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

Chemical Ideas 5.4

Hydrogen Bonding

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

0

50

100

150

200

250

300

350

400B

oil

ing

Po

int

(K)

CH4

SiH4

GeH4

SnH4

H2O

H2SH2Se

H2Te

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

This section of water is frozen

This section of water is liquid

The ice structure has large empty spaces which gives it a lower density than water.

Hydrogen Bonding in Hydrogen Fluoride

H F..

....

Fluorine atoms have three electron lone pairs for bonding to other HF molecules

H

F.

...

..

H

F.

...

..H

F.

...

..

H

F.

...

..

Hydrogen Bonding in Hydrogen Fluoride

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)

0

50

100

150

200

250

300

350

400B

oil

ing

Po

int

(K)

Boilin

g P

oin

t (K

)

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

The ice structure has large empty spaces which gives it a lower density 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

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