Chapter 6 - Shapes Of Molecules & Intermolecular Forces

Question 1

Electron pair repulsion theory (EPR)

Answer 1

• electrons have negative charges
• around a central atom they repel each other so they are arranged as far apart as possible

Question 2

4 bonding regions

Answer 2

tetrahedral
109.5

Question 3

2 bonding regions

Answer 3

linear
180

Question 4

3 bonding regions

Answer 4

trigonal planar
120

Question 5

6 bonding regions

Answer 5

octahedral
90

Question 6

what repels more - bonded or lone pairs?

Answer 6

lone pairs repel more
- for every lone pair the bond angle is reduced by 2.5

Question 7

3 bonded, 1 lone

Answer 7

pyramidal
107

Question 8

2 bonded, 2 lone

Answer 8

non-linear
104.5

Question 9

electronegativity

Answer 9

the ability for an atom to attract the electron pair in a covalent bond

Question 10

how is electronegativity measured and trend?

Answer 10

• Pauling electronegativity values
• increases as period does and going up a group as nuclear charge increases and atomic radius decreases
• fluorine is the most electronegative element with a value of 4.0

Question 11

effects of electronegativity difference

Answer 11

• larger the difference, the more an atom has control of electrons - becomes more ionic
• covalent - 0
• polar covalent - 0-1.8
• ionic - >1.8

Question 12

non-polar bonds

Answer 12

• electron pair shared equally
• when bonded atoms are the same e.g oxygen
• or when they have same/similar electronegativity
• small difference in C-H bond so considered electronegative

Question 13

polar bonds

Answer 13

• bonded electron pair is shared unequally due to different electronegativity values
• it’s polarised forming a small partial +ve & -ve charge

Question 14

dipole

Answer 14

separation of opposite charges

Question 15

polar molecules

Answer 15

when polar charges don’t cancel out due to molecule shapes

Question 16

intermolecular forces

Answer 16

weak interactions between dipoles of different molecules - 3 types:
- induced dipole-dipole interactions (London forces)
- permanent dipole-dipole interactions
- hydrogen bonding

Question 17

Induced dipole-dipole interactions (London forces)

Answer 17

• exists between all molecules
• electrons move around creating instantaneous dipoles
• induces one on a neighbouring molecule
• continues inducing on more molecules

Question 18

strength of London forces

Answer 18

• more electrons the larger the instantaneous dipoles
• greater induced dipole-dipole interactions
• stronger attractive forces between molecules
• more energy needed to overcome
• higher m.p/b.p

Question 19

permanent dipole-dipole interactions

Answer 19

• act between permanent dipoles in polar molecules
• stronger then London forces
• act in addition to London forces
• more energy needed to overcome
• higher m.p/b.p

Question 20

simple molecular substances

Answer 20

• made up of simple molecules
• form simple molecular lattice - molecules held together by weak intermolecular forces, atoms within molecules have strong covalent bonds

Question 21

m.p/b.p of simple molecular substances

Answer 21

• can exist as any state at room temp
• weak intermolecular forces need little energy to be broken so low melting and boiling points needed

Question 22

solubility of simple molecular substances

Answer 22

• non-polar dissolves in non-polar
• i.f. form between solute and solvent, weakens i.f. in simple molecular substance, breaks them then dissolves
• non-polar insoluble in polar
• polar soluble in polar sometimes
• depends on strength of dipole, if similar strengths then will dissolve

Question 23

electrical conductivity of simple molecular substances

Answer 23

• no mobile charged particles
• doesn’t conduct electricity

Question 24

hydrogen bond

Answer 24

• special type of permanent dipole-dipole between:
• electronegative atom - F, O, N with a lone pair
• H atom attached to an electronegative atom
• strongest intermolecular interaction

Question 25

anomalous properties of water

Answer 25

• solid (ice) is less dense then water (liquid)
• H bonds hold molecules apart in open lattice structure, further away then when liquid
• makes it less dense and floats on water
• relatively high m.p/b.p - forces added to London & permanent dipole-dipole forces
• more energy needed to break forces so high m.p/b.p