1. Hydrogen bonding occurs when a hydrogen atom bonded to an electronegative atom like nitrogen, oxygen or fluorine interacts with another electronegative atom via electrostatic attraction between the hydrogen and the lone pair of electrons. 2. This leads to higher boiling points and enthalpies of vaporization for compounds that can form multiple hydrogen bonds like water. It also allows for the formation of structures like DNA base pairs and protein secondary structures. 3. Hydrogen bonding plays important roles in determining physical properties of compounds and enabling key biological molecules like DNA and proteins to form their functional three-dimensional structures.

1. Hydrogen Bonding

2. Hydrogen bondingWhen hydrogen is bonded to a highly electronegative atom (such as nitrogen, oxygen, fluorine), the bonding electron pair is drawn towards the electronegative atom

3. Hydrogen bondingHydrogen has no inner shell electron and is very small in size, the positive charge density developed is highThe nucleus of hydrogen atom is exposed to attraction by nearby electron cloud, a lone pair electrons on the electronegative atom

4. Hydrogen bondingDefinition:Electrostatic attraction between hydrogen atoms bonded to small, strongly electronegative atoms (N, O and F) and the lone pair electrons on these electronegative atomsHydrogen bonding

5. Hydrogen BondingEssential requirements for formation of hydrogen bond:A hydrogen atom must be directly bonded to a highly electronegative atom (e.g. F, O and N)Anunbonded pair of electrons (lone pair electrons) is presented on the electronegative atom

6. Hydrogen bondingExamples

7. Special NotesIntermolecular hydrogen bond:

8. Hydrogen bond formed between two molecules

9. Intramolecularhydrogen bond:

10. Hydrogen bond formed between two different atoms in the same moleculeIntermolecular hydrogen bond is stronger than van der Waals’ forcesImportance of Hydrogen Bonding in Physical PhenomenaAnomalous Properties of the second period hydridesQuestions:1. Why do H2O, NH3 and HF have abnormally high boiling point?2. Explain why the order of boiling point is H2O > HF > NH3.

11. Q1: Why do NH3, H2O and HF have abnormally high boiling point?Explanation:N, O and F are highly electronegative atomsFormation of intermolecular hydrogen bonds in their hydrides.Intermolecular hydrogen bonds are much stronger than van der Waals’ forcesMore energy is needed to break the hydrogen bonds in NH3, H2O and HF

12. Q2: Explain why the order of boiling point is H2O > HF > NH3Explanation

13. H2O can form 2 hydrogen bond per molecule

14. NH3 and HF can form only 1 hydrogen bond per molecule

15. The boiling point of H2O is higher than NH3 and HFQ2: Explain why the order of boiling point is H2O > HF > NH3Comparing N and F, F is more electronegativeThe intermolecular hydrogen bond formed between HF molecules is stronger than NH3HF has a higher boiling point than NH3

16. Importance of Hydrogen Bonding in Physical PhenomenaEnthalpy of vaporizationEnergy required to vaporized one mole of liquidQuestion:Explain why H2O has the largest enthalpy of vaporization than H2S, H2Se and H2Te.

17. Explanation:

18. In H2O, there are intermolecular hydrogen bonds between molecules

19. However, in H2S, H2Se and H2Te, the interaction between molecules is dipole-dipole interactions

20. The strength of hydrogen bond is stronger than dipole-dipole interactions

21. To break the hydrogen bonds, more energy is required

22. H2O has the largest enthalpy change of vaporizationImportance of Hydrogen Bonding in Physical Phenomena Boiling Point and Solubility of AlcoholsThe boiling points of alcohols are higher than the thiols because there are intermolecular hydrogen bonds between alcohol molecules, but only dipole-dipole interactions between molecules of thiols Alcohols:Thiols:

23. Alcohols of low molecular masses are soluble in water because they can form intermolecular hydrogen bonds with water molecules

24. Importance of Hydrogen Bonding in Physical PhenomenaDimerization of Carboxylic AcidIt happens when carboxylic acids are dissolved in non-polar solvents or in vapour stateE.g. Ethanoic acid (CH3COOH)Ethanoic acid molecules formdimers through the formationof intermolecular hydrogenbonds

25. Importance of Hydrogen Bonding in Biochemistry Hydrogen bonding in ice and waterIntermolecular hydrogen bonds exist in both water and ice In water, molecules are in constant motion. Intermolecular hydrogen bondsare formed and broken continuously. The arrangement ofmolecules is random

26. In ice, the molecular motion is restricted. The molecules are arranged in a way to form the maximum number of hydrogen bonds