.15 Saline hydrides are known to react with water violently producing fire. Can CO 2, a well known fire extinguisher, be used in this case? Explain. Saline hydrides (i.e., NaH, LiH, etc.) react with water to form a base and hydrogen gas. The chemical equation used to represent the reaction can be written as: The reaction is violent and produces fire. CO2 is heavier than dioxygen. It is used as a fire extinguisher because it covers the fire as a blanket and inhibits the supply of dioxygen, thereby dousing the fire. CO2 can be used in the present case as well. It is heavier than dihydrogen and will be effective in isolating the burning surface from dihydrogen and dioxygen. 9.16 Arrange the following (i) CaH2, BeH2 and TiH2 in order of increasing electrical conductance. (ii) LiH, NaH and CsH in order of increasing ionic character. (iii) HH, DD and FF in order of increasing bond dissociation enthalpy. (iv) NaH, MgH2 and H2O in order of increasing reducing property. (i) The electrical conductance of a molecule depends upon its ionic or covalent nature. Ionic compounds conduct, whereas covalent compounds do not. BeH2 is a covalent hydride. Hence, it does not conduct. CaH2 is an ionic hydride, which conducts electricity in the molten state. Titanium hydride, TiH2 is metallic in nature and conducts electricity at room temperature. Hence, the increasing order of electrical conductance is as follows: BeH2 < CaH2 < TiH2 (ii) The ionic character of a bond is dependent on the electronegativities of the atoms involved. The higher the difference between the electronegativities of atoms, the smaller is the ionic character. Electronegativity decreases down the group from Lithium to Caesium. Hence, the ionic character of their hydrides will increase (as shown below). LiH < NaH < CsH (iii) Bond dissociation energy depends upon the bond strength of a molecule, which in turn depends upon the attractive and repulsive forces present in a molecule.

The bond pair in DD bond is more strongly attracted by the nucleus than the bond pair in HH bond. This is because of the higher nuclear mass of D2. The stronger the attraction, the greater will be the bond strength and the higher is the bond dissociation enthalpy. Hence, the bond dissociation enthalpy of DD is higher than HH. However, bond dissociation enthalpy is the minimum in the case of FF. The bond pair experiences strong repulsion from the lone pairs present on each F-centre. Therefore, the increasing order of bond dissociation enthalpy is as follows: FF < HH < DD (iv) Ionic hydrides are strong reducing agents. NaH can easily donate its electrons. Hence, it is most reducing in nature. 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. 9.19 Consider the reaction of water with F2 and suggest, in terms of oxidation and reduction, which species are oxidised/reduced. The reaction between fluorine and water can be represented as: This is an example of a redox reaction as water is getting oxidized to oxygen, while fluorine is being reduced to fluoride ion. The oxidation numbers of various species can be represented as:

Fluorine is reduced from zero to ( 1) oxidation state. A decrease in oxidation state indicates the reduction of fluorine. Water is oxidized from ( 2) to zero oxidation state. An increase in oxidation state indicates oxidation of water. 9.20 Complete the following chemical reactions. (i) (ii) (iii) Classify the above into (a) hydrolysis, (b) redox and (c) hydration reactions. (i) The reactions in which a compound reacts with water to produce other compounds are called hydrolysis reactions. The given reaction is hydrolysis. (ii) The reactions in which a compound reacts with water to produce other compounds are called hydrolysis reactions. The given reaction represents hydrolysis of AlCl3. (ii) The reactions in which a compound reacts with water to produce other compounds are called hydrolysis reactions. The given reaction represents hydrolysis of Ca3N2. 9.29 What properties of water make it useful as a solvent? What types of compound can it (i) dissolve, and (ii) hydrolyse? A high value of dielectric constants and dipole moment make water a universal solvent. Water is able to dissolve most ionic and covalent compounds. Ionic compounds dissolve in water because of the ion-dipole interaction, whereas covalent compounds form hydrogen bonding and dissolve in water.
9

Water can hydrolyze 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:

9.31 What is the difference between the terms hydrolysis and hydration? 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: Hydration is defined as the addition of one or more water molecules to ions or molecules to form hydrated compounds. For example:

9.32 How can saline hydrides remove traces of water from organic compounds? Saline hydrides are ionic in nature. They react with water to form a metal hydroxide along with the liberation of hydrogen gas. The reaction of saline hydrides with water can be represented as: (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 distills over. 9.33 What do you expect the nature of hydrides is, if formed by elements of atomic numbers 15, 19, 23 and 44 with dihydrogen? Compare their behaviour towards water. 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. It is an electron-rich hydride owing to the presence of excess electrons as a lone pair on nitrogen.

2) Hydride of potassium Dihydrogen forms an ionic hydride with potassium owing to the high electropositive nature of potassium. It is crystalline and non-volatile in nature. 3) Hydrides of Vanadium and Ruthenium Both vanadium and ruthenium belong to the dblock of the periodic table. The metals of dblock form metallic or nonstoichiometric hydrides. Hydrides of vanadium and ruthenium are therefore, metallic in nature having a deficiency of hydrogen. 4) Behaviour of hydrides towards water Potassium hydride reacts violently with water as: Ammonia (NH3) behaves as a Lewis base and reacts with water as: Hydrides of vanadium and Ruthenium do not react with water. Hence, the increasing order of reactivity of the hydrides is (V, Ru) H < NH3 < KH. 9.34 Do you expect different products in solution when aluminium(III) chloride and potassium chloride treated separately with (i) normal water (ii) acidified water, and (iii) alkaline water? Write equations wherever necessary.

10

Potassium chloride (KCl) is the salt of a strong acid (HCl) and strong base (KOH). Hence, it is neutral in nature and does not undergo hydrolysis in normal water. It dissociates into ions as follows: In acidified and alkaline water, the ions do not react and remain as such. Aluminium (III) chloride is the salt of a strong acid (HCl) and weak base [Al(OH) 3]. Hence, it undergoes hydrolysis in normal water. In acidified water, H+ ions react with Al(OH)3 forming water and giving Al3+ ions. Hence, in acidified water, AlCl3 will exist as and ions.

In alkaline water, the following reaction takes place:

9.36 What do you understand by the terms: (i) hydrogen economy (ii) hydrogenation (iii) syngas (iv) water-gas shift reaction (v) fuel-cell ? (i) Hydrogen economy Hydrogen economy is a technique of using dihydrogen in an efficient way. It involves transportation and storage of dihydrogen in the form of liquid or gas. Dihydrogen releases more energy than petrol and is more ecofriendly. Hence, it can be used in fuel cells to generate electric power. Hydrogen economy is about the transmission of this energy in the form of dihydrogen. (ii) Hydrogenation Hydrogenation is the addition of dihydrogen to another reactant. 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 Syngas is a mixture of carbon monoxide and dihydrogen. 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 as: This reaction is used to increase the yield of dihydrogen obtained from the coal gasification reaction as: (v) Fuel cells Fuel cells are devices for producing electricity from fuel in the presence of an electrolyte. Dihydrogen can be used as a fuel in these cells. It is preferred over other fuels because it is eco-friendly and releases greater energy per unit mass of fuel as compared to gasoline and other fuels.

11