Understanding Hydrogen Bonding in Chemistry: Properties and Applications

1. 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

2. Chemical Ideas 5.4 Hydrogen Bonding J Hudson 2005

3. 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.

4. Electronegativity Which covalent bonds would be the most polar?

5. Hydrogen Bonding The three types of bonds which give molecules significant hydrogen bonding are; (i) N – H (ii) O – H (iii) F – H • These three bonds all have; • A strong permanent dipole • A hydrogen atom • An atom with lone pair electrons

6. H2O H2Te H2Se H2S SnH4 GeH4 SiH4 CH4

7. 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

8. This section of water is frozen This section of water is liquid

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

10. Hydrogen Bonding in Hydrogen Fluoride .. H F .. .. Fluorine atoms have three electron lone pairs for bonding to other HF molecules

11. Hydrogen Bonding in Hydrogen Fluoride .. .. .. .. H H H H F F F F .. .. .. .. .. .. .. ..

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

13. Comparing Bonds

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

15. Physical Properties of Alcohols Alcohol molecules can hydrogen bond to each other and to water molecules. This explains their solubility in water.

16. 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

17. 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.

18. Hydrogen Bonding The three types of bonds which give molecules significant hydrogen bonding are; (i) (ii) (iii) • These three bonds all have; • A strong permanent dipole • A hydrogen atom • An atom with lone pair electrons

19. Boiling Point (K)

20. 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

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

22. Hydrogen Bonding in Hydrogen Fluoride .. .. .. .. H H H H F F F F .. .. .. .. .. .. .. .. Fluorine atoms have three electron lone pairs for bonding to other HF molecules

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

24. Comparing Bonds

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

26. 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