5.4.1 Hydrogen Bond

1. A hydrogen bond is a force of attraction between hydrogen atom, \(H\) that has bonded with an atom of high electronegativity such as nitrogen, \(N\), oxygen, \(O\) or fluorine, \(F\) with nitrogen, \(N\), oxygen, \(O\) or fluorine, \(F\) in another molecule.
2. Hydrogen bonds exist between molecules such as water, \(H _2 O\), hydrogen fluoride, \(HF\), ammonia, \(NH _3\) and ethanol, \(CH _3 OH\).
3. The hydrogen bond is a strong force of attraction or intermolecular attraction, but weaker than a covalent or an ionic bond.
   
   1. Hydrogen fluoride, \(HF\)
      
   2. Water, \(H_2 O\)
      

Role of Hydrogen Bonds in Daily Life

1. Wet hair sticks together
   
   1. When hair is wet, protein molecules will form hydrogen bonds with water molecules, \(H _2 O\) instead of forming hydrogen bonds among themselves.
   2. This causes the hair to stick together.
2. Wet finger helps to turn pages of a book which stick together
   
   1. Cellulose in paper consists of hydrogen atoms, \(H\) that are bonded to oxygen atoms, \(O\).
   2. When fingers are wet, water molecules, \(H _2 O\) on the wet finger will form hydrogen bonds with the cellulose in the paper. The paper will stick to the finger.
3. Substances such as sugar solution, honey and syrup are sticky
   
   1. Hydrogen atoms of substances such as sugar solution, honey and syrup molecules are attracted to oxygen atoms, \(O\) in water molecules, \(H _2 O\).
   2. The hydrogen bonds that are formed between these molecules and water molecules make them sticky.

Effect of Hydrogen Bonds on the Physical Properties of Substances

1. Hydrogen bonds cause covalent molecules such as ammonia, \(NH _3\), ethanol, \(C _2 H _5 OH\) and ethanoic acid, \(CH _3 COOH\) to dissolve in water.
   

   

2. Hydrogen bonds cause covalent compounds such as ethanol, \(C _2 H _5 OH\) and ethanoic acid, \(CH _3 COOH\) to have a higher boiling point as compared to other covalent molecules such as nitrogen, \(N _2\) and oxygen, \(O _2\).