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

Electronegativity An 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 AtomElectronegativity F4.0 O3.4 Cl3.2 N3.0 Br3.0 I2.7 S2.6 C H2.2

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

CH 4 SiH 4 GeH 4 SnH 4 H2OH2O H2SH2S H 2 Se H 2 Te

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

HF.. HF HF HF Hydrogen Bonding in Hydrogen Fluoride

Ice Both lone pairs are involved in hydrogen bonds Both hydrogen atoms are involved in hydrogen bonds

Comparing Bonds Type of attractionBond 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 Properties Propane M R = 44 Ethanol M R = 46 How do these two molecules differ in boiling point, viscosity and water solubility? Explain why… H - C-C- OH 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 Bonding An 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)

Boiling Point (K)

Hydrogen Bonding Hydrogen 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

Ice Both 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

How do these two molecules differ in boiling point, viscosity and water solubility? Explain why… H - C-C-C - H H H H Properties Propane M R = 44 Ethanol M R = 46 H - C-C- OH 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