Chemical Reactions and Equations

"Oxidation is a reaction in which oxygen is added or hydrogen is removed from a substance."

• Addition of Oxygen
  Oxygen adds on to magnesium to become magnesium oxide.
  
  
   
• Removal of Hydrogen
  Hydrogen is removed from hydroiodic acid to liberate free iodine.
  
  
  
  "Reduction is a reaction in which oxygen is removed from a substance or hydrogen is added to a substance."
  
  
• Removal of Oxygen
  Oxygen is removed from copper oxide to form copper metal.
  
  
  

• Addition of Hydrogen
  Hydrogen adds to chlorine to form hydrogen chloride gas.
  
  
  
  Nowadays, one can explain oxidation- reduction in terms of 'electrons transfer', which one will learn later on. Also, it is necessary to point out that oxidation and reduction reactions occur simultaneously. In brief, we call these reactions as "redox" reactions. Many reactions discussed earlier in this chapter can be broadly classified as redox and non-redox reactions. 
  

Example of Redox Reaction

Example of Non-redox Reaction

 

Reversible Reactions
Usually chemical reactions proceed only in one direction. But in some cases the reaction reverses itself. Example, when steam is passed over red hot iron, magnetic oxide of iron and hydrogen gas are formed. If hydrogen gas is passed over heated magnetic oxide of iron, the reverse reaction takes place, producing iron and steam.

 

Reversibility of a reaction is depicted by double arrow as or

Thermal Dissociation
This is also a type of reversible reaction, in which heat decomposes a substance. But if the products are not allowed to escape, then on cooling they recombine to form the original substance.

• Nitrogen dioxide decomposes above 50^oC to form nitric oxide and oxygen. But, if the products are cooled to below 50^oC, then the freshly formed nitric oxide and oxygen recombine to form nitrogen dioxide.
  
   
  
  

• When ammonium chloride is heated, it decomposes to form ammonia and hydrogen chloride. But when the products are cooled, they recombine to form ammonium chloride.
  
    
  
  It is to be remembered here that since NH₃ and HCl are gases, they should not be allowed to escape from the container after their formation.