THE S-BLOCK ELEMENTS - Lecture Notes

Brilliant STUDY CENTRE CHEMISTRY - ONLINE CLASS NOTES) CHAPTER - 10 S-BLOCK ELEMENTS Group 1 and 2 elements having outer EC ns1 and ns2 are s-block elements. GROUP-1 (Alkali metals) - ns1 They are Li, Na, K, Rb, Cs and Fr. Fr - radioactive t 1  22 min . They are alkali metals, their 2 compounds are strong bases. GENERAL PROPERTIES 1. They are silvery white soft metals, can be cut with a knife. They have low MP and BP due to weak metallic bonding, but good conductors. 2. Size and Reactivity : They have largest size and size increases. So IE value decreases, so they are most reactive elements. M  M 1e But Li-exception, due to small size and high IE Li form covalent compound. Eg : LiCl. 3. Order of hydration enthalpy of ions Li  Na  K  Rb  Cs 4. Alkali metals have low density Lowest d form Li = 0.53 g/cm3 Li < K < Na < Rb < Cs 5. They are normally kept in Kerosene, they are very reactive, and get tarnished in air, catch fire with moisture. But Li kept in paraffin wax. 6. Alkali metals are powerful reducing agent, they have tendency to loss electrons. Li most powerful reducing agent. 7. K and Cs used in photoelectric cells due to low I.E. 1

Brilliant STUDY CENTRE CHEMISTRY - ONLINE CLASS NOTES) 8. Flame colouration Li - Red/Crimson Na - Yellow/Golden K - Violet Rb - Dark red Cs - Blue Chemical properties Reaction with air M  O2 heat Li2O, Na2O, K2O.......... (s) oxides excess O2 Na2O2 K2O2 ............. peroxides excheesastO2 NaO2 KO2 RbO2 (s) sup eroxides Li has only one oxide Li2O - small size others have higher oxides. Most stable Li2O Na2O  H2O  2NaOH Na2O2  2H2O  2NaOH  H2O2 2KO2  2H2O  2KOH  H2O2  O2  Oxides and peroxides - colourless solids. But superoxides - yellow colour - Paramagnetic. Li  Air  Li2O  Li3N 2. Reaction with H2O Na / K  H2O fast NaOH  H2 KOH  H2 Li  H2O slow LiOH H 2 hot less stable 2LiOH  Li2O  H2O 2

Brilliant STUDY CENTRE CHEMISTRY - ONLINE CLASS NOTES) 3. Reaction with Acids 2Na / K  2HCl  2NaCl  2KCl  H2  4. Reaction with H2 2Na / 2K  H2  2NaH / 2KH 2Li  H2  2LiH Order of stability LiH > NaH > KH ................ 5. Reaction with alcohol/acetylene 2Na  2C2H5  OH  2C2H5  ONa  H2 2Na  HC  CH  NaC  CNa  H2 6. Reaction with liquid NH3 M   x  y  NH3  M  NH3   e   NH3  X g ammoniated electron bluecolour conductivity and paramagnetic As concentration of ‘M’ increases Colour changes - Blue  Bronze  Diamagnetic 2Na  2NH3  2NaNH2  H2  7. Reaction with halogens 2M  X2  2MX Stability : For fluorides stability decreases down the group  H 0 decreases (become less negative) f For others stability increases  H 0 increases. (become more negative) f Almost all MX soluble in H2O But LiF insoluble in H2O LiCl, LiBr - soluble in pyridine, ether etc Salts of oxo acids Na form NaHCO3, Na2CO3, NaHSO4, Na2SO4. NaHCO3 and Na2CO3 - stable Li2CO3  less stable - decomposes on heating 3

Brilliant STUDY CENTRE CHEMISTRY - ONLINE CLASS NOTES) Li2CO3 heat Li2O  CO2 LiHCO3 - will not exist as solid Anomalous nature of Li Reasons : Small size, high I.E., high polarising power The properties 1. Li hardest of alkali metal 2. Li form oxide and nitride 3. Li halides are covalent except LiF 4. LiCl - deliquescent solid - exist as LiCl . 2H2O 5. Li2CO3 - unstable 6. LiNO3  Li2O  NO2  O2 7. All have solid bicarbonates, Li-does not Diagonal relation of Li  Mg Reasons : Similar size, EN values are very close, same polarising power. Similarities are 1. Li and Mg very hard 2. They form oxide and nitride 3. Li2CO3 and MgCO3 less stable 4. They have no solid bicarbonate 5. LiCl & MgCl2  deliquescent solids. 6. Their nitrate on heating give NO2  7. Their reaction with H2O very slow. Extraction Alkali metals are powerful reducing agents, so they are extracted by electrolysis of their fused salts - Electrolytic reduction. Li - used as reducing agent, used in batteries and cell Na - used us reducing agent, For preparation of NaCN, NaNH2 ...... etc Liquid Na - used as coolant in nuclear reactors Liquid Na - K alloy used in thermometers. 4

Brilliant STUDY CENTRE CHEMISTRY - ONLINE CLASS NOTES) Compounds of alkali metals:- NaCl - Rock salt/common salt Manufactured from sea water by solar evaporation. Crude NaCl contain CaCl2 and MgCl2 - so become wet on keeping. Purified by passing HCl gas through saturated solution, NaCl crystals comes out - dried and KIO3 added  iodised salt. Uses As a taste maker Food preservative For preparation of Na2CO3, NaHCO3, NaCl ...... Sodium carbonate Na2CO3.10H2O (washing soda) But Na2CO3  soda ash Prepared by solvay process (Ammonia soda process) Reagents :- NH4OH, CO2 and Brine 2NH3  H2O  CO2  NH4  CO3 2  NH4  CO3  H2O  CO2  2NH4HCO3 2 NH3HCO3  NaCl  NaHCO3  NH4Cl Insoluble, fitter 2NaHCO3  Na 2CO3  H2O  CO2   Na 2CO3.10H2O 2NH4Cl  Ca OH2  2NH3  H2O  CaCl2 slaked lim e But K2CO3 can’t be prepared by solvay process, because KHCO3 soluble in medium. Uses 1. For softening of hard water 2. As a laboratory reagent 3. Na2CO3 + K2CO3 - fusion mixture 4. Manufacture of glass, paper, borax 5

Brilliant STUDY CENTRE CHEMISTRY - ONLINE CLASS NOTES) 5. Manufacture of washing soap Baking soda (NaHCO3) - sodium bicarbonate 1. Prepared by solvay process 2. Na2CO3  H2O  CO2  2NaHCO3 excess Uses 1. As antacid in medicine 2. As antiseptic powder - sidelitz powder 3. In fire extinguishers 4. In preparation of baking powder Sodium hydroxide (NaOH) - Caustic soda Manufactured by 1. Castner-Kellner process/Mercury cell 2. Nelson cell 3. Nafion cell In the above cells, brine electrolysed . NaCl  H2O electrolysis NaOH H2  Cl2 cathode anode  NaOH - dried converted to pellets Uses of NaOH As a laboratory reagent For purification of Bauxite Manufacture of soap, dyes, drugs etc Biological importance of Na and K Na+ ion in blood plasma - for transmission of nerve signals, Regulate transport of sugars, aminoacids in cells. K+ ion in cell fluids - activate enzymes, oxidation of glucose to ATP GROUP-2 (Outer EC - ns2) They are Be, Mg, Ca, Sr, Ba and Ra Ra - radioactive ; t 1  1600 years 2 6

Brilliant STUDY CENTRE CHEMISTRY - ONLINE CLASS NOTES) All except Be are alkaline earth metals, because Be is rare, its compounds are amphoteric, but compounds of others are basic, less basic than group 1. They exist in earth as carbonates and sulphates. General properties 1. Size and radii less than group 1, so high I.E, but IE values decreases down the group. They form dipositive ion. M  M2  2e But Be exception, small size and high I.E. So Be form covalent compounds Eg : BeCl2. But BeF2  ionic 2. Hydration enthalpy Be2  Mg2  Ca2  Sr2  Ba 2 So their salts exists as hydrated salts. Eg : MgCl2 . 6H2O .............. 3. They are weaker reducing agents than group 1 4. They have strong metallic bonds, so high M.P. 5. They have higher density values Lowest density for Ca = 1.55 g/cm3 6. Flame colouration Be and Mg  no colour to flame due to high I.E. Ca - Brick red; Sr - Crimson; Ba - Pale green/apple green Chemical reactions 1. Reaction with air 2M  O2  2MO; H 0  ve . So thermodynamically unstable f BeO - amphoteric MgO, CaO - basic oxides All MO  H2O  M OH 2 heat BeOH - amphoteric, others are bases 2 Mg(OH)2  Ca OH  ......  Ba OH 2 2 Basic character increases Be / Mg  N2  Be3N2 / Mg3N2 7

Brilliant STUDY CENTRE CHEMISTRY - ONLINE CLASS NOTES) 2. Reaction with H2O Mg / Ca  2H 2O  Mg  OH   H 2 2 hot or Ca (OH)2 Be - no reaction with H2O 3. Reaction with acids Mg / Ca  2HCl  MgCl2 / CaCl2  H2 Mg / Ca  H2SO4  MgSO4 / CaSO4  H2 But Be - react with base (no reaction with acid) Be + 2NaOH  Na2BeO2 + H2 4. Reaction with H2 M  H2  MH2 BeH2 - covalent, all others are ionic CaH2  Hydrolyth Structure of BeH2 - solid HH Be Be Be HH Polymeric structure with bridge bonds 5. Reaction with halogens M  X2  MX2  NH4  BeF4  BeF2  2NH4F 2 BeO  Cl2  C  BeCl2  CO  Structure of BeCl2 Cl Cl 1. Solid : Be Be Be (bridge bonds) Cl Cl 2. Vapour : Cl  Be  Cl at 1200 K (linear) 8

Brilliant STUDY CENTRE CHEMISTRY - ONLINE CLASS NOTES) Cl Be Cl Cl - at a temperature, below 1200 K Be Cl (Dimer) 6. Order of stability of CO32 & SO24 oxo salts BeCO3  MgCO3  CaCO3  SrCO3  BaCO3 BeSO4  MgSO4  ..................  BaSO4  Soluble inH2O Insoluble inH2O All nitrates of group 2 decompose on heating 2Mg  NO3   2MgO  4NO2  O2  2 Anomalous nature of Be Reasons : - Small size, high IE, high EN and no ‘d’ orbitals Properties are 1. Its oxide and hydroxide are amphoteric 2. It has no reaction with H2O 3. It has no reaction with acid but react with base 4. BeCO3 - unstable 5. Be has maximum co-ordination number = 4 due to absence of d orbitals Be Al diagonal relations: Reasons:- Similar size, EN-values same, same polarising power Similarities are:- 1. Their oxide and hydroxide are amphoteric 2. They have no reaction with H2O 3. They react with base, liberating H2 4. Their carbide on hydrolysis give CH4 5. BeCl2 and AlCl3 exist as dimer in vapour state 9

Brilliant STUDY CENTRE CHEMISTRY - ONLINE CLASS NOTES) 6. They become passive in con. HNO3 Difference between Be and Al Be - Maximum co-ordination number = 4 Al - Maximum co-orination number = 6 Na2[BeF4] Na3[AlF6] Be - sp3 Al - sp3d2 Compounds of Ca and Mg Epsom salt/Epsomite MgSO4.7H2O MgCO3  H2SO4  MgSO4  H2O  CO2 cool MgSO 4 .7H 2O Actual formula  Mg H2O6  SO4.H2O This is isomorphous with white vitriol ZnSO4.7H2O USES 1. As a mordent in dyeing 2. As a purgative in medicine 3. For tanning of leather Lime stone-CaCO3 quick lime CaO slaked lime Ca(OH)2, lime water Ca(OH)2 Lime stone - CaCO3 naturally occuring CaCO3  CaO  CO2 heat Quick lime Add CaO  H2O Ca(OH)2  heat slaked lim e Ca(OH)2  filter pasty mass  Clear liquid Ca(OH)2 Ca(OH)2 lime water Ca (OH) 2 CO2 CaCO3  H2O gas lime water white ppt 10

Brilliant STUDY CENTRE CHEMISTRY - ONLINE CLASS NOTES) Ca(OH)2 excess CO2  Ca(HCO3)2  CaCO3  H2O  CO2 white ppt Colourless & soluble USES Lime stone Manufacture of cement, glass As a flux In building industry As a abrasive Quick lime Manufacture of cement, Na2CO3 Purification of sugar Sodalime - Mixture of (NaOH + CaO) Slakes lime Manufacture of glass, white wash Bleaching powder, As mortar in building industry 2Ca(OH)2  2Cl2  CaOCl2  CaCl2  H2O Bleaching powder Lime water Used as laboratory reagent. Gypsum and Plaster of Paris CaSO4.2H2O 393K CaSO4. 12 H2O  1.5 H2O or Gypsum (CaSO4)2 . H2O White powder - plaster of paris Above 400 K CaSO4 -2 H2O Dead burnt plaster (anhydrite) 11

Brilliant STUDY CENTRE CHEMISTRY - ONLINE CLASS NOTES) USES Gypsum used in cement industry Plaster of Paris used for plastering Making statues, interior decoration Cement - Portland cement [Joseph Aspidin] Raw materials Lime stone, Clay and Gypsum added CaO : 50  60%  Ca2SiO4 : 26% SiO2 : 20  25%  Ca3SiO5 : 51% Al2O3 : 5 10% Ca3Al2O6 : 11% MgO : 2  3% Fe2O3 : 2  3% Clinker Clinker powdered Gypsum added  Portland cement 23% Setting When H2O added, hydration takes place and rearrangement - sets to hard mass. [Presence and % of gypsum control the setting lime] Biological Importance of Ca and Mg Mg  act as a co-factor for enzymes - ATP transfer  Present in chlorophyll - for absorption of light Ca  Present in bones and teeth Ca  Control - two hormones - Calcitonin and parathyroid. 12