CHEMICAL.........................................BONDING

Q. On the basis of VSEPR theory predict the shape of Hydronium (H₃O⁺) ion 
Ans. pyramidal

Q.Define formal charge of an atom.

Ans.The formal charge is the difference between the number of valence electrons in an isolated atom and the number of electrons assigned to that atom in a LEWIS STRUCTURE,

Q.Define Bond length& Bond dissociation enthalpy.

Ans. The equilibrium distance between the centre s of the nuclei of two bonded atoms is called bond length.

The energy required to break a bond is called bond dissociation enthalpy.

Q.On the basis of VSEPR theory give the geometry & bond angle of 

(i) BF₃ (ii) SiH₄ 

Ans. Trigonal planar 120°

Tetrahedral 109.5°

Q .What is the geometry of molecule of type AB₄E₂ & AB₃E.

Ans. Square planar & Pyramidal shape

Q (a)State one significance of Formal Charge.

Answer. It helps us to select the most stable structure out of all the different Lewis structures. (b)In nitrate ion what should be total of Formal Charge on each atom? Why?

Ans. The sum of formal charge is always equal to the charge present on the ion. Hence sum of charges in NO₃^- will be -1.

a)        NaCl gives a white ppt. with AgNO₃ but CCl₄ does not. Why?.

Answer. Being an ionic compound NaCl gives Cl negative ion and thus gives White ppt. with silver nitrate but CCl₄ is a covalent compound and thus do not give Cl negative ions.

Q.Dipole moment of NH₃ is greater than of NF₃.?

Ans. Resultant moment of N-H bond adds up to the bond moment of lone pair, that of 3 N-F bonds partly cancels the resultant moment of lone pair. Thus the dipole moment of NH₃ is greater than of NF₃.

Q. Explain why PCl₅ is trigonal bipyramidal and IF₅is square pyramidal?

Ans. PCl₅ has 5 bond pairs of electrons around central atom while in IF₅ there are 5 bond pair & 1 lone pair of electrons around central atom.

Q .Give Reason

(i)N₂ is inert at room temperature.

(ii) Overlapping of S orbital with any other orbital forms sigma bonds.

Ans .Due to high bond order (3) , it has high bond dissociation energy ,which is not possible at room temperature

(ii)Because s orbital has same electron distribution in all axis/ directions

Q Out of NH₃ & BF₃ Which is polar & Why?

Ans. NH₃ has net dipole moment because resultant of 3N-H bond & lone pair lie in the same direction. While BF3 being trigonal planar has zero dipole moment.

Q.When does ionic character develop in covalent bond?

Ans. When the difference in electro negativity of the combining atom is more than 1.9 it develops 50 % ionic character . 

Q  .Account for the following :

(i) AlF₃ is a high melting point solid whereas SiF₄ is a gas?

Ans.AlF₃ is an ionic solid due to large difference in electro negativity of Al and F whereas SiF₄ is a covalent compound and hence only weak Vander Waals force exists between the molecules.

(ii) Out of peroxide ion and superoxide ion which has larger bond length and why?

Ans. The bond orders of(superoxide ion) O₂^- is 1.5 while that of(peroxide ion ) O₂^2- is 1.0 .Smaller the bond order, greater the bond length. Hence, O₂^2- has larger bond length.

Q. What do you mean by canonical structures? Draw canonical structures of Carbonate ion & Benzene. [1+2]

Ans. Sometimes a molecule possess several structures and the actual one is in between of them. This is called Canonical structure.

Q .Define polarizing power and polalrisability,

Ans . The ability of an atom to deform the electron cloud of an atom is called polarizing power . while the ease with which any atom can be polarized by its neighbouring bonded atom is called polarizability.

Q .Why PCl₅  is more covalent than PCl₃ ?

Ans . Because P⁵⁺ has more polarising power than P³⁺.

Q .Why is SF₄ more reactive than SF₆ ?

Ans . SF₄ has two unpaired electrons which it can donate further to extend its valency , hence it is reactive .’S’ in SF₆ has maximum 6 0.S.

Q .Explain the following order of bond angle ,
NO2+		NO2		NO2—
Ans. This is because NO + has no lone pair of e-, and hence its linear.NO						has one unshared e- while
	>	2	>		2		-	than in case of NO2.
NO2 has one unshared e- pair.Greater the repulsion on N-O bond in case of NO2

Q . Why is CO₂ a linear molecule while SO₂ a non-linear molecule?

Ans. In SO₂ molecule there are two bond pairs and one lone pair of electron .Due to repulsion by lone pair its bond angle decreases from 120° to 119.5° hence it is not linear whereas in CO₂ molecule there is no lone pair of electron due to which it has linear geometry.

Q . Why He₂ molecule does not exist?

Ans. It has been found that when two helium atoms approach each other four new attractive forces and five new repulsive forces come into play. Therefore repulsive forces predominate and the potential energy of the system increases which leads to instability. Or Bond order is Zero.

Q Explain the formation of H₂ molecule on the basis of valence bond theory.

Ans. When two H atoms come closer the following forces operate between them: a. Attractive :

i.nucleus of atom and its electron

ii.nucleus of one atom and electron of other. b.repulsive :

i.electrons of two atoms. ii.nucleus of two atoms.

It has been found experimentally that magnitude of attractive forces is more than repulsive forces hence two atoms come closer to each other and the potential energy of the system decreases. A stage is reached when attractive forces balance repulsive forces and the system acquires minimum energy .At this stage two hydrogen atoms are said to be bonded together

Q. i) Why is bond angle in PH₄⁺ higher than that in PH₃?

ii) Arrange the bonds in order of increasing ionic character in the molecules: (L3) LiF, K₂O, N₂, SO₂ and ClF₃.

Ans- N₂ < SO₂ < ClF₃ < K₂O < LiF.

Q4 i) Explain why BeH2 molecule has a zero dipole moment although the Be–H bonds are polar.

 Ans. The Lewis structure for BeH2 is as follows:

There is no lone pair at the central atom (Be) and there are two bond pair. It has a linear structure. Dipole moments of each H–Be bond are equal and are in opposite directions.

Therefore, they nullify each other and the BeH₂ molecule has zero dipole moment.