Inorganic Chemistry Reasoning Questions

Here are some FAQs of inorganic reasoning for PMT/NEET/IITJEE

  1. Why is density of Potassium less than Sodium?
    • Poor packing of Potassium crystals
  1. Lithium has most negative standard reduction potential?
    • Very small size and very high hydration energy
  1. Why do alkali metals show flame colouration?
    • Low Ionisation energy
  1. Why is solution of alkali metals in liquid ammonia blue coloured?
    • Due to presence of ammoniated electrons
  1. Why do lower members of alkali metals form peroxides and super oxides when heated in air?
    • Large cations are stabilised by large anions(peroxide and superoxide anions).
  1. Why does Potassium reacts faster with water than Sodium or Lithium
    • As we go down the group MP dec and the reaction is exothermic, hence, heavier metals melt faster and the surface area increases therefore rate of reaction increases.
  1. Why are halides of Lithium covalent?
    • small size of Li, higher polarisation.
  1. Why is LiCl soluble in organic solvents?
    • Covalent character.
  1. Why is Be not considered as alkaline earth metal?
    • It is amphoteric and not alkaline
  1. Why do Mg and Be not show flame colouration?
    • Higher ionisation energy
  1. Why can’t we prepare Potassum bicarbonate by Solvay’s process?
    • KHCO3 is very soluble hence doesn’t precipitate out.
  1. Why are BeCl2 covalent?
    • Large polarisation effect. exists as polymer.
  1. Why are halides of alkaline earth metals hydrated?
    • Small size of alkaline earth is balanced by water of crystallisation.
  1. Why is BeCO3 is stored in atmosphere of CO2?
    • Very low thermal stability.
  1. Why is CaCl2 added to NaCl during extraction of Sodium?
    • To decrease MP of NaCl. (It decreases the fuel consumption to melt NaCl and Cl2 and Na produced become easier to handle at lower temp)
  1. Why is MP of Gallium very low?
    • Gallium exists as discrete Ga2 molecules which have VanderWall forces among them.
  1. Why is MP of Boron very high?
    • Exists as icosahedral crystals with each B making 3 strong covalent bonds.
  1. Why AlCl3 a covalent molecule becomes ionic in aqueous medium?
    • High hydration energy of Al3+ ions provide for bond dissociation energy.
  1. BCl3 doesn’t exist as dimer whereas AlCl3 exists as Al2Cl6
    • B can’t accommodate 6 large sized halogens around it due to small size of B.
  1. Aluminium doesn’t react with conc sulphuric acid although it is highly electropositive element?
    • It forms a protective layer fo Al2O3 over its surface.
  1. Borax is used in analytical chemistry. Why?
    • On heating Borax, it gives B2O3 and NaBO2, which give colour with transition metals. (Borax bead test)
  1. Boric acid is slippery to touch in solid state?
    • In solid state it exists as planar molecule due to H-bonding.
  1. Boric acid is weak mono basic lewis acid. Explain.
    • H3BO3 acts as acid by accepting a lone pair from the water in aqueous medium and not be donating a H+ ions of its own.
  1. Ionisation energy of Pb is more than that of Sn. Why?
    • Due to poor shielding effect of 4f orbitals of Pb.
  1. Why is MP of Pb more than that of Sn?
    • Due to poor shielding effect of 4f orbitals of Pb, it has stronger metallic bond.
  1. SiO does not exists?
    • Si being large can’t make multiple bonds and also doesn’t show inert pair effect.
  1. CO2 is a gas while SiO2 is solid. Why?
    • SiO2 exists as 3-D giant polymeric molecule. With each Si making 4 strong covalent bonds. Whereas CO2 exists as discrete small molecules which have weak vanderWall attractions among them.
  1. PbI4 and PbBr4 does not exist?
    • Pb can not accommodate 4 large sized Br- or I- around it.
    • Br and I are good reducing agents which readily reduce Pb4+ to Pb2+.
  1. SnCl4 is liquid while SnCl2 is solid?
    • SnCl4 is covalent while SnCl2 is ionic(Fazan’s rule)
  1. SnCl2 is reducing in nature while PbO2 is oxidising in nature?
    • The most stable oxidation state of Sn is +4 where as that of Pb is +2(due to inert pair effect).
  1. CCl4 is not hydrolysed easily, where as SiCl4 and halides of other members are hydrolysed easily. Why?
    • C doesn’t have vacant d orbitals hence can’t accept lone of oxygen of water, whereas Si and other members have vacant d orbitals and can accept lone pair of oxygen and can extend their coordination number beyond 4.
  1. Trimethyl amine is basic and pyramidal, whereas trisilyl amine is not basic and planar. Why?
    • In case of trim ethyl amine, lone of Nitrogen is localised and the N is sp3 hybridised, whereas in trisilyl amine, lone pair of N is dispersed over Si hence N becomes sp2 hybridised and planar is shape. As the lone pair is delocalised, it is quiet less basic.
  1. Nitrogen is a gas while phosphorus is a solid. Why?
    • Nitrogen exists as diatomic molecule while Phosphorus exists as tetratomic or polymer. So the intermolecular forces are very less in case of Nitrogen. (P can’t make multiple bonds hence no P2 )
  1. Ionisation energy of nitrogen family is greater than oxygen family. Why?
    • Half filled configuration of nitrogen family.
  1. Nitrogen has lesser tendency to show catenation than P. why? (or O vs S)
    • N-N bond strength is less than that of P-P. (Similarly O-O < S-S)
  1. Nitrogen doesn’t form pentahalides as phosphorous does. Why?
    • Nitrogen doesn’t have vacant d orbitals hence can’t extend its coordination number beyond 4.
  1. Why is Calcium phosphide used in Holme’s signal?
    • When Ca3P2 reacts with water it gives out diphosphene in small quantity along with phosphene. P2H4 catches fire spontaneously in air, making PH3  and C2H2  burn along with it.(C2H2 is produced from CaC2 )
  1. Why does PCl5 readily decompose?
    • Two axis bonds in PCl5 are longer and weaker. These break easily. Hence, PCl2 readily decomposes to PCl3 + Cl2.
  1. Why does PCl5 appear ionic in solid state?
    • PCl5 exists as [PCl4]+[PCl6] in solid state. It is due to this that it appears ionic in nature.
  1. Why does NO2 becomes colourless and diamagnetic upon long standing?
    • NO2 has an odd electron, hence it dimerises to give N2O4 which is colourless and diamagnetic.
  1. Why does old samples of Nitric acid appear yellow?
    • HNO3 decomposes in presence of sunlight to give NO2 which dissolves in nitric acid. NO2  being brown in colour, upon dissolution imparts yellow colour to nitric acid.
  1. Why does basic character of hydrides decreases down the group? (NH3, PH3, AsH3…etc)
    • As we go down the group size of atom increases, so the lone pair electrons get dispersed over a larger area and get stabilised. Hence the tendency to donate a pair of electron decreases.
  1. Sulphur shows paramagnetic behaviour is vapour phase?
    • In vapour phase, sulphur exists as S2 molecules which have 2 unpaired electrons just as O2 .
  1. Acidic strength of Hydrides of Oxygen family increases down the group. Why? (H2O, H2S, H2Se …etc)
    • Bond length increases down the group so bond dissociation energy decreases.
  1. Why are halogens coloured?
    • The last electrons in the halogens absorb energy from the visible spectrum and get excited to higher levels. The reflected light shows the complementary colours.
  1. Why is electron affinity of Cl more than that of F?
    • F being too small, the incoming electron faces repulsions from existing electrons, whereas size of Cl is optimum to attract the incoming electron with minimum repulsions.
  1. Bond dissociation energy of F2 is quiet low. Why?
    • F being too small, there exist a repulsion between the nuclei of two F atoms.
  1. In interhalogen compounds F is not a central atom. Why?
    • F being most electronegative element, can not show positive oxidation states.
  1. BrO3 is most oxidising among halogen oxides. Why?
    • Middle row anomaly.
  1. Why is HF a weak acid?
    • High bond dissociation energy
    • Strong H-bonds
  1. Why don’t He and Ne make compounds?
    • High ionisation energies
  1. Why is H2O2 not stored in glass bottles?
    • Glass is made of basic substances which catalyse the decomposition of H2O2.
  1. Why are old paintings are washed with H2O2 solution?
    • Old paintings have Pb which turns black by atmospheric H2S forming black PbS. When washed with H2O2 solution, PbS is converted to PbSO4 which is white.
  1. Atomic sizes doesn’t decrease much after midway in d-block (left to right). Why?
    • Increase in effective nuclear charge is almost offset by increase in screening effect of doubly filled d-orbitals.
  1. Third ionisation energy of Fe is less than that of Mn. Why?
    • Mn loses half filled configuration on third ionisation whereas Fe gains half filled configuration after third ionisation.
  1. Why there is dip in MP of first and second transition series at Mn and Tc?
    • Half filled configurations are more stable and less available for delocalisation, hence the metallic bond strength is less.
  1. Why does MP of transition elements first increase then decrease?
    • Strength of metallic bond depends upon unpaired electrons, which first increase then decrease as we go left to right.
  1. Sizes of 3rd and 4th transition series are almost same. Why?
    • Lanthanoid Contraction.
  1. Why does basic character of hydroxides of lanthanoids decreases?
    • Covalent character increases due to lanthanoid contraction.
  1. Why do Hf and Zr exist together in nature?
    • Similar sizes (differ only by 1pm)
  1. Why do Ni2+ compounds are more stable than Pt2+ while Ni4+ compounds are less stable than Pt4+?
    • Sum of first two ionisation energies of Ni is less than that of Pt.
    • Sum of first four ionisation energies of Pt is less than that of Ni.
  1. Why is Cr2+ reducing and Mn3+ oxidising when both have d4 configuration?
    • Cr2+ is reducing as its configuration changes from d4 to d3, the latter Solution having a half-filled t2g level. On the other hand, the change from Mn3+ to Mn2+ results in the half-filled (d5) configuration which has
    • extra stability
  1. Why is KMnO4 Coloured? (or K2Cr2O7)
    • Due to charge transfer
  1. Why is Mn2+ is almost colourless?
    • Half filled configuration is quiet stable hence, d-d transition is almost absent. (it is very very slightly green coloured).
  1. Why do transition elements act as catalysts?
    • Provide large surface area for adsorption thereby increasing concentration and wearing the bonds
    • Ability to show variable oxidation states hence change the path of reaction.
    • Ability to form complexes thereby releasing large energies for activation energy.
  1. Why is Na2Cr2O7 is converted to K2Cr2O7?
    • Na2Cr2O7 is hygroscopic in nature, hence can not be used as primary standard, where as K2Cr2O7 is not hygroscopic and can be used as primary standard.

Here is the downloadable file:   Inorganic reasoning

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