Types of Chemical Reactions-II

Chemical Reactions and Equations-II

Types of Chemical Reactions

Introduction

Chemical reactions involves making and breaking of bonds between the reacting atoms to produce new substances. Here we will focus on what are the different types of chemical reactions and what are the factors involved in bond formation.
1. Combination reaction: In this type of reactions two or more reactants combine to form a single product. Large amount of energy is released during this reaction.

A   +   B  →        C  (Reactants)      (Product)

Example: Quick lime (CaO) reacts with water to produce slaked lime Ca (OH)₂ and large amount of energy is released.

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

               (Quick lime)                                      (Slaked lime)

             (Calcium oxide)                              (Calcium hydroxide)

   2. Decomposition reaction: It involves the breaking down of a single compound into two or more simpler substances. The reactant can be decomposed either by heat (thermal decomposition), electricity, and sunlight (photo decomposition).

A        →          B   +   C (Reactant)             (Products)

     a)  Thermal decomposition: A reactant breaks down into two or simpler substances or products on heating.

                                                                    i. Decomposition of Ferrous sulphate

                  2FeSO₄ (s) →   Fe₂O_{3 (S) +}   SO_{2 (g)} (↑) +  SO_{3 (g)} (↑)

Observation- Ferrous sulphate crystals (FeSO_4.7H₂O) lose water when heated and the color of crystals changes from green to white (FeSO₄). It then decomposes to ferric oxide, sulphur dioxide and sulphur trioxide.

                                                                 ii.   Decomposition of Calcium carbonates (Limestone)

                             CaCO_{3 (S)}   → CaO (s)   +   CO₂ (g) (↑)

                                 Used in manufacture of cement

                                                                 iii.   Decomposition of Lead nitrate

        2Pb (NO₃)_{2 (S)}      →   2PbO_(S)      +      4NO₂ (g) (↑)       +        O₂ (g) (↑)

        (Lead nitrate)          (Lead oxide)     (Nitrogen dioxide)            (Oxygen)

Observation- The emission of brown fumes is characteristics of this reaction.

      b)   Electric decomposition: A reactant breaks down into simpler substances or product on passage of electricity.

i.                    Decomposition of water

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

Observation – On passing electricity through water hydrogen is liberated at cathode (negative electrode) and oxygen is liberated at anode (positive electrode) in the ratio 2:1.

      c)      Photo decomposition: A single substance breaks down into two or more substances by sunlight.

i.       2AgCl (s)         →     2Ag (s)   +   Cl₂ (g)

                   (Silver chloride)           (Silver)

                          (White)                  (Grey)

ii.       2AgBr (s)         →     2Ag (s)   +     Br₂ (g)

                  (Silver bromide)             (Silver)           (Bromine)

                       (Yellow)                     (Grey)

Note: These reactions are used in black and white photography.

    3.  Displacement reaction: More reactive element displaces the less reactive element from its salt solution. The displacement of elements is based on the reactivity series.

A    +     BSO4              →     ASO4    +    B                                       (Salt solution)

Reactivity series: The reactivity series is a list of metals arranged in the order of their decreasing activities.

K                          (Most reactive) Na Ca Mg Al Zn Fe Pb H Cu Hg Ag Au                        (Least reactive)

Example:    Zn +              CuSO₄      →          ZnSO₄      +      Cu

                 (Zinc)      (Copper sulphate)     (Zinc sulphate)  (Copper)

    4. Double displacement reaction: Exchange of ions takes place between the compounds to form new compounds. A precipitate is formed as a result of double displacement reaction.

Precipitate – Insoluble solid that settles down at the bottom when two compounds react. It is shown by ↓ in the chemical reaction.

A+ B-    +    C+ D-     →      AD   +    CB

Example:  Reaction between lead nitrate and potassium iodide so as to give lead iodide (Yellow ppt.) and potassium nitrate.

       Pb(NO₃)_{2 (aq)}       +         KI _(aq)        →        PbI₂ (↓)    +        KNO₃

    (Lead nitrate)       (Potassium iodide)     (Lead iodide)  (Potassium nitrate)

                                                               (Yellow precipitate)

5.      Oxidation and Reduction Reactions:

a)      Oxidation: It can be defined as, the process in which oxygen is added and hydrogen is removed. Also the process in which a substance loses electron(s).

b)      Reduction: It can be defined as, the process in which hydrogen is added and oxygen is removed. Also the process in which a substance gains electron(s).

c)      Oxidising Agent: A substance which helps in the oxidation of another substance. Oxidising agent itself gets reduced.

d)     Reducing Agent: A substance which helps in the reduction of another substance. Reducing agent itself gets oxidised.

    6. Exothermic Reactions: The reactions in which heat/energy is released along with the formation of products.

Example: Respiration is an exothermic process. Food contains carbohydrates; these carbohydrates are broken into glucose. This glucose combines with oxygen in the cells and provides energy.

C₆H₁₂O_{6 (s)}    +    6 O_{2 (g)}      →    6 CO_{2 (g)}    +   6 H₂O (l)    +   Energy

                          Glucose            Oxygen

     7. Endothermic Reactions: The reactions in which heat/energy is needed for the reaction to occur.

Example: Photosynthesis is an endothermic process.

6 CO_{2 (g)}    +   6 H₂O (l)   + Energy       →           C₆H₁₂O_{6 (s)}    +    6 O_{2 (g)}

_{Prepared by Ms. Priti Maan}