5.4 chemical bonding 🫂

Question 1

explain why all chemical bonds are electrostatic in nature

Answer 1

• ionic bonds are strong electrostatic attraction between oppositely charged ions
• covalent bonds are electrostatic attraction between the shared pair of electrons and the positively charged nuclei (protons inside nucleus)
• metallic bonds are strong electrostatic attraction between a lattice of cations and sea of delocalised electrons

Question 2

what does the strength of metallic bond depend on?

Answer 2

no. of valence electrons aavailable for delocalisation per atom / size of metal cation

Question 3

what does the strength of ionic bond depend on?

Answer 3

lattice energy

Question 4

define lattice energy

Answer 4

lattice energy is the energy released when one mole of the solid ionic compound is formed from its constituent gaseous ions

Question 5

what does the magnitude of lattice energy depend on?

Answer 5

q+q- / (r+)+ r- (cation and anion radius)

Question 6

define sigma and pi bond

Answer 6

sigma bonds are a head-on overlap of orbitals with 1 region of overlap

pi bonds are a side-on overlap of orbitals with 2 regions of overlap (formed only when a sigma bond already exists)

Question 7

define dative / coordinate bonds

Answer 7

electrostatic forces of attraction that exist between the donor and acceptor atoms nuclei and the shared pair of electrons

Question 8

define electronegativity

Answer 8

electronegativity is the ability of an atom to attract a shared pair of electrons towards itself in a covalent bond

Question 9

how does a polar bond arise?

Answer 9

the pair of electrons are not equally shared between the two atoms, giving rise to a permanent dipole in the bond, making the bond polar

Question 10

define bond (dissociation) energy

Answer 10

the amount of energy needed to break one mole of covalent bonds in gaseous state under standard conditions

Question 11

factors affecting strength of covalent bond

Answer 11

1) number of bonds between atoms
2) effectiveness of orbital overlap (atomic radius) and bond length
- larger atomic radius, larger orbitals that are more diffused, less effective orbital overlap, bond length longer, weaker bond
3) polar VS non-polar
- increased electrostatic force of attraction between oppositely charged dipoles

Question 12

describe instantaneoous dipole-induced dipole (id-id) interactions and how it arises

Answer 12

id-id refers to the electrostatic attraction that exists between oppositely-charged poles of temporary charges in molecules.
it arises from momentary polarisation of the electron cloud at any instant when the molecules are in continual, random motion

Question 13

factors affecting strength of id-id

Answer 13

1) total number of electrons in an atom
- greater number of electrons, larger size of electron cloud, greater momentary polarisation, stronger id-id
2) shape of molecules (organic molecules only)
- linear molecule has greater surface area than a branched molecule with same number of atoms to facilitate a greater degree of id-id

Question 14

describe permanent dipole-permanent dipole (pd-pd) interactions and how it arises

Answer 14

pd-pd refers to the electrostatic forces of attraction between oppositely-charged permanent dipoles of polar molecules.
it arises due to differences in electronegativity of the atoms and the dipole moments do not cancel out, resulting in a net dipole moment.

Question 15

describe hydrogen bonds and how it arises

Answer 15

hydrogen bonds refers to the electrostatic forces of attraction involving the hydrogen atom from a H-F/O/N covalent bond and a lone pair from F/O/N of another molecule.
It arises because F/O/N is small and highly electronegative, leading to a very polar F/O/N - H bond . electrostatic forces of attraction form between the poorly shielded hydrogen atom on one molecule and a lone pair of electrons from the F/O/N atom in another molecule.

Question 16

effect of hydrogen bonds on physical properties

Answer 16

1) higher mp/bp
2) anomalous Mr due to dimerisation
3) difference in isomers due to intra and inter-molecular hydrogen bonding

Question 17

why are most ionic compounds soluble in water?

Answer 17

in water, strong ion-dipole interactions between ions and water (solvent) molecules and this process releases sufficient energy for the detachment of ions from the ionic lattice for solvation

Question 18

why are certain ionic compounds not soluble in water?

Answer 18

lattice energy of the ionic compound is too exothermic, hence energy released in the formation of ion-dipole interactions between ions and water molecules is insufficient to overcome the strong ionic bonds

Question 19

what unique property of water is a result of hydrogen bonding?

Answer 19

ice (water in solid state) is less dense than water in liquid state because of its open lattice structure

Question 20

why can atoms period 3 and above expand the octet configuration?

Answer 20

availability of vacant low-lying d orbitals to accomodate electrons for chemical bonding

Question 21

steps to draw dot-cross

Answer 21

1. identify central atom (first in formula, least electronegative, not H and O)
2. for polyatomic ions, negative charge extra electron to most electronegative atom; positive charge one less electron of least electronegative atom
3. make the surrounding atoms have stable octet configuration
4. octet rule can be flouted only for period 3 and above elements (can have less than 8 also)
5. dative bond for atoms with more than 8 electrons

Question 22

explain the VSEPR (valence shell electron repulsion) theory

Answer 22

the VSEPR theory states that electron pairs are arranged around the central atom in a molecule as far apart as possible so as to minimise repulsion between them.
bond pair-bond pair > bond pair-lone pair > lone pair-lone pair

Question 23

explain why the shape of the molecule is [trigonal pyramidal]

Answer 23

the 4 electron pairs around nitrogen are directed in a tetrahedral arrangement to minimise repulsion. Since there are 3 bond pairs and 1 lone pair of electrons, ammonia is trigonal pyramidal in shape.

Question 24

how do you determine whether a MOLECULE is polar?

Answer 24

1) there are polar bonds present in the molecule
2) dipole moments do not cancel off because bonds are asymmetrical (and surrounding atoms are not all the same), hence there is a net dipole moment

Question 25

what does extent of ionic character in covalent bond depend on?

Answer 25

increases with increased electronegativity difference between the covalently bonded atoms

Question 26

how does covalent character in an ionic bond arise?

Answer 26

it arises from the polarisation of electron cloud, involving distortion of the anion electron cloud by a neighbouring cation. this induces covalent character in the ionic bond.

Question 27

what are other factors affecting bond angles (aside from shape?)

Answer 27

1) electronegativity of central atom: a more electronegative central atom attracts the shared pair of electrons more closely to itself, increasing bp-bp repulsion hence bond angle increases
2) electronegativity of surrounding atoms: electronegative surrounding atoms attract the shared pair of electrons away from central atom, decreasing bond angle
3) size of atoms bonded to central atom: larger atoms could experience mutual repulsion of electron clouds that can result in a larger bond angle

Question 28

factors affecting extent of covalent character in the ionic bond

Answer 28

1) polarising power of the cation = charge density = charge / cationic radius
2) polarisability of anion = size of anion