Chemical Reactions: Transforming Atoms through Bond Changes

During a chemical reaction, neither atoms of one element change into those of another element nor do atoms disappear from the mixture or appear from elsewhere. In fact, chemical reactions involve the breaking and making of bonds between atoms to produce new substances.

Chemical Reaction: Combination Reaction and Exothermic Reaction

• Combination Reaction: When two or more substances (elements or compounds) combine to form a single product, the chemical reactions are called combination reactions. For example,
• Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

                      CaO (s) + H2O (l) → Ca(OH)2 (aq) + Heat

                  (Quick lime)                                   (Slaked lime)

• In this chemical reaction, calcium oxide and water combine to form a single product, calcium hydroxide.

• Burning of coal

                     C(s) + O2 (g) → CO2 (g)

• Formation of water from H2 (g) and O2 (g)

                     2H2 (g) + O2 (g) → 2H2O (l)

• • Exothermic chemical reaction: Reactions in which heat is released along with the formation of products are called exothermic chemical reactions. 

• Example:

I. Burning of natural gas

       CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)

II. Respiration 

• During digestion, food is broken down into simpler substances. 
• Example: rice, potatoes and bread contain carbohydrates. 
  • These carbohydrates are broken down to form glucose. 
  • This glucose combines with oxygen in the cells of our body and provides energy.

        C6H12O6 (aq) + 6O2 (aq) → 6CO2 (aq) + 6H2O (l) + energy

        (Glucose)

III. The decomposition of vegetable matter into compost.

Chemical Reaction: Decomposition and Endothermic Processes

• In this chemical reaction a single reactant breaks down to give simpler products. 
• Example:

I. 2FeSO4 (s)    →    Fe2O3 (s) + SO2 (g) + SO3 (g)

• Ferrous sulphate crystals (FeSO4, 7H2O) lose water when heated and the green colour of the crystals changes.  
• It then decomposes to ferric oxide (Fe2O3), Sulphur dioxide (SO2) and Sulphur trioxide (SO3).

II. Decomposition of Calcium Carbonate (Limestone) to calcium oxide (called lime or quicklime) and carbon dioxide on heating is an important decomposition chemical reaction used in various industries. 

1. 1. 1. It has many uses such as in the manufacture of cement. When a decomposition reaction is carried out by heating, it is called thermal decomposition.

             CaCO3 (s)    →    CaO(s) + CO2 (g)

          III.  2Pb(NO3)2 (s)  →     2PbO(s)                 +             4NO2 (g)                 +                 O2 (g)

                (Lead nitrate)         (Lead oxide)      (Brown fumes of Nitrogen dioxide)          (Oxygen)

IV. 2AgCl(s) → 2Ag(s) + Cl2 (g) 

          White silver chloride turns grey in sunlight.

V. 2AgBr(s)  2Ag(s) + Br2 (g)

• The above reactions of silver chloride and silver bromide are used in black and white photography.
• Endothermic Reactions: The decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants. 
  • Thus, the chemical reactions in which energy is absorbed are known as endothermic reactions.

Chemical Reaction: Displacement Reaction

• A chemical reaction in which a more reactive element displaces a less reactive element from a compound is known as displacement reaction. For example,
• Fe(s)     +     CuSO4 (aq)               →              FeSO4 (aq) + Cu(s)

                                                                                 (Iron sulphate)

• The iron nail becomes brownish in colour and the blue colour of copper sulphate solution fades. 
• In this chemical reaction, iron has displaced or removed another element, copper, from copper sulphate solution. 
• Zn(s)     +     CuSO4 (aq)          →         ZnSO4 (aq)   +   Cu(s)

                           (Copper sulphate)                (Zinc sulphate)

• Pb(s)     +     CuCl2 (aq)        →       PbCl2 (aq)     +     Cu(s)

                          (Copper chloride)            (Lead chloride)

• Zinc and lead are more reactive elements than copper.

Chemical Reaction: Double Displacement Reaction 

• Chemical reactions in which there is an exchange of ions between the reactants are called double displacement reactions. 

• Example:

               Na2SO4 (aq)          +            BaCl2 (aq)      →      BaSO4 (s)            +            2NaCl (aq) 

           (Sodium sulphate)       (Barium chloride)      (Barium sulphate)      (Sodium chloride)

• The white precipitate of BaSO4  is formed by the reaction of SO4-2 and Ba2+which is insoluble in water) is formed. 
• This insoluble substance formed is known as a precipitate. 
• Any chemical reaction that produces a precipitate can be called a precipitation reaction.
• The other product formed is sodium chloride which remains in the solution.

Chemical Reaction: Oxidation and Reduction (Redox Reactions) 

• One reactant gets oxidised while the other gets reduced during a reaction. Such chemical reactions are called oxidation-reduction reactions or Redox reactions. 

• Example:
• When copper powder is heated, the surface of copper powder becomes coated with black copper(II) oxide. 
  1. This is because oxygen is added to copper and copper oxide is formed.

               2Cu + O2   →  2CuO

• If hydrogen gas is passed over this heated material (CuO), the black coating on the surface turns brown as the reverse reaction takes place and copper is obtained.

                 CuO + H2   →  Cu + H2O

• If a substance gains oxygen or loses hydrogen during a reaction, it is oxidised. 
• If a substance loses oxygen or gains hydrogen during a reaction, it is reduced. 
• During this reaction, the copper(II) oxide is losing oxygen and is being reduced. 
• The hydrogen is gaining oxygen and is being oxidised.

ZnO + C → Zn + CO

• In this reaction carbon is oxidised to CO and ZnO is reduced to Zn.

MnO2 + 4 HCl → MnCl2 + 2 H2O + Cl2 

• In this reaction HCl is oxidised to Cl2 whereas MnO2 is reduced to MnCl2.

Chemical Reaction: Effects of Oxidation in Everyday Life

• Corrosion: When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion. 
  • Example: Iron articles get coated with a reddish brown powder when left for some time by the process known as rusting of iron. 
  • The black coating on silver and the green coating on copper are other examples of corrosion.
• Rancidity: When fats and oils are oxidised, they become rancid and their smell and taste change. 
  • Usually substances which prevent oxidation (antioxidants) are added to foods containing fats and oil. 
  • Keeping food in air tight containers helps to slow down oxidation. 
  • Example: chips manufacturers usually flush bags of chips with gas such as nitrogen to prevent the chips from getting oxidized. 

DOWNLOAD PDF