Chemistry Notes for NEET Chapter 17 Hydrogen and its Compounds PDF

60 Chapter E3 17 Hydrogen and Its compounds ID hydride only with strongly electropositive metals due to smaller value of its Hydrogen electron affinity (72.8 kJ mol 1 ). (1) Position of hydrogen in the periodic table Hydrogen is the first element in the periodic table. Hydrogen is placed in no spec...

60 Chapter E3 17 Hydrogen and Its compounds ID hydride only with strongly electropositive metals due to smaller value of its Hydrogen electron affinity (72.8 kJ mol 1 ). (1) Position of hydrogen in the periodic table Hydrogen is the first element in the periodic table. Hydrogen is placed in no specific group due to its property of giving electron (When (i) Hydrogen is placed in group I (Alkali metals) as, like other alkali D YG (a) It has one electron in its (Outer) shell- 1s 1 U H  is formed) and also losing electron (When H  is formed). 1 metals which have (inert gas) ns configuration. (b) It forms monovalent H   (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).  ion like Li , Na  (c) Its valency is also 1. (2) Discovery and occurrence : It was discovered by Henry Cavendish in 1766. Its name hydrogen was proposed by Lavoisier. Hydrogen is the 9 most abundant element in the earth’s crust. th Hydrogen exists in diatomic state but in triatomicstate it is called as Hyzone. Systematic name of water is oxidane. (3) Preparation of Dihydrogen : Dihydrogen can be prepared by the following methods, (i) By action of water with metals (d) Its oxide (H 2 O) is stable as Li 2 O, Na 2 O. (a) Active metals like Na, K react at room temperature (e) It is a good reducing agent (In atomic as well as molecular state) like Na, Li  (ii) Hydrogen also resembles halogens (Group VII A) as, U (a) It is also diatomic (H 2 ) like F2 , Cl 2  (b) It also forms anion H  like F  , Cl   by gain of one ST electron. 2 M  2 H 2O 2 MOH  H 2 (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. 3 Fe  4 H 2O(steam) (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 (ii) By the action of water on alkali and alkaline earth metals NaH  H 2 O NaOH  H 2 Cl  3 s 2 3 p 5. CaH 2  2 H 2 O Ca(OH )2  2 H 2 (e) (IE) of H (1312 kJ mol 1 ) is of the same order as that of of H  is very small compared to that of alkali metal ion. H forms stable Fe3 O4  4 H 2 Ferrosofer ric oxide hydrides F, Cl,  which are also one electron deficient than octet, F  2 s 2 2 p 5 ; halogens. (iii) (IE) of H is very high in comparison with alkali metals. Also size [M = Na, K etc.] (iii) By reaction of metals like Zn, Sn, Al with alkalies (NaOH or KOH)  Zn  2 NaOH  Na 2 ZnO2  H 2 sod. zincate  Al  2 NaOH  H 2 O  2 NaAlO2  Sod. meta - aluminate 2H 2 (c) By electrolysis of water : Electrolysis of acidified water using platinum electrodes is used for the bulk preparation of hydrogen.  Si  2 NaOH  2 H 2 O  Na 2 SiO3  3 H 2 Silicon  Sn  2 NaOH  Na 2 SnO 2  H 2  (d) From hydrocarbons : Hydrocarbons (alkanes) react with steam at high temperature to produce carbon monoxide and hydrogen, e.g., Sod. stannite (iv) By action of metal with acids : All active metals which lie above hydrogen in electrochemical series, can displace hydrogen gas from dilute mineral acids like HCl, H 2 SO 4. Fe  2 HCl FeCl2  H 2 Catalyst 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 (v) By the electrolysis of acidified water is obtained from hydrocarbons. At anode (vi) Laboratory method : In laboratory, it is obtained by action of granulated zinc with dilute H 2 SO 4. Zn  dil.H 2 SO 4 ZnSO 4  H 2 (e) It is also produced as a by-product of the brine electrolysis process for the manufacture of Cl 2 and NaOH. 60  H / Electrolysis 2 H 2 O   2 H 2   O2  At cathode 1270 K CH 4 (g)  H 2 O(g)    CO (g)  3 H 2 (g) (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, E3 Tin It must be noted that Atomic radius (pm) – 37 (a) Pure zinc is not used for the preparation of H 2 as rate of Ionic radius of H  ion (pm) – 210 reaction of pure Zn with dil. H 2 SO 4 is quite slow. Ionisation energy (kJ mol 1 ) – 1312 instead of H 2. Electron affinity (kJ mol 1 ) –72.8 ID (b) Conc. H 2 SO 4 is not used because then SO 2 gas is evolved Electronegativity – 2.1 (vii) Preparation of pure hydrogen: It can be obtained by (a) The action of pure dil. H 2 SO 4 on pure magnesium ribbon. 2000 K H 2    H  H. Its bond dissociation energy is very high, U Mg  H 2 SO 4 MgSO4  H 2 (5) Chemical properties of dihydrogen : Dihydrogen is quite stable and dissociates into hydrogen atoms only when heated above 2000 K, D YG (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 KAlO2  3 H 2  (viii) Commercial production of dihydrogen (a) Bosch process : In this method, water gas is mixed with twice its volume of steam and passed over heated catalyst Fe2 O3 in the presence of a promoter Cr2O3 or ThO 2 at 773 K when CO 2 and H 2 are obtained. CO 2 is removed by dissolving it in water under pressure (20-25 atm) and U H 2 left undissolved is collected. 1270 K C  H 2 O   CO  H 2   ST Water gas 773 K H 2  CO  H 2 O   CO 2  2 H 2 Fe2O3 , Cr2O3 H 2 H  H ; H  435.9 kJ mol 1. K. 3 Fe  4 H 2O Fe3 O4  4 H 2 The ferrosoferric oxide (Fe3 O4 ) so produced is reduced back to iron with water. this reaction is known as Vivification reactions Fe3 O4  4 H 2 3 Fe  4 H 2 O Fe3 O4  4 CO 3 Fe  4 CO 2 to its high bond (i) Action with metals : To forms corresponding hydrides. Heat Heat 2 Na  H 2   2 NaH ; Ca  H 2   CaH 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 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. (ii) Reaction with Non-metals 970 K 2 H 2  O 2    2 H 2 O Fe, Mo N 2  3 H 2   2 NH 3 750 K , Pressure Dark H 2  F2   2 HF About 18% of the world’s production of H 2 is obtained from coal. (b) Lane’s process : By passing steam over spongy iron at 773-1050 Due dissociation energy, it is not very reactive. However, it combines with many elements or compounds. Sunlight H 2  Cl 2   2 HCl 673 K , Pressure H 2  Br2 2 HBr 673 K H 2  I 2    2 HI Pt The reactivity of halogen towards dihydrogen decreases as, F2  Cl 2  Br2  I 2 Br2 reacts only upon heating while the reaction with I 2 occurs in the hydrogen rise through the solution and the colour is discharged due to the reduction on KMnO 4 by nascent hydrogen. KMnO4  presence of a catalyst. (iii) Reaction with unsaturated hydrocarbons : H 2 reacts with Zn  H 2 SO 4 ZnSO 4  unsaturated hydrocarbons such as ethylene and acetylene to give saturated hydrocarbons. 473 K Ethylene Ethane Ni or Pt or Pd HC  CH  2 H 2    CH 3  CH 3 Acetylene 473 K Ethane This reaction is used in the hydrogenation or hardening of oils. The vegetable oils such as groundnut 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. (solid) (i) As a reducing agent (ii) In the hydrogenation of vegetable oils such as D YG U Electric H 2 (g)    2 H (g) : H  435.90 KJ mol 1 arc This arrangement is also called atomic hydrogen torch. ST Ortho hydrogen Fig. 17.2 Para hydrogen stable. U compounds (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. Different forms of hydrogen (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 H2 Nuclei (a) At 0°K, hydrogen contains mainly para hydrogen which is more (iv) In the manufacture of synthetic petrol many (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, hydrogen. Ortho hydrogen ⇌ Para hydrogen. The amount of ortho and para hydrogen varies with temperature as, (iii) As a rocket fuel in the form of liquid H 2 of 2 KMnO4  3 H 2 SO 4  10[H ] K2 SO 4  2 MnSO4  8 H 2 O ID (6) Uses of Dihydrogen (v) In the preparation NH 3 , CH 3 OH, Urea etc. 2[H ] Nascent hydrogen (i) Molecules of hydrogen in which the spins of both the nuclei are in the same directions, called ortho hydrogen. (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 Ni Vegetable oil H 2    Fat 473 K (liquid) No reaction E3 Ni or Pt or Pd H 2 C  CH 2  H 2    CH 3  CH 3 H 2  H 2 SO 4 Molecular 60 As a result, F2 reacts in dark, Cl 2 in the presence of sunlight, Tungsten rod 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 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 (b) At the temperature of liquefaction of air, the ratio of ortho and para hydrogen is 1:1. (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. (4) Hydrides : Hydrogen forms binary hydrides of the type MH x or M m H n with (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). 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 non-volatile, non-conducting crystalline solids. However, BeH 2 and MgH2 have covalent polymeric structure. These ionic hydrides have rock-salt structure. Thermal stability of 1 and 2 group hydrides are in the order; st LiH > NaH > KH > RbH > CsH nd CaH 2  SrH 2  BaH2 structure. A common example of such molecular hydride is diborane, B2 H 6. Electrolysis of solution of saline hydride in molten alkali halide produces H 2 at anode. Saline hydrides react explosively with water. (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, PH3 Phosphane NaH(s)  H 2O(aq) NaOH(aq)  H 2 (g) reduced by the hot metal hydride. Only sand is useful, as it is a solid. Alkali metal hydrides are used for making LiAlH4 , 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 (dblock) 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 , TiH2 , ZrH2 , HfH2 , VH, VH2 , NbH , NbH 2 , TaH 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 Symbol Protium or Hydrogen Mass number Relative abundance Nature radioactive or non-radioactive 1 1 99.985% Non-radioactive 1 2 0.015% Non-radioactive 1 3 10 15 % Radioactive 1 1 H or H Deuterium 2 1 H or Tritium 3 1 H or D ID Table 17.2 Physical constants of H , D and T ZrHx (1.30  x  1.75) , TiH x (1.8  x  2.0). Most of these hydrides are good conductors of electricity in solid D YG Metallic hydrides can be used to store hydrogen especially in cars working on fuel cells. (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 CH 4 , NH 3 , H 2O, HF etc. The stability of these hydrides decreases down the group. For example, NH 3  PH3  AsH3  SbH 3  BiH3. In a U period the stability increases with increasing electronegativity. For example, CH 4  NH 3  H 2O  HF. Molecular hydrides are classified as electron rich, electron precise and electron deficient hydrides. ST (a) Electron rich molecular hydrides : These hydrides have one or more lone pairs of electrons around the central more electronegative element. For example   |.. Property Molecular mass H  O H , H  N  H , H  F : H (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. (c) Electron deficient molecular hydrides : These hydrides have lesser number of electrons than that required for writing the conventional Lewis 2 2 2 H2 D2 T2 2.016 4.028 6.03 20.63 Melting point (K) 13.8 18.7 Boiling point (K) 20.4 23.9 25.0 0.117 0.197 0.250 Heat of vaporisation (kJ mol -1 ) 0.994 1.126 1.393 Bond energy (kJ mol -1 ) 435.9 443.4 446.9 U composition e.g.,.. Atomic number T and MH 3. All these hydrides are non-stoichiometric with variable  ozane Isotopes of Hydrogen The f-block metals form hydrides of limiting compositions of MH 2 state. NH 3 E3 The fire so produced cannot be extinguished by CO 2 as it gets H 2O oxidane 60 BeH 2 , MgH2 and LiH have significant covalent character. -1 Heat of fusion (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 Cl 2 is 13.4 times faster between D 2 and Cl 2 under similar conditions. Such differences in 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 distorted and the H  O  H bond angle is 104.5°, which is less than the normal tetrahedral angle (109.5°). The Lone Pair of geometry of the molecule is regarded as angular.. Electron.. or bent. In water, each O  H bond is polar because of the high electronegativity of oxygen (3.5) in comparison to that of hydrogen (2.1). The :O: resultant dipole moment of water molecule is 1.84D. o H In ice, each oxygen atom is tetrahedrally H 104.5 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 that water has maximum 3 density (1g cm ) at 4°C (277 K). (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 : (2) Heavy water : Chemically heavy water is deuterium oxide (D2 O). It was discovered by Urey. SO 2  H 2 O H 2 SO 3 Mg3 N 2  6 H 2 O 3 Mg(OH )2  2 NH 3 It is obtained as a by-product in some industries where H 2 is produced by the electrolysis of water. 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 CaH 2  2 H 2 O Ca(OH)2  2 H 2 CaO  H 2 O Ca(OH )2 Na 2 CO 3  2 H 2 O 2 NaOH  H 2 CO 3 60 SiCl4  4 H 2 O Si(OH )4  4 HCl Ca 3 P2  6 H 2 O 3Ca(OH )2  2 PH3 Deuteriosu lphuric acid Al4 C 3  12 D2 O  3CD4 4 Al(OD)3 CaC 2  2 H 2 O Ca(OH )2  C 2 H 2 Deuteromet hane C 2 D2  Deuterioacetylene Ca(OD)2 (3) Physical properties : Water is colourless, odourless and tasteless liquid at ordinary temperature. At 273K water is in equilibrium with ice and vapour this point is known triple point. Table 17.3 Some physical constants of H O and D O at 298 K Ordinary water 2 Heavy water D O 2 HO 2 18.015 20.028 Maximum density (g cm 3 ) 1.000 1.106 Melting point (K) 273.2 Boiling point (K) 373.2 Heat of fusion (kJ mol 1 ) at 6.01 (kJ mol 1 ) at 373K bonded to SO 42  ion. (c) In some compounds, water molecule occupies, interstitial sites in the crystal lattice e.g., BaCl2.2 H 2 O. (5) Hard and Soft water 374.4 6.28 Water which produces lather with soap solution readily is called soft water. e.g. distilled water, rain water and demineralised water. 41.61 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. – 285.9 – 294.6 (i) Cause of hardness of water : The hardness of water is due to the presence of bicarbonates, chlorides and sulphates of calcium and magnesium. 1.008  10 14 1.95  10 15 40.66 Heat of formation (kJ mol 1 ) Ionisation constant 276.8 D YG Heat of vaporisation water are co-ordinated to Cu 2  while the fifth molecule is hydrogen U Molecular mass 273K ID 2 Constant (v) Water forms hydrates with metal salts : There are three main types of hydrates. (a) Compounds in which water molecule are co-ordinated to the metal ion (complex compounds) [ Ni(OH 2 )](NO 3 )2 , Fe(OH 2 )6 ]Cl 3 etc. (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 E3 CaC 2  2 D2 O ST U (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. (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 electrical conductivity and it dissociates as, H 2 O  H 2 O ⇌ H 3 O   OH  Hard water does not produce lather because the cations 2 (Ca and Mg 2 ) present in hard water react with soap to form insoluble precipitates, M 2 From hard water (ii) Amphoteric nature : Water can act both as an acid and a base and is said to be amphoteric. However, water is neutral towards litmus and its pH is 7. (iii) Oxidising and reducing nature : Water can act both as an oxidising and a reducing agent in its chemical reactions. e.g. 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 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. Hydronium ion KW  1.0  10 14 mol 2 L2 at 298K  2C17 H 35 COONa (C17 H 35 COO )2 M  2 Na  (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. (iii) Softening of water : The process of the removal of hardness from water is called softening of water. (a) Removal of temporary hardness : It can be removed by the following methods,  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 filtration. Ca(HCO 3 )2  CaCO 3  CO 2  H 2 O Heat 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, BaO2.8 H 2O must be used. PPt. Heat Mg(HCO 3 )2   MgCO3  CO 2  H 2 O Mag. bicarbonate (a) BaO2.8 H 2O  H 2 SO 4 BaSO4   H 2O2  8 H 2O PPt.  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 (b) 3 BaO2  2 H 3 PO4 Ba3 (PO4 )2  3 H 2O2 Ba3 (PO4 )2  3 H 2 SO 4 3 BaSO4  2 H 3 PO4 60 Cal. bicarbonate (ii) By the action of sulphuric acid or phosphoric acid on hydrated barium peroxide BaO2.8 H 2O Phosphoric acid is preferred to H 2 SO 4 because soluble impurities hard water. like barium persulphate (from Ca(HCO 3 )2  Ca(OH )2  2CaCO 3  2 H 2 O Lime decompose H 2 O2 while H 3 PO4 acts as preservative (negative catalyst) Insoluble Mg(HCO 3 )2  Ca(OH 2 )  MgCO3  CaCO 3  2 H 2 O Soluble Lime (Insoluble) (iii) Industrial method : On a commercial scale, H 2 O 2 can be (b) Removal of permanent hardness : Permanent hardness can be removed by the following methods, chlorides and sulphates of Ca and Mg into their respective carbonates which get precipitated. MgSO 4  Na 2 CO 3  MgCO3  Na 2 SO 4 ppt. D YG 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  (From hard water) Cal zeolite U Sodium zeolite Na 2 Z  Mg 2 (From hard Sodium water) zeolite  MgZ  2 Na  Magnesium zeolite ST where Z  Al2 Si 2 O8. xH 2 O 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 Peroxy disulphuri c acid This is drawn off from the cell and hydrolysed with water to give H 2O2. H 2 S 2 O8  2 H 2 O  2 H 2 SO 4  H 2 O 2  Permutit method : This is a modern method employed for the softening of hard water. hydrated sodium aluminium silicate (Na 2 Al2 Si 2 O8. xH 2 O) is called permutit. These complex salts are also known as zeolites. 2 H 2 SO 4    H 2 S 2 O 8 (aq.) H 2 (g) Elecrolysis The resulting solution is distilled under reduced pressure when H 2 O 2 gets distilled U ppt. prepared by the electrolysis of 50% H 2 SO 4 solution. In a cell, peroxy disulphuric acid is formed at the anode. ID  By washing soda method : In this method, water is treated with a calculated amount of washing soda (Na 2 CO 3 ) which converts the CaCl 2  Na 2 CO 3  CaCO 3  2 NaCl for H 2 O2. E3 Soluble BaO2.8 H 2O  H 2 SO 4 ) tends to while H 2 SO 4 with high boiling point, remains undistilled. (iv) By redox process : Industrially H 2 O2 is prepared by the autooxidation 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 H 2 O2. OH O C2H5 C2H5 O2 + H2O2 OH O 2-Ethylanthraquinol 2-Ethylanthraquinone H2/Pd The H 2 O2 formed (about 1%) is extracted with water and concentrated. (2) Physical properties (i) Pure hydrogen peroxide is a pale blue syrupy liquid. (ii) It freezes at – 0.5°C and has a density of 1.4 in pure state. (iii) Hydrogen peroxide is diamagnetic. (iv) It is more highly associated via hydrogen bonding than water. (v) Although it is a better polar solvent than H 2 O. However, it can’t be used as such because of strong autooxidation ability. (vi) Dipole moment of H 2 O2 is 2.1 D. (3) Chemical properties (i) Decomposition : Pure H 2 O 2 is an unstable liquid and decomposes into water and O 2 either upon standing or upon 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 in alkaline medium. e.g. 2 KI  H 2 O 2  2 KOH  I 2 [In neutral medium] H 2 O2 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 2 O2 when needleshaped crystals of 100% H 2 O2 separate out. These crystals are removed, 2 FeSO 4  H 2 SO 4  H 2 O 2  Fe2 (SO 4 )3  2 H 2 O [In acidic medium] dried and melted to get 100% H 2 O2. (6) Storage of H O : H 2 O2 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 H 3 PO4 are often used as stablizers to check its 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 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 From oxidising agent or even neutral medium. In acidic medium, H 2 O 2  2 H   O 2  2e  In alkaline medium, 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 Thus, the bleaching action of H 2 O 2 is due to oxidation. It oxidises U ID the colouring matter to a colourless product, Colouring matter + O Colour less matter. H 2 O 2 is used to bleach delicate materials like ivory, silk, wool, leather etc. (v) Acidic nature : Anhydrous hydrogen peroxide is acidic in E3 H 2 O 2  O  H 2 O  O 2. It can act as a reducing agent in acidic, basic 2 60 2 MnSO 4  H 2 O 2  2 NaOH  MnO2  Na 2 SO 4  2 H 2 O [In alkaline medium] (iii) Reducing nature : H 2 O 2 has tendency to take up oxygen from strong oxidising agents and thus, acts as a reducing agent, character (Ka  1.55  10 12 at 298 K). its dissociation in aqueous solution may be given as D YG H 2O2  H 2O H 3 O   HO2 It forms two types of salts NaOH  H 2 O2 NaHO2 effluents containing CN  ions. H 2 O2 oxidises CN  ions to harmless products.  H 2O Sod. hydroperox ide (Acidic salt) 2 NaOH  H 2 O2 Na 2 O2  2 H 2 O Sod. peroxide (Normal salt) U (vi) Addition reactions : Hydrogen peroxide is capable of adding itself to ethylenic linkage. CH 2 OH CH 2 ||  H 2 O2 | CH 2 CH 2 OH Ethylene Ethylene glycol (4) Structure of H O : Hydrogen peroxide is non-linear, non-planar molecule. It has a open book structure. The O  O  linkage is called peroxy linkage. The structure is shown below. (111.5)° (90.2)° 147.5 pm H 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 (94.8)° 95.0 pm O H O 145.8 pm (101.9)° 98.8 pm O (5) Concentration of H O : Dilute H 2 O2 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 agent. Further concentration to 99% is obtained by distillation under reduced pressure. Last traces of moisture in 99% of H 2 O2 are removed or anhydrous 2 2  In solids, water molecules can also be present as zeolite water and  Ice is a good thermal insulator.  30% H O is called perhydrol. Its volume strength is 100 and 2 H In solid phase (110 K) In gas phase Colloidal Palladium > Palladium > Platinum > Gold > Nickel. clathrate water. H O  Metals like Pd, Pt, Au etc., have the property of absorbing large quantity 2 ST 2  Hydrogen forms more compounds than even carbon. molarity is 8.8. 2