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Slides 1 – 16 Presentation
Slides 17 – 26 To be printed as a handout
Slides 4 and 14 are intended to be short
discussion activities for small groups
J Hudson 2005
ElectronegativityAn atom or ion’s electronegativity is its ability to pull electrons towards itself in a covalent bond. The most electronegative elements are found towards the top right corner of the periodic table.
Which covalent bonds would be the most polar?
Electronegativity
Atom Electronegativity
F 4.0
O 3.4
Cl 3.2
N 3.0
Br 3.0
I 2.7
S 2.6
C 2.6
H 2.2
Hydrogen Bonding
These three bonds all have;
• A strong permanent dipole
• A hydrogen atom
• An atom with lone pair electrons
The three types of bonds which give molecules significant hydrogen bonding are; (i) N – H (ii) O – H (iii) F – H
Hydrogen BondingHydrogen bonding in water results in some unusual properties;
• Higher than expected boiling point
• High specific heat capacity
(absorbs a lot of heat energy with only a small change in temperature)
• Ice is less dense than water
Hydrogen Bonding in Hydrogen Fluoride
H F..
....
Fluorine atoms have three electron lone pairs for bonding to other HF molecules
IceBoth lone pairs are involved in hydrogen bonds
Both hydrogen atoms are involved in hydrogen bonds
Comparing Bonds
Type of attraction Bond enthalpy/kJ mol-1
O–H covalent bond +464
Hydrogen bond +10 to +40
Instantaneous dipole – induced dipole forces
Less than +10
H - C-C-C - H
H H H
H H H
Properties
Propane
MR = 44
Ethanol
MR = 46
How do these two molecules differ in boiling point, viscosity and water solubility? Explain why…
H - C-C- OH
H H
H H
Alcohol molecules can hydrogen bond to each other and to water molecules. This explains their solubility in water.
Physical Properties of Alcohols
Results of Hydrogen Bonding• Wool and nylon fibres can hydrogen bond to water – these fabrics can absorb water
• Polythene has no hydrogen bonding – polythene clothes would get very sweaty and sticky
• Ice floats on water making life possible
Chemical Ideas 5.4 Hydrogen BondingAn atom or ion’s electronegativity is its ability to pull electrons towards itself in a covalent bond. The most electronegative elements are found towards the top right corner of the periodic table.
Hydrogen Bonding
These three bonds all have;
• A strong permanent dipole
• A hydrogen atom
• An atom with lone pair electrons
The three types of bonds which give molecules significant hydrogen bonding are; (i) (ii) (iii)
Hydrogen BondingHydrogen bonding in water results in some unusual properties;• Higher than expected boiling point
• High specific heat capacity (absorbs a lot of heat energy with only a small change in temperature)
• Ice is less dense than water
Hydrogen Bonding in Hydrogen Fluoride
H
F..
....
H
F..
.. ..
H
F..
..
..H
F..
Fluorine atoms have three electron lone pairs for bonding to other HF molecules
IceBoth lone pairs are involved in hydrogen bonds
Both hydrogen atoms are involved in hydrogen bonds
Comparing Bonds
Type of attraction
Bond enthalpy/kJ mol-
1
O–H covalent bond
+464
Hydrogen bond +10 to +40Instantaneous dipole – induced dipole forces
Less than +10
How do these two molecules differ in boiling point, viscosity and water solubility? Explain why…
H - C-C-C - H
H H H
H H H
Properties
Propane
MR = 44
Ethanol
MR = 46
H - C-C- OH
H H
H H
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