Shapes and Intermolecular interactions

Question 1

Using VSEPR theory, work out the shape of Trifluoroborane-ammonia (BF₃H₃N)
Tip - The bond between the nitrogen and the boron is a co-ordinate bond, where nitrogen is the donor and boron is the acceptor

Answer 1

Electrons from Boron: 3
Number of B-F bonds: 3
Number of B-N coordinate bond: 2
Total electrons at B: 8
FOUR pairs - therefore AB₃C tetrahedral

Electrons from Nitrogen: 5
Number of N-H bonds: 3
Number of B-N coordinate bonds: 0
Total electrons at N: 8
FOUR pairs - therefore AB₃C tetrahedral

Question 2

What are two things to keep in mind when determining the shape of molecules with multiple bonds

Answer 2

1. Only the first bond between two atoms significantly affects the structure (single over double bonds)
2. Subtract central atom electrons used to form second or third bonds

Question 3

When using VSEPR theory, how do you account for double and triple bonds in the structure

Answer 3

Usually you would add all the bonds together, to work out the number of electrons at the central atom
However, for a double bond you -1 (so technically add nothing) and for a triple bond you -2 (so technically -1)

Question 4

Phosphorus oxychloride has the formula POCl₃
Using VSERP, work out its shape

Answer 4

Electrons from Phosphorus: 5
N.o P-Cl bonds: 3
N.o P-O bonds: 1
-1 for the double bond
FOUR elctron pairs, so AB₃C, basically tetrahedra

Question 5

We have worked out the phosphorus oxychloride (POCl₃) has a tetrahedral structure
Why then does it not have bond angles of 109.5°

Answer 5

There are two electron pairs in the double bond, therefore more replusion. Hence greater OPCl angle

Question 6

CO₃²¯ has 3 different resonance structures as show. Using VSEPR theory, work out its shape

Answer 6

Electrons from Carbon: 4
Number of C-O bonds: 3
-1 for C-O double bond
Total electrons at C: 6
THREE pairs so AB₃ - trigonal planar

Question 7

CO₃²¯ has a trigonal planar structure and has bond angles of 120°, despite having a double bond
How is this the case

Answer 7

The double bond is delocalised

Question 8

Why is the carbon diamond strucuture such a good conductor of heat

Answer 8

Because there is so many bond in the structure, when one carbon starts vibrating because heat is applied, it moves through the structure very easily

Question 9

Diamond has 4 bonds per molecule, whereas graphite has 3
Yet, thermodynamically graphite is more stable than diamond, why?

Answer 9

Van der Waals forces betwen the graphine sheets

Question 10

What is a gas
Moving from a gas to a solid entrophy…

Answer 10

gases are compressible fluids
Their molecules are in constant random motion through relatively free space (molecules ar far apart unless the gas is under high pressure)
Entrophy increases

Question 11

What is a liquid

Answer 11

Incompressible fluid
Molecules are in constant random motion, but the molecules are closer together, so there is less free space between them

Question 12

What are solids

Answer 12

Are incompressible solids
Molecules are fixed in positions close together, however there is a small amont of vibrational energy

Question 13

Why are bonds an idealistic model when considering electrons

Answer 13

Real molecules do not have localised pairs of electrons as drawn in lewis strctures
They instead can be pictured as ‘clouds’ of electron density surrounding the nuclei

Question 14

How do dispersive forces come about

Answer 14

The electrons move around, so at any particular time there is likely to be an uneven distribution that creates an instantaneous dipole
This dipole induces an instantaneous dipole in an neighbouring molecules and the two are then attracted together by electrostatic interactions
These are dispersion (London) forces

Question 14

What are two key facts about dispersive forces

Answer 14

Always exist for all molecules
Always attractive (independent of molecule oritetation

Question 15

Why do dispersive forces increase with molecular weights

Answer 15

Since there are more electrons, larger molecules are more easily polarised to give a dipole

Question 16

For molecules of a similar weight, why would dispersive forces differ

Answer 16

The more compact (smaller SA) a molecule is, the less easily it is polarised and so the dispersion forces are weaker

Question 17

How do Dipole-Dipole forces come about

Answer 17

If two atoms are bonded together have different electronegativities, this will create a polar bond which give a permanent dipole
Two molecules are attracted together by electrostatic interaction as a result of the dipoles. These are dipole-dipole (Keesom) forces

Question 18

Why overall are dipole-dipole forces tend to be weaker than dispersion forces

Answer 18

due to the oritentation dependence of the permanent dipole

Question 19

What is a dipole-induced force

Answer 19

A molecule with a permanent dipole can induce an instantaneous dipole in a neighbouring molecules
The two molecules are then attracted together by electostatic interactions
These are dipole-induced-dipole (Debye) forces

Question 20

Why is a molecule containin an electronegative atom be more difficult to polarised

Answer 20

Due to electrons being tightly held by the electronegative atom

Question 20

If a molecule is very large then the contribution from a polar bond created by an electronegative atom may …

Answer 20

… be less significant than the dispersion forces in the overal intermolecular interactions

Question 20

What is hydrogen bonding

Answer 20

Is weak to moderate attractive force that exists between a hydrogen tom covalently bodned to a very electronegative atom X, and a lone pair of electrons on another small electronegative atom, Y
It is a special case of dipole-dipole interactions (usually N, O, F)

Question 20

Why is ice less dense than water, even though it is a solid

Answer 20

Ice has a hydrogen bonded network which are approximately linear (180°) and form a 3-D netwok with an open structure
Upon melting 273K some (but not all) of the hydrogen bonds are lost so that the density actually increases, which is unusual upon chaning from a solid to a liquid

Question 21

Define Van der Waals forces

Answer 21

Refer to intermolecular interactions, which includes dispersion forces, dipole-dipole forces and dipole-induced-dipole forces (hydrogen bonds are considered a special case and not part of these forces because of how strong they are)

Question 22

How can a molecule have a dipole but not have dipole-dipole forces

Answer 22

Take for example CCl₄, because the δ⁺ carbon centre is full enclosed by a δ¯ ring, but there is no overall dipole on the molecule as a whole

Question 23

Which is the strongest intermolecular interaction

Answer 23

Strongest: Hydrogen bonding
Dipole-dipole forces
(Dispersion forces are very variable in strength)

Question 24

What is a chemical bond

Answer 24

Is formed betwen two atoms if the resulting arrangement of the atoms and their electrons have a lower energy than the seperate atoms themselves

Question 25

Atoms can obtain the noble gas confiuration (stable) by sharing electrons (covalent bonding) or by transferring electrons (ionic bonding)
Oxidation is …
Reduction is …

Answer 25

Oxidation is the Loss of electrons
Reduction is the Gain of electrons

Question 26

What are the key features of Ionic compounds

Answer 26

• Ionic compounds form large 3D matrices
• There are strong electrostatic interactions between ions
• As a result they have a high melting point
• Water can usually dissolve ionic compounds by solvating ions

Question 27

Due to the strong electrostatic interactions in ionic compounds, their melting points are really high. However, they easily dissolve in water, how?

Answer 27

The bonds formed between the δ¯ oxygen and the positive sodium ions, and the δ⁺ hydrogens and the negative chloride ions, are stronger than the intermolecular forces in ionic bonding
The sodium and chloride ions become hydrated by water

Question 28

With solubility the general rule is ‘like dissolves like’
What does this mean

Answer 28

The solubility of a substance is dependent upon the intermolecular interactions that occur
Polar molecules with dissolve in polar solvenets
Non-polar molecules will dissolve in non-polar solvents