Intermolecular forces
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which of these molecules has a permanent dipole (H 2 O, CO 2 , CCl 4 , NH 3 ).Also state: shapes, angles and identify any polar bonds.
description
Intermolecular forces. L.O.: Describe intermolecular forces in terms of permanent and instantaneous (van der Waals’ forces) dipoles. In pairs discuss: which of these molecules has a permanent dipole (H 2 O, CO 2 , CCl 4 , NH 3 ). Permanent dipole–dipole forces between HCl molecules. - PowerPoint PPT Presentation
Transcript of Intermolecular forces
which of these molecules has a permanent dipole (H2O, CO2, CCl4, NH3).Also state: shapes, angles and identify any polar bonds.
1) Choose a board (eg. 1,2 3 or 4)
2)Deduce the shape and angles of the molecules on the board. Also, identify polar bonds and polar molecules.
Intermolecular forcesIntermolecular forces
L.O.:
Dipole-dipole London forces
H-bonding
Permanent dipole–dipole forces between HCl molecules
A permanent dipole-dipole force is a weak attractive force between permanent dipoles in neighbouring polar molecules.
Induced dipole-dipole interactions (London forces)
The more electrons in each molecule, the stronger the attractive forces between the molecules.
Why do the boiling points of the noble gases increase as the atomic number of the noble gases increase?
Hydrogen bonding
Describe hydrogen bonding between molecules containing –OH , –NH groups or F.
Describe and explain the anomalous properties of water resulting from hydrogen bonding.
Look at this example of hydrogen bonding between 2 molecules of water.
In pairs discuss:1)What is a hydrogen bond?2)What is required to make a hydrogen bond?
Hydrogen bonding: a type of intermolecular force in which a hydrogen atom covalently bonded to an electronegative atom interacts with another electronegative atom.
To form a hydrogen bonding we must have: a hydrogen atom that is bonded to a very electronegative atom (O,N and F).A very electronegative atom with a lone pair of electrons.
What trend would you expect in the boiling points of:
1)Ne, Ar, Kr, Xe2)CH4, SiH4, GeH4, SnH4,
3)NH3, PH3, AsH3, SbH3,
4)HF, HCl, HBr HI5)H2O, H2S, H2Se, H2Te,
BOILING POINTS OF HYDRIDESBOILING POINTS OF HYDRIDES
Mr
BO
ILIN
G P
OIN
T /
C°
100
0
-160
14050 100
The boiling points of the hydrides increase with molecular mass. CH4 has the lowest boiling point as it is the smallest molecule.
CH4
SiH4
GeH4
PbH4
Larger molecules have greater intermolecular forces and therefore higher boiling points
GROUP IV
BOILING POINTS OF HYDRIDESBOILING POINTS OF HYDRIDES
Mr
BO
ILIN
G P
OIN
T /
C°
100
0
-160
14050 100
NH3 has a higher boiling point than expected for its molecular mass. There must be an additional intermolecular force.
NH3
GROUP V
BOILING POINTS OF HYDRIDESBOILING POINTS OF HYDRIDES
Mr
BO
ILIN
G P
OIN
T /
C°
100
0
-160
14050 100
H2O has a very much higher boiling point for its molecular mass. There must be an additional intermolecular force.
H2O
GROUP VI
BOILING POINTS OF HYDRIDESBOILING POINTS OF HYDRIDES
Mr
BO
ILIN
G P
OIN
T /
C°
100
0
-160
14050 100
HF has a higher boiling point than expected for its molecular mass. There must be an additional intermolecular force.
HF
GROUP VII
BOILING POINTS OF HYDRIDESBOILING POINTS OF HYDRIDES
GROUP IV
GROUP V
GROUP VI
GROUP VII
Mr
BO
ILIN
G P
OIN
T /
C°
100
0
-160
14050 100
H2O
HF
NH3
The higher than expected boiling points of NH3, H2O and HF are due to intermolecular HYDROGEN BONDING
BOILING POINTS OF HYDRIDESBOILING POINTS OF HYDRIDES
Mr
BO
ILIN
G P
OIN
T /
C°
100
0
-160
14050 100
GROUP IV
GROUP V
GROUP VI
GROUP VII
The boiling points of H2O, HF, NH3, are higher than what we would expect if only London forces were operating. This is because hydrogen bonding is present between the molecules in each of these compounds.
HYDROGEN BONDING - HYDROGEN BONDING - ICEICE
each water molecule is hydrogen-bonded to 4others in a tetrahedral formation
ice has a “diamond-like” structure
volume is larger than the liquid making it
when ice melts, the structure collapsesslightly and the molecules come closer; theythen move a little further apart as they getmore energy as they warm up
this is why…a) water has a maximum density at 4°Cb) ice floats.
hydrogen bonding
lone pair
HYDROGEN BONDING - HYDROGEN BONDING - ICEICE
hydrogen bonding
Liquid water: hydrogen bonds break and reform
Ice: water molecules are in fixed positions. Molecules are slightly less packed than in liquid water.
Hydrogen bonding in water
Draw H-bonding between:Group 1: a) 2 molecules of H2O
b) 2 molecules of CH3OH,
Group 2: 2 molecules of NH3
Group 3: 2 molecules of Me2NHGroup 4: 1 molecule of H2O and CH3COCH3Group 5: 2 molecules of CH3CO2H
1) In which of the following does hydrogen bonding not occur between molecules: H2O, NH3, HBr, HF
2) Explain why hydrogen bonds do not form between a) molecules of CH4 b) molecules of CCl4
3) Explain why water molecules form on average two hydrogen bonds per molecule, where the ammonia molecule, NH3, forms only one
For H-bonding look out for O-H, N-H bonds or F
The ice lattice is an open network of H2O molecules
In DNA, three hydrogen bonds link C and G
In DNA, two hydrogen bonds link A and T
Double helix in DNA
Explain why the molecule PH3 has a pyramidal shape and a bond angle of 107o (3 or 4 marks)
o 4 electron pairs surround the central atom. 3 bonding pairs and a lone pairo electron pairs repel other electron pairso lone pair repel moreoelectron pairs get as far apart as possible
