Class 11 Chemistry KVS Marking Scheme 2022 2020 2021

23. (i) CaCO3 (s) -> CaO (s)  + CO2 (g) 1 mark
      (ii) Because the reaction is reversible. As CO2 is formed it combines with CaO  and CaCO3  is reformed    CaO(s)  + CO2(g) -> CaCO3(s) 1 mark
    (iii) When the top of the chimney was kept opened the gas CO2 escaped and CaO was left .Thus the reaction continued in the forward direction. 1 mark
  (iv) Awareness of chemical principles is helpful in industry. 1 mark
24. (a) Correct statement  1 mark
Correct definition  1 mark
       (b)  6C + 3H2 -> C6H6      ∆fHO=?              (1)   ½ mark
C6H6 + 15/2 O2 -> 6CO2 + 3H2O   ∆cHo = -3267 KJ mol-1     (2)        ½ Mark
C + O2  -> CO2   ∆ fHO    = -393.5 KJ mol-1 (3)        ½ mark
H2+ ½  O2 -> H2O fHO  = -285.83 KJ mol-1 (4)     ½ mark
Eqn   (3) x 6  + eqn (4) x3  - eqn (2) . we get  1 ½ mark
6C + 3H2  -> C6H6  , Δ fHO  = 48.51  KJ mol-1 ½ mark

OR
Free Energy –  it is the net energy available to do useful work .     1 mark
Bond Enthalpy – Its is the change in enthalpy when one mole of covalent bond of a gaseous covalent compounds is broken to form products in the gas phase.                          1 mark
∆GO = -2.303 RT log K                                                                                                  1 mark
= - 2.303 x 8.314 J K-1 x 300 K x log 10                                   1 mark
=55.27  KJ mol -1                                                                                         1mark
25. (i)  (a)    Due to small size and high electronegativity , carbon forms  Pπ- Pπ double   bonds with O- atoms to form CO2 molecules .These molecules are held together by weak vander waals forces.
Du to larger size & lower electronegativity than carbon silicon forms four  single covalent bonds with O-atoms which are tetrahedrally arranged around it leading to the formation of a three dimensional network structure.
( b)  Diamond is the hardest substance.
(c )  on a weight to weight basis .The electrical conductivity of aluminum is twice that of  copper.                                                                                                                               
(ii )  (a) 2BF3 + 6Li H  -> B2H6  +  6 LiF
       ( b) B2H6 + 6H2O -> 2H3BO3 + 6 H2                                                                                                                                                                                                                                                            1*3+1*2=5 mark

OR
(a) H3BO3 - -- Δ(  HBO2  -Δ--(B2O3
      (b ) Al2O3. 2H2O (s)  + 2NaOH (aq) (  2NaAlO2  + 3H2O
  Sodium metal aluminate
                   (c )  CO + ZnO ->  Zn + CO2
Inert pair effect - The reluctance of ns2 electron pair to participate in bond formation. Catenation – The self-linking property of atoms of an element.
26.  (i)  (a)  Correct Chemical reaction 1 mark
    (b) Correct Chemical reaction 1 mark
(c ) Correct Chemical reaction 1 mark