Module 2: Chapter 6 (Shapes of molecules and intermolecular forces)

Question 1

What are the 3D symbols used when drawing molecules?

Answer 1

solid line = a bond in the plane of paper
solid wedge = comes out of the plane
dotted wedge = going into the plane

Question 2

Draw the example of two electron pairs: BeCl2, state bond angle and the shape

Answer 2

bond angle = 180

linear

Question 3

Draw the example of 3 electron pairs: BF3, state bond angle and the shape

Answer 3

bond angle = 120

trigonal planar

Question 4

Draw the example of 4 electron pairs: CH4, state bond angle and the shape

Answer 4

bond angle = 109.5

tetrahedral

Question 5

Draw the example of 6 electron pairs: SF6, state bond angle and the shape

Answer 5

bond angle = 90

octahedral

Question 6

What is the effect of lone pairs?

Answer 6

• lone pairs are slightly closer to the central atom, occupies more space
• results in lone pair repelling stronger than the bonded pair
• they repel the bonding pairs slightly closer together, decreasing the bond angle 2.5 per lone pair

Question 7

Draw the example of 4 electron pairs with one being a lone pair: NH3, state bond angle and the shape

Answer 7

bond angle = 107

pyramidal

Question 8

Draw the example of 4 electron pairs with 2 being lone pairs: H20, state bond angle and the shape

Answer 8

bond angle = 104.5

non linear

Question 9

How do you deal with multiple bonds or polyatomic ions?

Answer 9

multiple bonds treated as a bonding pairs, same with polyatomic ions

Question 10

Draw the example of CO2, state bond angle and shape

Answer 10

bond angle = 180

linear

Question 11

Draw the example of carbonate, state bond angle and shape

Answer 11

CO3^2-
bond angle = 120
trigonal planar

Question 12

Draw the example of ammonium, state bond angle and shape

Answer 12

NH4+
bond angle = 109.5
tetrahedral

Question 13

Draw the example of sulphate, state bond angle and shape

Answer 13

SO4^2-
bond angle = 109.5
tetrahedral

Question 14

What is electronegativity?

Answer 14

Electronegativity is a measure of the attraction of a bonded atom for the pair of electrons in a covalent bond

Question 15

How is electronegativity measured?

Answer 15

pauling scale

Question 16

Describe electronegativity of elements

Answer 16

• non metals: nitrogen, oxygen, fluorine and chlorine are the most electronegative (tend to form δ-), all above 3.0
• group 1 metals including lithium, sodium and potassium are the least electronegative (δ+), all below 1.0

Question 17

What is the list of comparative elements for electronegativity?

Answer 17

δ+ = C,H

δ- (in descending order of electronegativity) = 
F
O
Cl, N (similar value)
Br
I

Question 18

Describe bond polarity

Answer 18

In a non-polar bond, bonded electrons are shared equally between bonded atoms. This happens when:
• bonded atoms are the same
• have a similar/equal electronegativity (for example a C-H bond is ALWAYS NON-POLAR)

In a polar bond, bonded electrons are shared unequally. This happens when:
• atoms have different electronegativity values -> more electronegative atom has a small partial negative charge and the other a small partial positive charge -> separation of opposite charges is called a dipole

Question 19

What is the exception for polar molecules?

Answer 19

• dipoles can cancel each other out if the shape is symmetrical, results in the molecules itself being non-polar with no overall dipole
• for example: CO2 (linear) and CCl4 (tetrahedral)
• if there were lone pairs it would be polar

Question 20

What are intermolecular forces?

Answer 20

• weak interactions between neighbouring molecules
• broken when a molecular substance melts, boils or dissolves
• significantly weaker than the covalent bonds
• three main types: london forces, permanent dipole-dipole and hydrogen bonding

Question 21

Describe london forces

Answer 21

• exist between all molecules, polar or non-polar
• movement of electrons produces a change in dipole in a molecule and at any point an instantaneous dipole will exist, constantly changing
• the instantaneous dipole induces a dipole on a neighbouring molecule, which induces further molecules and attract one another
• they are only temporary, in the next instant could disappear

Question 22

What factors increase the strength of london forces?

Answer 22

The more electrons in each molecule
• larger the instantaneous and induced dipoles
• greater the interactions
• stronger attractive forces

For example as you go down the noble gases group, boiling point increases as there are more electrons and therefore stronger induced dipoles

Question 23

Describe permanent dipole-dipole interactions

Answer 23

• only act between permanent dipoles in polar molecules

- more energy than London forces needed to overcome forces -> gives substances higher boiling points

Question 24

What is a hydrogen bond?

Answer 24

A hydrogen bond is an attraction between a
lone pair of electrons on a highly electronegative
atom (O, N or F) in one molecule and an
electron-deficient hydrogen atom (H attached to
a highly electronegative O, N or F atom) in
another molecule

Question 25

Define a simple molecular substance

Answer 25

made up of simple molecules, i.e. small covalently bonded units with a definite molecular formula

Question 26

Describe the melting and boiling points of simple molecular substances

Answer 26

• in a solid substance the molecules form the structure a simple molecular lattice, where molecules are held in place by weak molecular forces
• little energy is required to break these forces and therefore typically have low melting and boiling points

Question 27

Describe electrical conductivity of simple molecular substances

Answer 27

• non conductors

- no mobile charge carriers (either delocalised electrons or mobile ions)

Question 28

Why is liquid water denser than solid water?

Answer 28

• In ice each water molecule is involved in 4 stable H-bonds in a tetrahedral arrangement.
• Extensive H-bonds lead to a crystalline structure
• This creates an open lattice with quite large gaps which lowers the density (the gaps are empty. NOT filled with air!)
• When heated some of the H-bonds in ice break. H2O molecules fill some of the gaps increasing the density

Question 29

Describe surface tension in water

Answer 29

Surface tension in water is caused by hydrogen bonding.

• Surface tension is caused by molecules on surface experiencing unbalanced hydrogen bonding forces pulling them in.
• Molecules in the bulk experience balanced forces in every direction.

Question 30

Describe the high melting and boiling point of water

Answer 30

• hydrogen bonds are extra forces over and above London
• more energy is needed to break the H bonds in water
• when the ice lattice breaks the rigid arrangement of hydrogen bonds in ice is broken and when water boils the H-bonds break completely

Question 31

What is the solubility of non-polar simple molecular substances?

Answer 31

• Soluble in non-polar solvents as intermolecular forces form between molecules and the solvent, weaken lattice forces and eventually breaking them all
• tend to be insoluble in polar solvents as intermolecular bonding within polar solvents may be too strong to be broken

Question 32

What is the solubility of polar simple molecular substances?

Answer 32

• soluble in polar solvents as molecules can attract each other
• solubility mainly depends on the strength of the dipole

Question 33

Explain why water molecule are polar

Answer 33

O is more electronegative than H (O attracts
the bonded pair of electrons in the covalent
bond between O and H more than H)

H2O is non-symmetrical. Dipoles do not
cancel.