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Friday, 9 November 2018

Chemical properties of Gr. 15 elements

Chemical properties of Gr. 15 elements


 Chemical properties:
i) Action of air: 
         All elements react with oxygen to form oxides. Reactivity with oxygen increases down the group. 
                         N₂ + O → 2NO

ii)Action of oxidizing agents: 
   a) oxidizing agent i.e. hot concentrated Nitric acid not reacts with N but reacts with P, As, gives oxy acids, with the Sb gives oxide & with Bi gives nitrate.
 
                    P₄  +  20 HNO₃  →  4HPO       +    20 NO   +   4HO
                                             (Phosphoric acid)
                   
                   As  +  20 HNO   4H AsO   +   20 NO  +   4HO
                                              (Arsenic acid)
 
                  4Sb + 20 HNO  →   Sb O₁₀     +   20 NO   +  10H
                                                  (Antimony Oxide)
\
                      Bi +  6HNO Bi (NO)₃   +   3HO
                                             (Bismuth nitrate)
 
b)Oxidizing agent i.e. hot conc. sulphuric acid not reacts with nitrogen but reacts with P & As gives oxy acids with Sb & Bi gives sulphate.                     P+ 10HSO₄     4HPO + 10SO + 4HO

                2Sb  +  6HSO      Sb (SO ) + 3SO+  6HO 

               As   + 10 HSO  4H AsO + 10SO₂  +  4HO

                2Bi   +  6HSO  →  Bi(SO)   +  3SO₂  +  6HO

Friday, 2 November 2018

Trends in physical properties

Trends in physical properties

(Atomic & ionic radii, density, electronegativity, etc. ) 

                   
                     ↓ indicates  down the groups in periodic table.


 Ionization enthalpy:
         Ionization enthalpy of Gr.15 elements decrease down the group due to increase in atomic size. But it is more than Gr.14 & 16 elements due to extra stability of half filled p-orbital & relatively smaller size of Gr.15 elements. First three ionization enthalpies are in order ofΔH₁<ΔH<ΔH.

Atomic volume & melting points:
        Down the group atomic volume increases. P & As has low atomic volume & P has large atomic volume than As due to weak van der Wals force of attraction. In As atomic volume is lower than P because it has higher van der Wals force of attraction. Down the group melting point not regularly increases but boiling point increases regularly.

Oxidation State: 
          Common oxidation states are +3, +5, & -3 due to valence electrons in electronic configuration ns² np³. Oxidation state down the group increases +3 to +5 then decreases to +3 due to inert pair effect.           N cannot form +5 oxidation state or compound like NF₅ or NCl due to absence of d–orbital. While other elements can form i.e. PCl, etc. Nitrogen also shows negative oxidation state & positive oxide state.


NEET 2018
Q: The correct order of N-compounds in its decreasing order of oxidation states is
    (1) HNO₃, NO, N, NHCl
    (2)
HNO₃ , NO, NHCl , N
    (3) HNO₃ , NHCl, NO, N
    (4) NHCl, N, NO, HNO₃

 Ans:  (1) HNO₃, NO, N, NHCl

       HNO₃ = (+1)+ N+ (-2)3 =0     
                                          N = +5
                                                                      
        NO = N+ (-2) = 0        
                          N = +2

        N=   N= 0        

        NHCl = N+(+1)4+(-1) = 0 
                                         N = - 3 

AIIM 2017:
Q: Which of the following fluorides does not exist?

     (a) NF
           (b) PF         (c) AsF              (d) SbF 

Ans: (a) NF          
                 N does not form pentahalides due to the absence of d-orbital in its valence shell. While P, As and Sb form pentahalides due to the presence of vacant d-orbitals in their respective valence shell.

Atomic & ionic radii or size, density, electronegativity, etc.

For Gr. 17:
Electron gain enthalpy:
           Electron gain enthalpy of halogen elements are negative due to maximum heat released, on moving down the group it becomes less negative. But electron gain enthalpy of fluorine is less than chlorine is due to small atomic size (2p) & electronic repulsion in F.          The order is Cl>F>Br>I 
 

Oxidation States:
          Halogen has valence electronic configuration ns²np
, to achieve stable Nobel gas electronic configuration (ns² np⁶), halogen requires 1ē. Hence common oxidation state is –1.  
            F only shows –1 oxidation state due to absence of d–orbital and other elements shows –1 to +7 oxidation state. Cl & Br also shows +4 & +6 oxidation state in their oxides & oxyacid form. Higher variable oxidation states are due to excitation of electrons from s / p–orbital to empty d–orbital for bond formation. 

NEET 2018:
Q. Which of the following statements is not true for halogens?
    1) All form monobasic oxyacids.
    2) All are oxidizing agents.
    3) All but fluorine show positive oxidation states.
    4) Chlorine has the highest electron-gain enthalpy.

Ans: 3) All but fluorine show positive oxidation states. 
 
For Gr.18:
 Atomic radius:
           Atomic radii of gr.18 are larger than gr.17. Because of electronic repulsion between completely filled electrons in valence shell of noble gases. Also moving down radii increases due to addition of new shells.

Electron gain enthalpy:
        It has large positive value of electron gain enthalpy because of stable electronic configuration they do not accept the electrons.