Hydrogen and Its Compounds Chapter 17 PDF

60 678 Hydrogen and Its compounds Chapter E3 17 Hydrogen and Its compounds (1) Position of hydrogen in the periodic table ID (e) (IE) of H (1312 kJ mol 1 ) is of the same order as Hydrogen that of halogens. Hydrogen is the first element in the periodic table. Hydrogen is placed in no specific group...

60 678 Hydrogen and Its compounds Chapter E3 17 Hydrogen and Its compounds (1) Position of hydrogen in the periodic table ID (e) (IE) of H (1312 kJ mol 1 ) is of the same order as Hydrogen that of halogens. Hydrogen is the first element in the periodic table. Hydrogen is placed in no specific group due to its also losing electron (When H  is formed). U property of giving electron (When H  is formed) and D YG (i) Hydrogen is placed in group I (Alkali metals) as, (a) It has one electron in its (Outer) shell- 1s 1 like other alkali metals configuration. which have (iii) (IE) of H is very high in comparison with alkali metals. Also size of H  is very small compared to that of alkali metal ion. H forms stable hydride only with strongly electropositive metals due to smaller (inert gas) ns 1 (b) It forms monovalent H  ion like Li  , Na   (c) Its valency is also 1. U (d) Its oxide (H 2 O) is stable as Li2 O, Na 2 O. value of its electron affinity (72.8 kJ mol 1 ). (iv) In view of the anomalous behaviour of hydrogen, it is difficult to assign any definite position to it in the periodic table. Hence it is customary to place it in group I (Along with alkali metals) as well as in group VII (Along with halogens). (2) Discovery and occurrence : It was discovered by Henry Cavendish in 1766. Its name hydrogen was proposed by Lavoisier. Hydrogen is the 9th most abundant element in the earth’s crust. Hydrogen exists in diatomic state but in triatomicstate it is called as Hyzone. Systematic name of water is oxidane. (ii) Hydrogen also resembles halogens (Group VII A) as, (3) Preparation of Dihydrogen : Dihydrogen can be prepared by the following methods, ST (e) It is a good reducing agent (In atomic as well as molecular state) like Na, Li  (a) It is also diatomic (H 2 ) like F2 , Cl 2  (b) It also forms anion H  like F  , Cl   by gain of one electron. (c) H  has stable inert gas (He ) configuration as CH 4 , C 2 H 6 like halogens CCl 4 , SF2 Cl 2 etc. (d) H is one electron short of duplet (Stable configuration) like F, Cl,  which are also one electron deficient than octet, F  2 s 2 2 p 5 ; Cl  3 s 2 3 p 5. (i) By action of water with metals (a) Active metals like Na, K react at room temperature 2M  2 H2O 2 MOH  H2 [M = Na, K etc.] (b) Less active metals like Ca, Zn, Mg, Al liberate hydrogen only on heating. 2 Al  3 H 2 O Al 2 O 3  3 H 2 (c) Metals like Fe, Ni, Co, Sn can react only when steam is passed over red hot metals. Hydrogen and Its compounds Fe 3 O4  4 H 2 Ferrosoferric oxide (viii) Commercial production of dihydrogen (ii) By the action of water on alkali and alkaline earth metals hydrides NaH  H 2 O NaOH  H 2 (a) Bosch process : In this method, water gas is mixed with twice its volume of steam and passed over heated catalyst Fe 2O3 in the presence of a promoter or ThO 2 Cr2O3 at 773 K when CO 2 collected. alkalies (NaOH or KOH) 1270 K C  H 2 O   CO  H 2    Zn  2 NaOH  Na 2 ZnO 2  H 2 Water gas sod. zincate H 2  CO  H 2 O  CO 2  2 H 2 2H 2 Fe2O3 , Cr2O3  Si  2 NaOH  2 H 2 O  Na 2 SiO 3  3 H 2 About 18% of the world’s production of H 2 is obtained from coal.  Sn  2 NaOH  Na 2 SnO 2  H 2  Sod. stannite (iv) By action of metal with acids : All active At cathode At anode Fe 3 O4  4 H 2 3 Fe  4 H 2 O U  H / Electroly sis 2 H 2O   2 H 2   O2  Fe3O4  4 CO 3 Fe  4 CO2 D YG (vi) Laboratory method : In laboratory, it is obtained by action of granulated zinc with dilute H 2SO 4. Zn  dil. H 2 SO 4 ZnSO 4  H 2 It must be noted that (a) Pure zinc is not used for the preparation of as rate of reaction of pure Zn with dil. H 2SO 4 is U (b) Conc. H 2SO 4 is not used because then SO 2 gas is evolved instead of H 2. ST (vii) Preparation of pure hydrogen: It can be obtained by (a) The action of The ferrosoferric oxide (Fe 3 O4 ) so produced is as Vivification reactions (v) By the electrolysis of acidified water quite slow. 3 Fe  4 H 2O Fe3O4  4 H2 reduced back to iron with water. this reaction is known Fe  2 HCl FeCl 2  H 2 H2 (b) Lane’s process : By passing steam over spongy iron at 773-1050 K. ID metals which lie above hydrogen in electrochemical series, can displace hydrogen gas from dilute mineral acids like HCl, H 2SO 4. E3 2 NaAlO 2  Sod. meta - aluminate 773 K Silicon Tin are under pressure (20-25 atm) and H 2 left undissolved is (iii) By reaction of metals like Zn, Sn, Al with Al  2 NaOH  H 2 O  H2 obtained. CO 2 is removed by dissolving it in water CaH 2  2 H 2 O Ca(OH )2  2 H 2  and 60 3 Fe  4 H 2O(steam ) 679 pure dil. H 2SO 4 on pure magnesium ribbon. Mg  H 2SO 4 MgSO 4  H 2 (b) Hydrogen of high purity (> 99.95%) is obtained by electrolysing warm aqueous barium hydroxide between nickel electrodes. (c) By the action of water on sodium hydride. NaH  H 2O NaOH  H 2  (d) By the action of KOH (aq.) on aluminium. 2 Al  2 KOH  2 H 2O 2 KAlO 2  3 H 2  (c) By electrolysis of water : Electrolysis of acidified water using platinum electrodes is used for the bulk preparation of hydrogen. (d) From hydrocarbons : Hydrocarbons (alkanes) react with steam at high temperature to produce carbon monoxide and hydrogen, e.g., 1270 K CH 4 (g)  H 2 O(g)    CO (g)  3 H 2 (g) Cataly st The mixture of CO and H 2 so obtained can be converted into hydrogen as in Bosch process. About 77% of the world’s production of H 2 is obtained from hydrocarbons. (e) It is also produced as a by-product of the brine electrolysis process for the manufacture of Cl 2 and NaOH. (4) Physical properties of dihydrogen : It is a colourless, tasteless and odourless gas. It is slightly soluble in water. It is highly combustible. The Physical constants of atomic hydrogen are, Atomic radius (pm) – 37 Ionic radius of H  ion (pm) – 210 Ionisation energy (kJ mol 1 ) – 1312 680 Hydrogen and Its compounds Electron affinity (kJ mol 1 ) –72.8 Electronegativity – 2.1 (5) Chemical properties of dihydrogen : Dihydrogen is quite stable and dissociates into hydrogen atoms only when heated above 2000 K, oil or cotton-seed oil are unsaturated in nature because they contain at least one double bond in their molecules. Dihydrogen is passed through the oils at about 473 K in the presence of catalyst to form solid fats. The vegetable ghee such as Dalda, Rath, etc. are usually prepared by this process. 2000 K H 2    H  H. Its bond dissociation energy is very Ni Vegetable oil  H 2    Fat 473 K. Due to its high compounds. (i) Action with metals : To forms corresponding Heat Heat  CaH 2.  2 NaH ; Ca  H 2  hydrides. 2 Na  H 2  With transition metals (elements of d – block) such as Pd, Ni, Pt etc. dihydrogen forms interstitial hydrides in which the small molecules of dihydrogen occupy the (i) As a reducing agent (ii) In the hydrogenation of vegetable oils (iii) As a rocket fuel in the form of liquid H 2 (iv) In the manufacture of synthetic petrol (v) In the preparation of many compounds such as NH 3 , CH 3 OH , Urea etc. (vi) It is used in the oxy-hydrogen torch for welding if temperature around 2500°C is required. It is also used in atomic hydrogen torch for welding purposes in which temperature of the order of 4000°C is required. ID interstitial sites in the crystal lattices of these hydrides. As a result of formation of interstitial hydrides, these metals adsorb large volume of hydrogen on their surface. This property of adsorption of a gas by a metal is called occlusion. The occluded hydrogen can be liberated from the metals by strong heating. (6) Uses of Dihydrogen 60 bond dissociation energy, it is not very reactive. However, it combines with many elements or (solid ) (liquid ) E3 high, H 2 H  H ; H  435.9 kJ mol 1 970 K 2 H 2  O 2    2 H 2 O Fe, Mo D YG N 2  3 H 2   2 NH 3 750 K , Pressure Dark H 2  F2   2 HF Sunlight H 2  Cl 2   2 HCl 673 K , Pressure H 2  Br2 2 HBr 673 K H 2  I 2    2 HI Pt Different forms of hydrogen U (ii) Reaction with Non-metals U The reactivity of halogen towards dihydrogen decreases as, F2  Cl 2  Br2  I2 As a result, F2 reacts in dark, Cl 2 in the presence (1) Atomic hydrogen : It is obtained by the dissociation of hydrogen molecules. The atomic hydrogen is stable only for a fraction of a second and is extremely reactive. It is obtained by passing dihydrogen gas at atmospheric pressure through an electric arc struck between two tungsten rods. The electric arc maintains a temperature around 4000 – 4500°C. As the molecules of dihydrogen gas pass through the electric arc, these absorb energy and get dissociated into atoms as Electric H 2 (g)    2 H (g) : H  435.90 KJ mol 1 arc This arrangement is also called atomic hydrogen torch. ST of sunlight, Br 2 reacts only upon heating while the reaction with I 2 occurs in the presence of a catalyst. H2 (iii) Reaction with unsaturated hydrocarbons : reacts with unsaturated hydrocarbons such as ethylene and acetylene to give saturated hydrocarbons. Ni or Pt or Pd H 2 C  CH 2  H 2    CH 3  CH 3 Ethylene 473 K Ethane HC  CH  2 H 2   CH 3  CH 3 Ni or Pt or Pd Acetylene 473 K Ethane This reaction is used in the hydrogenation or hardening of oils. The vegetable oils such as groundnut Tungsten rod H2 H H Tungsten rod Fig. 17.1 Atomic hydrogen torch (2) Nascent hydrogen : The hydrogen gas prepared in the reaction mixture in contact with the substance with which it has to react, is called nascent hydrogen. It is also called newly born hydrogen. It is more reactive than ordinary hydrogen. For example, if Hydrogen and Its compounds ordinary hydrogen is passed through acidified KMnO 4 (pink in colour), its colour is not discharged. On the other hand, if zinc pieces are added to the same solution, bubbles of hydrogen rise through the solution and the colour is discharged due to the reduction on KMnO 4 by nascent hydrogen. Zn  H 2 SO 4 ZnSO 4  No reaction (a) All main group elements except noble gases and probably indium and thallium. (b) All lanthanoids and actinoids. (c) Transition metals (Sc, Y, La, Ac, Tc, Zr, Hf and to a lesser extent V, Nb, Ta, Cr, Cu and Zn). In group 6 only Cr forms hydride (CrH). 2[H ] Nascent hydrogen 2KMnO 4  3 H 2 SO 4  10[H] K2 SO 4  2 MnSO 4  8 H 2 O (3) Ortho and para hydrogen : A molecule of dihydrogen contains two atoms. The nuclei of both the atoms in each molecule of dihydrogen are spinning. Depending upon the direction of the spin of the nuclei, the hydrogen is of two types, Nuclei 60 H 2  H 2 SO 4 Molecular (4) Hydrides : Hydrogen forms binary hydrides of the type MH x or Mm Hn with Hydrides are classified into three main categories. (i) Saline or ionic hydrides : Most of the s-block metals form this type of hydrides. These are nonvolatile, non-conducting crystalline solids. However, E3 KMnO 4  681 BeH 2 and MgH 2 have covalent polymeric structure. These ionic hydrides have rock-salt structure. Thermal stability of 1st and 2nd group hydrides are in the order; Ortho hydrogen Fig. 17.2 ID LiH > NaH > KH > RbH > CsH CaH 2  SrH 2  BaH 2 Para hydrogen BeH 2 , MgH 2 character. U (i) Molecules of hydrogen in which the spins of and LiH have significant covalent both the nuclei are in the same directions, called ortho hydrogen. D YG (ii) Molecules of hydrogen in which the spins of both the nuclei are in the opposite directions, called para hydrogen. Ordinary dihydrogen is an equilibrium mixture of ortho and para hydrogen. Ortho hydrogen ⇌ Para hydrogen. The amount of ortho and para hydrogen varies with temperature as, U (a) At 0°K, hydrogen contains mainly para hydrogen which is more stable. (b) At the temperature of liquefaction of air, the ratio of ortho and para hydrogen is 1:1. ST (c) At the room temperature, the ratio of ortho to para hydrogen is 3:1. (d) Even at very high temperatures, the ratio of ortho to para hydrogen can never be more than 3:1. Thus, it has been possible to get pure para hydrogen by cooling ordinary hydrogen gas to a very low temperature (close to 20 K) but it is never possible to get a sample of hydrogen containing more than 75% of ortho hydrogen. i.e., Pure ortho hydrogen can not be obtained. Electrolysis of solution of saline hydride in molten alkali halide produces H 2 at anode. Saline hydrides react explosively with water. NaH (s)  H 2O(aq) NaOH (aq)  H 2 (g) The fire so produced cannot be extinguished by CO 2 as it gets reduced by the hot metal hydride. Only sand is useful, as it is a solid. Alkali metal hydrides are used for making LiAlH 4 , NaBH 4 etc. Alkali metal hydrides are also used for the removal of last traces of water from organic compounds. (ii) Metallic or interstitial hydrides : Elements of groups 3, 4, 5 (d-block) and f-block elements form metallic hydrides. In group 6, only Cr forms hydride (CrH). Metals of group 7, 8, 9 do not form hydrides. This region of periodic table from group 7 to group 9 is known as hydride gap. Examples of hydrides of group 3 to 5 are, ScH 2 , YH 2 , YH 3 , LaH 2 , LaH 3 , TiH 2 , ZrH2 , HfH 2 , VH , VH 2 , NbH , NbH 2 , TaH The f-block metals form hydrides of limiting compositions of MH 2 and MH 3. All these hydrides are non-stoichiometric with variable composition e.g., 682 Hydrogen and Its compounds ZrHx (1.30  x  1.75 ) , TiH x (1.8  x  2.0). r r e or nonradioactive Most of these hydrides are good conductors of electricity in solid state. Protium Metallic hydrides can be used to store hydrogen especially in cars working on fuel cells. n Deuteriu Property.. D YG (b) Electron precise molecular hydrides : Elements of group 14 form such hydrides. The bond length increases on going down the group. A common example of electron precise molecular hydrides is CH 4. U (c) Electron deficient molecular hydrides : These hydrides have lesser number of electrons than that required for writing the conventional Lewis structure. A common example of such molecular hydride is diborane, B2 H 6. ST (d) Systematic names of molecular hydrides : The systematic names of these hydrides are obtained from the name of the element and the suffix –ane. For example, H 2O oxidane NH 3 ozane Isotopes of Hydrogen Isotopes are the different forms of the same element, which have the same atomic number but different mass numbers. Table 17.1 Isotopes of hydrogen Name Symbo l % Radioactive H2 D2 T2 2.016 4.028 6.03 18.7 20.63 20.4 13.8 23.9 25.0 Heat of fusion (kJ mol ) 0.117 0.197 0.250 Heat 0.994 1.126 1.393 435.9 443.4 446.9 -1 of vaporisation (kJ mol -1 ) -1 Bond energy (kJ mol ) Isotopic effect : In general chemical properties of isotopes are same but quantiative differences are noticed amongst them. For example, the reaction between H 2 and Cl2 is 13.4 times faster between D 2 and Cl2 under similar conditions. Such differences in H PH 3 Phosphane Melting point (K) ID | H  O H , H  N  H , H  F :.. 10 15 3 U  1 Table 17.2 Physical constants of H2 , D2 and T2 Boiling point (K)  Non- T increases with increasing electronegativity. For example, CH 4  NH 3  H2O  HF. Molecular hydrides  0.015% radioactive 3 1 H or Molecular mass (a) Electron rich molecular hydrides : These hydrides have one or more lone pairs of electrons around the central more electronegative element. For example 2 D Tritium group. For example, NH 3  PH 3  AsH 3  SbH 3  BiH 3. In a period the stability are classified as electron rich, electron precise and electron deficient hydrides. Non- 60 the 1 E3 down 99.985% radioactive 2 1 H or CH 4 , NH 3 , H 2O, HF etc. The stability of these hydrides decreases 1 H m (iii) Molecular or covalent hydrides : Hydrogen form molecular compounds with p-block elements (B, C, N, O, F; Si, P, S, Cl; Ga, Ge, As, Sb, Br; In, Sn, Sb, Te, I; Tl, Pb, At). common examples of such hydrides are 1 1 1 H or or Hydroge Atomic Mass Relative Nature numbe numbe abundanc radioactive chemical properties, which are due to difference in the mass numbers of isotopes is known as isotopic effect. Water Water is the oxide of hydrogen. It is an important component of animal and vegetable matter. Water constitutes about 65% of our body. It is the principal constituent of earth’s surface. (1) Structure : Due to the presence of lone pairs, the geometry of water is Lone Pair distorted and the H  O  H bond.. of angle is 104.5°, which is less..Electron than the normal tetrahedral angle (109.5°). The geometry of :O the molecule is regarded as : angular or bent. In water, each H 104.5 H o is polar because of the high O  H bond electronegativity of oxygen (3.5) in comparison to that of hydrogen (2.1). The resultant dipole moment of water molecule is 1.84D. In ice, each oxygen atom is tetrahedrally surrounded by four hydrogen atoms; two by covalent bonds and two by hydrogen bonds. The resulting structure of ice is open structure having a number of vacant spaces. Therefore, the density of ice is less than that of water and ice floats over water. It may be noted Hydrogen and Its compounds that water has maximum density (1 g cm 3 ) at 4°C (277 K). 683 K W  1. 0  10 14 mol 2 L2 at 298K (ii) Amphoteric nature : Water can act both as an Heavy water (D2O) is used (a) as a moderator and coolant in nuclear reactors (b) in the study of mechanism of chemical reactions (c) as a starting material for the preparation of a number of deuterium compounds, e.g., SO 3  D2 O D2 SO 4 Deuteriosu lphuric acid Al 4 C3  12 D2 O CaC 2  2 D2 O 3CD4  Deuteromethane C 2 D2  Deuterioa cety lene 4 Al(OD)3 Ca(OD)2 2 Na  2H 2 O Oxidi sin g agent 2 F2  2H 2 O Re ducing agent 2 NaOH  H 2 4 HF  O 2 (iv) Hydrolytic reactions : Water can hydrolyse many oxides, halides, hydrides, carbides, nitrides, phosphides, carbonates etc. to give an acid or a base or both as shown below : Mg 3 N 2  6 H 2 O 3 Mg(OH )2  2 NH 3 CaH 2  2 H 2 O Ca(OH )2  2 H 2 ID At 273K water is in equilibrium with ice and vapour this point is known triple point. Table 17.3 Some physical constants of H2O and D2O at 298 K Heavy water D2O 18.015 20.028 CaO  H 2 O Ca(OH )2 Na 2 CO 3  2H 2 O 2 NaOH  H 2 CO 3 SiCl 4  4 H 2 O Si(OH )4  4 HCl U Ordinary water H2O Ca 3 P2  6 H 2 O 3Ca(OH )2  2 PH 3 CaC 2  2H 2 O Ca(OH )2  C2 H 2 D YG Molecular mass act both as an oxidising and a reducing agent in its chemical reactions. e.g. SO 2  H 2O H 2 SO 3 (3) Physical properties : Water is colourless, odourless and tasteless liquid at ordinary temperature. Constant (iii) Oxidising and reducing nature : Water can 60 It is obtained as a by-product in some industries where H 2 is produced by the electrolysis of water. acid and a base and is said to be amphoteric. However, water is neutral towards litmus and its pH is 7. E3 (2) Heavy water : Chemically heavy water is deuterium oxide (D2 O). It was discovered by Urey. 1.000 1.106 (v) Water forms hydrates with metal salts : There are three main types of hydrates. Melting point (K) 273.2 276.8 Boiling point (K) 373.2 374.4 (a) Compounds in which water molecule are coordinated to the metal ion (complex compounds) [Ni(OH 2 )](NO 3 )2 , Fe(OH 2 )6 ]Cl3 etc. Heat of 6.01 6.28 40.66 41.61 Maximum density 3 (g cm ) 1 fusion (kJ mol ) at 273K Heat of vaporisation U (kJ mol 1 ) at 373K Heat of formation – 285.9 – 294.6 1.008  10 14 1. 95  10 15 (c) In some compounds, water molecule occupies, interstitial sites in the crystal lattice e.g., BaCl 2.2 H 2 O. (kJ mol ) ST ordinated to Cu 2  while the fifth molecule is hydrogen bonded to SO 42  ion. 1 Ionisation constant (b) Compound in which water molecule may be hydrogen bonded to oxygen to form oxo-anion. For example in CuSO 4.5 H 2 O , 4 molecules of water are co- (5) Hard and Soft water (4) Chemical properties : Water shows a versatile chemical behaviour. It behaves as an acid, a base, an oxidant, a reductant and as ligand to metals. Water which produces lather with soap solution readily is called soft water. e.g. distilled water, rain water and demineralised water. (i) Dissociation of water : Water is quite stable and does not dissociate into its elements even at high temperatures. Pure water has a small but measurable Water which does not produce lather with soap solution readily is called hard water. e.g. sea water, river water, well water and tap water. electrical (i) Cause of hardness of water : The hardness of water is due to the presence of bicarbonates, chlorides and sulphates of calcium and magnesium. conductivity H 2O  H 2O ⇌ H 3 O  Hydronium ion and  OH it  dissociates as, 684 Hydrogen and Its compounds Hard water does not produce lather because the cations (Ca 2 and Mg 2 ) present in hard water react with Sodium stearate (soap ) Metal stearate ( PPt.) Where M = Ca or Mg Therefore, no lather is produced until all the calcium and magnesium ions are precipitated. This also results into wastage of lot of soap. (ii) Type of hardness of water : The hardness of water is of two types, (a) Temporary hardness : This is due to the presence of bicarbonates of calcium and magnesium. It is also called carbonate hardness. (b) Permanent hardness : This is due to the presence of chlorides and sulphates of calcium and magnesium. It is also called non-carbonate hardness. permutit. These complex salts are also known as zeolites. The permutit as loosely packed in a big tank over a layer of coarse sand. Hard water is introduced into the tank from the top. Water reaches the bottom of the tank and then slowly rises through the permutit layer in the tank. The cations present in hard water are exchanged for sodium ions. Therefore this method is also called ion exchange method. Na 2 Z  Ca 2  CaZ  2 Na  Sodium zeolite (From hard water) Cal zeolite Na 2 Z  Mg 2 (From hard Sodium water) zeolite  MgZ  2 Na  Magnesium zeolite where Z  Al 2 Si 2 O8. xH 2 O U (a) Removal of temporary hardness : It can be removed by the following methods,  Permutit method : This is a modern method employed for the softening of hard water. hydrated sodium aluminium silicate (Na 2 Al 2 Si 2 O8.xH 2 O) is called ID (iii) Softening of water : The process of the removal of hardness from water is called softening of water. ppt. 60  2C17 H 35 COONa (C17 H 35 COO )2 M  2 Na  E3 From hard water ppt. MgSO 4  Na 2 CO 3  MgCO 3  Na 2 SO 4 soap to form insoluble precipitates, M 2 CaCl 2  Na 2 CO 3  CaCO 3  2 NaCl filtration. D YG  By boiling : During boiling, the bicarbonates of Ca and Mg decompose into insoluble carbonates and give CO 2. The insoluble carbonates can be removed by Heat Ca(HCO 3 )2   CaCO 3  CO 2  H 2 O Cal. bicarbonat e PPt. Heat Mg(HCO 3 )2   MgCO 3  CO 2  H 2 O Mag. bicarbonat e PPt. U  Clark’s method : This process is used on a commercial scale. In this process, calculated amount of lime Ca(OH )2  is added to temporary hard water. Ca(HCO 3 ) 2  Ca(OH ) 2  2CaCO 3  2 H 2 O Soluble Lime Insoluble ST Mg(HCO 3 )2  Ca(OH 2 )  MgCO 3  CaCO 3  2 H 2 O Soluble Lime (Insoluble ) (b) Removal of permanent hardness : Permanent hardness can be removed by the following methods,  By washing soda method : In this method, water is treated with a calculated amount of washing soda (Na 2 CO 3 ) which converts the chlorides and sulphates of Ca and Mg into their respective carbonates which get precipitated. Hydrogen peroxide Hydrogen peroxide (H 2 O 2 ) was discovered by French chemist Thenard. (1) Preparation : It is prepared by (i) Laboratory method : In laboratory, H 2 O 2 is prepared by Merck’s process. It is prepared by adding calculated amounts of sodium peroxide to ice cold dilute (20%) solution of H 2 SO 4. Na 2 O 2  H 2 SO 4  Na 2 SO 4  H 2 O 2 (ii) By the action of sulphuric acid or phosphoric acid on hydrated barium peroxide BaO 2.8 H 2O (a) BaO 2.8 H 2O  H 2 SO 4 BaSO 4   H 2O2  8 H 2O It must be noted that anhydrous barium peroxide does not react readily with sulphuric acid (because a coating of insoluble barium sulphate is formed on its surface which stops further action of the acid). Therefore, hydrated barium peroxide, BaO 2.8 H 2O must be used. (b) 3 BaO 2  2H3 PO4 Ba3 (PO4 )2  3 H 2O2 Ba3 (PO4 )2  3 H2SO 4 3 BaSO 4  2H3 PO4 Phosphoric acid is preferred to H 2 SO 4 because soluble impurities like barium persulphate (from BaO 2.8 H 2O  H 2 SO 4 ) tends to decompose H 2O2 while Hydrogen and Its compounds H3 PO4 acts as preservative (negative catalyst) for H 2O2. 685 in alkaline medium. e.g. 2 KI  H 2 O 2  2 KOH  I 2 [In neutral medium] 2 FeSO 4  H 2 SO 4  H 2 O 2  Fe 2 (SO 4 )3  2 H 2 O [In (iii) Industrial method : On a commercial scale, H 2 O 2 can be prepared by the electrolysis of 50% acidic medium] H 2 SO 4 solution. In a cell, peroxy disulphuric acid is MnSO 4  H 2 O 2  2 NaOH  MnO 2  Na 2 SO 4  2 H 2 O formed at the anode. [In alkaline medium] (iii) Reducing nature : H 2 O 2 has tendency to take 2 H 2 SO 4    H 2 S 2 O 8 (aq.) H 2 (g) Elecroly sis Peroxy disulphuri c acid up oxygen from strong oxidising agents and thus, acts as a reducing agent, H 2 O 2  O  H 2 O  O 2. It can act H 2 S 2 O 8  2 H 2 O  2 H 2 SO 4  H 2 O 2 The resulting as a reducing agent in acidic, basic or even neutral medium. solution is distilled under reduced pressure when H 2 O 2 60 This is drawn off from the cell and hydrolysed with water to give H 2 O 2. From oxidising agent In acidic medium, H 2 O 2  2 H   O 2  2e  remains undistilled. In alkaline medium, (iv) By redox process : Industrially H 2O2 is H 2O2. H 2 O 2  2OH   2 H 2 O  O 2  2e  (iv) Bleaching action : H 2 O 2 acts as a bleaching agent due to the release of nascent oxygen. H 2 O 2  H 2 O  O ID prepared by the auto-oxidation of 2-alkylanthraquinols. The process involves a cycle of reactions. The net reaction is the catalytic union of H 2 and O 2 to give E3 gets distilled while H 2SO 4 with high boiling point, Thus, the bleaching action of H 2 O 2 is due to OH O C 2 H5 O2 + H2O2 O 2Ethylanthraquinone D YG OH 2-Ethylanthraquinol U C2H5 H2/Pd oxidation. It oxidises the colouring matter to a colourless product, Colouring matter +O Colour less matter. H 2 O 2 is used to bleach delicate materials like The H 2O2 formed (about 1%) is extracted with water and concentrated. (2) Physical properties (i) Pure hydrogen peroxide is a pale blue syrupy liquid. U (ii) It freezes at – 0.5°C and has a density of 1.4 in pure state. (iii) Hydrogen peroxide is diamagnetic. ST (iv) It is more highly associated via hydrogen bonding than water. (v) Although it is a better polar solvent than H 2O. However, it can’t be used as such because of strong autooxidation ability. (vi) Dipole moment of H 2O2 is 2.1 D. (3) Chemical properties (i) Decomposition : Pure H 2 O 2 is an unstable ivory, silk, wool, leather etc. (v) Acidic nature : Anhydrous hydrogen peroxide is acidic in character (Ka  1.55  10 12 at 298 K). its dissociation in aqueous solution may be given as H 2 O2  H 2 O H 3 O   HO 2 It forms two types of salts NaOH  H 2 O2 NaHO 2  H 2 O Sod. hy droperox ide (Acidic salt) 2 NaOH  H 2 O2 Na 2 O2  2 H 2 O Sod. peroxide (Normal salt) (vi) Addition reactions : Hydrogen peroxide is capable of adding itself to ethylenic linkage. CH 2 OH CH 2 ||  H 2O2 | CH 2 CH 2 OH Ethy lene Ethy lene gly col (4) Structure of H2O2 : Hydrogen peroxide is nonlinear, non-planar molecule. It has a open book structure. The O  O  linkage is called peroxy linkage. The structure is shown below. (111.5) ° liquid and decomposes into water and O 2 either upon standing or upon H heating, 2 H 2 O 2  2 H 2 O  O 2 ; H  196.0 kJ (ii) Oxidising nature : It is a powerful oxidising agent. It acts as an oxidising agent in neutral, acidic or (90.2)° O 147.5 pm H 95.0 pm O In gas phase (94.8 )° H O 145.8 pm H 98.8 pm (101.9 )° O In solid phase (110 K) 686 Hydrogen and Its compounds agent. Further concentration to 99% is obtained by distillation under reduced pressure. Last traces of moisture in 99% of H 2O2 are removed or anhydrous H 2O2 is obtained by cooling it to 263 K in a cold bath of ether and dry ice followed by seeding with a few crystals of solid H 2O2 when needle-shaped crystals of 100% H 2O2 separate out. These crystals are removed, dried and melted to get 100% H 2O2. (6) Storage of H2O2 : H 2O2 is not stored in glass bottles since the alkali metal oxides present in glass catalyse its decomposition. It is, therefore, stored in paraffin wax coated glass, plastic or teflon bottles. Small amounts of acid, glycerol, alcohol, acetanilide and H3 PO4 are often used as stablizers to check its carbon.  Metals like Pd, Pt, Au etc., have the property of absorbing large quantity of hydrogen at normal or higher temperature. Colloidal Pd can absorb 2950 times its own volume of hydrogen and Pd metal can absorb 900 times its own volume of hydrogen. This phenomenon is known as occlusion of hydrogen. the occlusion property of these metals is in the order Colloidal Palladium > Palladium > Platinum > Gold > Nickel. 60 H 2O2 is concentrated to about 50% by slow evaporation on a water bath. It is further concentrated to 90% in a vacuum desiccator using conc. H 2 SO 4 as dehydrating  In solids, water molecules can also be present as zeolite water and clathrate water.  Ice is a good thermal insulator. E3 (5) Concentration of H2O2 : Dilute  30% H2O2 is called perhydrol. Its volume strength is 100 and molarity is 8.8. U ID decomposition. Uses of hydrogen peroxide (i) For bleaching delicate articles like wool, hair, feather, ivory, etc. (ii) For restoring colour of old lead paintings whose white lead has blackened due to formation of PbS by H 2 S of atmosphere. Hydrogen peroxide ST U D YG converts the black lead sulphide to white lead sulphate (iii) As an aerating agent in production of spong rubber. (iv) As an antiseptic and germicide for washing wounds, teeth and ears, under the name of perhydrol. (v) In the manufacture of sodium perborate, sodium percarbonate. These are used in high quality detergents. (vi) As an antichlor. (vii) As an oxidant for rocket fuel. (viii) In the detection of Ti, V and Cr ions with which it forms peroxides of characteristics colours. (ix) In the production of epoxides, propylene oxide and polyurethanes. (x) In the synthesis of hydroquinone, pharmaceuticals (cephalosoporin) and food products like tartaric acid. (xi) For pollution control of domestic effluents where it restores the aerobic conditions of sewage wastes. For pollution control of industrial effluents containing CN  ions. H 2O2 oxidises CN  ions to Hydrogen 1. Which is used hydrogen generators [CPMT 1999] (a) NaH (b) HI (c) S 6 H 3 (d) None of these 2. Metal hydride on treatment with water gives [Bihar CEE 1995] (a) H 2O2 3. 4. (b) H 2O (c) Acid (d) Hydrogen Hydrogen burns in air with a [RPET 2003] (a) Light bluish flame (b) Yellow flame (c) Green flame (d) None of these Which pair does not show hydrogen isotopes [UPSEAT 2003] 5. harmless products. (a) Ortho hydrogen and para hydrogen (b) Protium and deuterium (c) Deuterium and tritium (d) Tritium and protium Which is distilled first [Pb. PMT 2002] (a) Liquid CO 2 (b) Liquid N 2 (c) Liquid O 2 6. (d) Liquid H 2 On reaction with Mg, very dilute nitric acid produces [CPMT 2003]  Hydrogen forms more compounds than even (a) NH 3 (b) Nitrous oxide (c) Nitric oxide (d) Hydrogen