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Hydrogen Bonding
Learning intention
Learn about this strong type of intermolecular forces which exists between molecules containing N-H, O-H or F-H bonds.
Relating physical properties to intermolecular forces
Learning intention
Learn how to explain differences in physical properties such as viscosity, melting point and boiling point in terms of differences in strength of intermolecular forces.
Intermolecular - Hydrogen Bonding
Consider the compounds formed between elements in group 4 of the Periodic table and hydrogen
The group 4 hydrides are CH4, SiH4, GeH4, SnH4
They are all covalent molecular so have low melting points and boiling points.
The boiling point increases as you go down the group.
Group 4
050
100150200250
CH4 SiH4 GeH4 SnH4
Boi
ling
Poi
nt (K
)
Group 4
As you go down the group the central atom gets bigger.
There are more electrons so a greater chance of an uneven distribution of electrons within the atom.
The London’s forces between the molecules gets stronger as you go down the group.
More energy is needed to separate the molecules from each other.
Intermolecular – Hydrogen Bonding
A similar pattern would be expected in the other covalent molecular hydrides
The group 5 hydrides NH3, PH3, AsH3 and SbH3
The group 6 hydrides H2O, H2S, H2Se and H2Te
The group 7 hydrides HF, HCl, HBr and HI
NH3, has a higher boiling point than expected.
Group 5
0
100
200
300
NH3 PH3 AsH3 SbH3
Boi
ling
Poin
t (K
)
Group 5
H2O has a higher boiling point than expected.
Group 6
0
100
200
300
400
H2O H2S H2Se H2Te
Boilin
g Po
int (
K)
Group 6
HF has a higher boiling point than expected.
Group 7
0
100
200
300
400
HF HCl HBr HI
Boilin
g Po
int (
K)
Group 7
Intermolecular - Hydrogen Bonding
Boiling Points of Hydrides
0
100
200
300
400
Series Number
Bo
ilin
g P
oin
t (K
)
Group 4
Group 5
Group 6
Group 7
NH3
H2O
HF
It is more difficult to separate NH3, H2O and HF molecules from each other than expected.
Intermolecular - Hydrogen Bonding
These compounds all have H atoms directly bonded to very electronegative atoms.
In HF the H-F bond is polar covalent.
The F has a much higher electronegativity than H.
The pair of shared electrons in the covalent bond spend more time closer to the fluorine than the hydrogen.
The H-F bond is polar. Hδ+ - Fδ-
Intermolecular - Hydrogen Bonding
The HF molecules can attract each other
HHδδ++ - F - Fδδ-- HHδδ++ - F - Fδδ--HHδδ++ - F - Fδδ--
This is called hydrogen bonding.
Hydrogen bonding is weak but is stronger than very weak London’s forces.
Intermolecular - Hydrogen Bonding
NH3 has H atoms directly bonded to very electronegative N atoms.
N-
H+ H+ H+
H+
N- H+
H+N- H+
H+ H+
There are Hydrogen bonds as well as London’s forces between the ammonia molecules.
Intermolecular - Hydrogen Bonding
H2O has H atoms directly bonded to very electronegative O atoms.
O-
H+ H+ H+O-
H+O- H+
H+
There are Hydrogen bonds as well as London’s forces between the water molecules.
Proteins consist of long chain atoms containing polar C=O and H-N bonds.
Hydrogen bonds help give enzymes their shape.
Water
Oxygen has 2 lone pairs of electrons which can forma hydrogen bonds with two hydrogen atoms.
Each water molecule, in theory, could be surrounded by 4 hydrogen bonds
O
H H
-
++
Hydrogen bonding in water
Water
Water has its greatest density at a temperature of 4oC. When, as water cools further, the molecules start to move further apart, due to the hydrogen bonding, until a more open structure is formed at its freezing point. So ice floats!!
Density of water
New Higher Chemistry E Allan J Harris
Video trends\3dwater.wmv
Water 3D model
Hydrogen bonding in ice
Hydrogen bonding in solid water gives rise to an open structure. This is why ice is less dense than liquid water.
Water in Space.wmv
Hydrogen bonding is also responsible for holding the two strands of nucleic acids together in DNA
Viscosity
Density of water
Viscosity is related to the molecular mass and the number of –OH present.
Hydrogen bonding between the molecules will increase its viscosity.
New Higher Chemistry E Allan J Harris
Water
Water has a high surface tension. The molecules on the surfacehave in effect, hydrogen bonds. This has the effect of pullingthe surface molecules closer together.
Surface tension
Bond Strengths
Bond Type Strength (kJ mol –1)
Metallic 80 to 600
Ionic 100 to 500
Covalent 100 to 500
Hydrogen 40
Dipole-Dipole 30
London’s forces 1 to 20
Behaviour in electrical fields
New Higher Chemistry E Allan J Harris
Video clip
•Cloth nappies cost between £100-£400 as opposed to disposable at £800-£1,200 for the 2.5 years of normal nappy use.
•3 billion nappies are thrown away in the UK each year with 90% going to landfill. They can take up to 500 years to decompose.
•Disposables make up 4% of total household waste and up to 50% of that of families with one baby
•Disposable nappies use up to 5 times more energy to produce than cotton ones – that's including the washing process .
•Seven million trees are felled every year in Canada and Scandinavia to supply the pulp for disposables sold in the UK.
Nappies
Disposable nappy
Chemical BackgroundGroups called sodium carboxylate are attached along the backbone.
sodium carboxylate
Sodium polyacrylate is a polymer with a molecular weight of over one million!
Sodium poly(acrylate) absorbs 500 times its own mass of water.
-
water
++
Na+
Sodium poly(acrylate) absorbs 500 times its own mass of water.
-+
+++
Sodium poly(acrylate) absorbs 500 times its own mass of water.
-
-
-
-
-
-
Predicting solubility from solute and solvent polarities
Learning intention
Learn how the polarity of both the solute and solvent molecules influences solubility.
Solvent Action
Solvent Action
Water is a polar molecule so it is a polar solvent.
A liquid that a substance dissolves in is called a SOLVENT. Solvents can be either polar or non-polar molecules.
Immiscible liquids do not mix, e.g. oil and water, however, non-polar liquids are miscible with each other.
Polar solvents will usually dissolve polar molecules.
Non-polar solvents will usually dissolve non-polar molecules.
Water has a polar covalent bondingbetween O and H. O
H
H
-
+
+
+
+
-
Dissolving in Water
Hydratedions
Ionic Compound dissolvingin water
-
++
-
++
- ++ -+ -
--
+
+ -+
+
-
+ +
+-+
+
+ -
+
-
+ +
Dissolving in Water
Dissolving in Water
Hydratedions
Pure Hydrogen chloride is polar covalent. When water is added it breaks to produce ions
Cl-
-
+
+ -+
+
-
+ +
+-+
+
H+ -
+
-
+ +
H Cl+ -
-
+ +
-
+ +
Dissolving in WaterGenerally, covalent molecules are insoluble in water. However, small moleculeslike ethanol (C2H5OH), with a polar O-H functional group, will dissolve,
-
+ +
H2OEthanol
+
O
H
H H
H
H
C C
H
-
• Dissolving salt
• solubility KMnO4 and I2(upper layer is water solvent lower layer is hexane)
