Chemical Bonding and Molecular Structure

Question 1

Fajan’s rules

Answer 1

1. Smaller the size of the cation and greater the size of the anion, greater is the extent of polarisation
2. Larger the charge on the cation and anion, greater is the extent of polarisation in an ionic compound
3. A cation with outer electronic configuration of s²p⁶d¹⁰ possesses a greater polarising power as compared to the cation having the same size and same charge but having outer electronic configuration s²p⁶
4. Higher the dielectric constant of the medium, lesser is the extent of polarisation in a compound

Question 2

Hydrogen bond

Answer 2

The weak attractive force which binds the partially positively charged hydrogen atom of one molecule, with the partially negatively charged atom of some other molecule of similar or different type, or with some other negative centre of the same molecule; is referred to as hydrogen bond or hydrogen bonding

Question 3

Conditions for hydrogen bonding

Answer 3

1. High electronegativity of the atom bonded to hydrogen: higher the electronegativity of the atom bonded to hydrogen, greater is the strength of the hydrogen bond. For example F, O or N
2. Small size of the electronegative atom: If the electronegative atom is large, the attraction on the bonded pair will be less.
   For example, N and Cl possess the same electronegativity value but only NH3 shows hydrogen bonding while HCl does not, due to large size of the Cl atom

Question 4

Intermolecular hydrogen bonding- definition

Answer 4

When hydrogen bonding exists between two molecules of the same or different substances, it is known as intermolecular hydrogen bonding.

Question 5

Why does ice float on water?

Answer 5

1. In the lattice of ice, H2O molecules are not packed closely, and vacant spaces exist between the molecules.
2. This is why, for a given mass, ice occupies more volume than the liquid water.
3. Consequently, the density of ice is less and it floats over water.

Question 6

Why is the maximum density of water at 4 degrees celsius and not 0 degree celsius?

Answer 6

On heating ice, the hydrogen bonds break up progressively, and the molecules start packing more closely in the vacant spaces

Question 7

Intramolecular hydrogen bonding

Answer 7

When hydrogen bonds exist within the same molecule, it is termed as intramolecular hydrogen bonding.

Question 8

Influence of hydrogen bonding on the molecular state

Answer 8

Two or more molecules of a substance may get associated resulting in the formation of clusters of molecules

Question 9

Influence of hydrogen bonding on the melting and boiling points

Answer 9

The compounds containing intermolecular hydrogen bonding possess abnormally higher melting and boiling points. This is because a large amount of energy is required to break intermolecular hydrogen bonds.

Question 10

Influence of hydrogen bonding on solubility in water

Answer 10

Compounds whose molecules are able to form hydrogen bonding with water are soluble in it

Question 11

VSEPR Theory postulates

Answer 11

1. the geometry and shape of the molecule is dependent on the number of valence shell electron pairs present on the central atom
2. the valence shell electron pairs repel one another and try to remain in space as far apart as possible to acquire a state of maximum stability
3. the repulsive interactions are different and follow the order: lone pair-lone pair > lone pair-bond pair > bond pair-bond pair

Question 12

Why are repulsive interactions between bond pairs minimum?

Answer 12

This is because bond pairs are sandwiched between two atoms and are unable to undergo repulsive interactions to a larger extent.

Question 13

Why are repulsive interactions between bond pairs maximum?

Answer 13

The lone pairs are attached to one atom only and are comparatively more free to interact.

Question 14

Bond angle in:
1. BeF2
2. BF3
3. CH4
4. NH3
5. H2O

Answer 14

1. 180°
2. 120°
3. 109°28’
4. 107°
5. 104.5°

Question 15

Shape of NH3

Answer 15

Tetrahedral

Question 16

Shape of H2O

Answer 16

Tetrahedral