chemical bonding(2)

Question 1

define intermolecular forces of attraction

Answer 1

they are electrostatic forces of attraction which arise from interactions of the net dipole of simple molecules.

they are generally weaker than interatomic bonds.

Question 2

define instantaneous dipole-induced dipole interactions

Answer 2

they are the electrostatic forces of attraction that exist between oppositely-charged poles of temporary dipoles in molecules/

Question 3

state how id-id interactions are formed.

Answer 3

1) as molecules are in constant random motion, the electron cloud of each molecule is distorted and unsymmetrically distributed at any instant.

2) the electron cloud is momentarily polarised, resulting in the formation of instantaneous dipoles.

3) a molecule which has an instantaneous dipole can induce a temporary dipole on its neighbouring molecules upon close approach.

4) the interaction between the molecule and its closest neighbours is known as instantaneous dipole-induced dipole interactions.

Question 4

state the factors affecting the strength of id-id interactions.

Answer 4

1. total no. of electrons in a molecule/atom
   the greater the total no. of electrons present, the** larger the size of the electron cloud.** this leads to greater momentary polarisation of the electron cloud, forming** stronger** instantaneous dipole-induced dipole interactions.
2. shape of molecule:
   for 2 molecules with the same no. of atoms, a linear molecule has greater surface area than a branched molecule, resulting in stronger id-id interactions.

Question 5

define permanent dipole-permanent dipole interactions

Answer 5

they are the electrostatic forces of attraction that exist between oppositely-charged permanent dipoles of polar molecules.

Question 6

state the rule of thumb for pd-pd interactions

Answer 6

for molecules with similar Mr, the strength of pd-pd interactions is relatively stronger than id-id.

if it is different Mr, compare id-id by seeing which molecule has more electrons.

the difference in strength of id-id interactions is a more improtant factor than presence of pd-pd interactions.

Question 7

define hydrogen bonds

Answer 7

they are electrostatic forces of attraction between a H atom already covalently bonded to a small, highly electronegative atom(F/O/N) and the lone pair of electrons on a second small, highly electronegative atom(F/O/N)

hydrogen bonds can either be intermolecular or intramolecular

generally, hydrogen bonds are stronger than pd-pd interactions and id-id interactions.

must know how to draw hydrogen bond

Question 8

how to find average number of hydrogen bonds per molecule

Answer 8

take the smallest no.

Question 9

how to use hydrogen bonding to find out which atom/molecule has a higher boiling/melting point?

Answer 9

more extensive hydrogen bonding will lead to more energy required to overcome stronger hydrogen bonds, leading to higher boiling point.

Question 10

how do hydrogen bonds result in ice having an open but rigid structure?

Answer 10

1) hydrogen bond formation causes water to expand when it freezes
2) the orientiation of water molecules needed to maximise the intermolecular hydrogen bonds results in ice having an open but rigid structure.

Question 11

how do hydrogen bonds result in ice being less dense than water

Answer 11

1. in the solid state, the 2 lone pairs of electrons on O atom and the 2 H atoms in each water molecule results in a regular tetrahedral structure where the water molecules are held together by hydrogen bonds.
2. this results in an open structure where water molecules are further apart in the solid state than when in the liquid state.
3. this results in ice being less dense than water.

when ice melts, some of these hydrogen bonds are broken and the sturcture becomes less regular. the water molecules can now pack more closely together and hence, the liquid has a higher density at 0 degree celcius.

Question 12

define the dimerization and state the conditions for it

Answer 12

dimerization as a result of hydrogen bonding between 2 organic acid molecules, leading to the formation of a dimer where the organic acids have an apparent relative molecular mass that is double than expected.

conditions are:
organic acids in the gas phase on in non-polar organic solvents

Question 13

why does dimerisation not occur in water.

Answer 13

when organic acids are dissolved in water, dimerization does not occur as hydrogen bonds are formed between the acid and the more abundant water molecules instead.

Question 14

how does hydrogen bonding affect physical properties.

Answer 14

for isomers, the difference in extents of intermolecular and intramolecular hydrogen bonding will result in a difference between their physical properties such as boiling point and solubility.

isomers are compounds having the same molecular formula but different structural formula.

Question 15

what is the criteria for a substance to be soluble?

Answer 15

it is only soluble when the strength of the solute-solvent interactions is greater than or comparable to that of the solute-solute and solvent-solvent interactions, where energy released from the formation of solute-solvent interactions is sufficient to overcome the solute-solute and solvent-solvent interactions.

Question 16

when will a substance be insoluble?

Answer 16

a substance is insoluble in a solvent if the strength of solute-solvent interactions is much less than that of the solute-solute and solvent-solvent interactions. Energy released from the formation of solute-solvent interactions is insufficient to overcome the solute-solute and solvent-solvent interactions. ,

Question 18

define what a giant molecular lattice structure is

Answer 18

it consists of a lattice of atoms held together by strong covalent bonds in an extensive network.

prominent examples include Diamond, silicon and silicon dioxide.

Question 19

explain why a GMLS has high melting/boiling point.

Answer 19

Large amount of energy is needed to overcome the strong covalent bonds between atoms in a giant extensive network.

Question 20

explain why a GMLS is not a conductor of electricity in any state

Answer 20

it has an absence of free mobile ions or delocaised electrons to conduct electricity as all valence electrons are used for covalent bonding.

Question 21

explain why a GMLS is insoluble in both water and non-polar solvent

Answer 21

the energy released in the formation of solute-solvent interactions is insufficient to overcome the strong covalent bonds in the giant molecular structure.

Question 22

explain why a GMLS is hard, strong and non-malleable.

Answer 22

atoms are held together by strong covalent bonds in a giant extensive network.

Question 23

define a giant molecular layered sturcture

Answer 23

it consists of atoms held together by strong covalent bonds in giant extensive flat parallel layers with the adjacent layers held together by weak instantaneous dipole-induced dipole interactions.

Question 24

why is there a non-bonding valence electron in each C atom?

Answer 24

Within each layer, each C atom is convalently bonded to 3 other C atoms in a trigonal planar manner.
Each C atom has 1 non-bonding valence electron which becomes delocalised over the plane of layer due to overlapping p orbitals.

Question 25

why does a GMLLS have high melting/boiling point?

Answer 25

A **large amount of energy** is needed to overcome the strong covalent bonds between atoms in the **giant extensive flat parallel layers** during melting/boiling.

Question 26

why is a GMLLS a good electrical conductor parallel to the layer?

Answer 26

the **non-bonding valence electrons** ofthe carbon atoms are **delocalised over each plane of layers** and function as **charge carriers** to conduct electricity.

Question 27

why is a GMLLS not an electrical conductor perpendicular to the layers?

Answer 27

there is **an absence of free mobile ions or delocalised electrons** to conduct electricity.

Question 28

why is a GMLLS generally insolublr in both water and non-polar solvents?

Answer 28

**weak interactions** formed between graphite and water/non-polar solvents molecules **release insufficient energy** to **overcome strong covalent bonds** in the giant molecular structure.

Question 29

explain why a GMLLS is soft and slippery.

Answer 29

As **adjacent layers** of carbon are held together by **weak id-id interactions**, the layers can **easily slides over one another when a force is applied**.

Question 30

define what a simple molecular lattive structure is.

Answer 30

it consists of discrete molecules held together by weak intermolecular forces of attraction. within each molecule are covalent bonds between atoms. between molecules are weak id-id interactions/pd-pd interactions/hydrogen bonding.

Question 32

why is a SMLS a non-conductor of electricity in any state.

Answer 32

There is **absence of free mobile ions or delocalised electrons as charge carriers** to conduct electricity.

Question 33

why is a SMLS soluble in non-polar solvents.

Answer 33

the **energy released** from the formation of **id-id interactions between solute molecules and the non-polar solvent molecules** is **sufficient** to overcome the **similar intermolecular id-id interactions in the solute and in the solvent**.

Question 34

why is a SMLS not soluble in water

Answer 34

the **id-id/pd-pd interactions** formed between the solute molecules and the water molecules released **insufficient energy** to **overcome the stronger hydrogen bonding** present in water.

Question 35

why is a SMLS soluble in water

Answer 35

for moleules capable of forming hydrogen bonds with water, energy released from the formation of hydrogen bonds between the solute molecules and water molecules is sufficient to overcome the intermolecular hydrogen bonds in the solute and in water.