Chapter 1 Chemical reaction and Equation

Magnesium ribbon is cleaned because magnesium react with atmospheric oxygen and CO₂ form stable compound. i.e. magnesium oxide & magnesium carbonate which causes hinderance in burning.

         2Mg         +      O₂        ----→         2MgO

  Magnesium         Oxygen                Magnesium oxide

1. Hydrogen  +  Chlorine  →  Hydrogen chloride

        H₂   +   Cl₂   →   2HCl
 

2. Barium chloride  +  Aluminium sulphate  →  Barium sulphate  +  Aluminium chloride

        3BaCl₂   +   Al₂(SO₄)₃   →   3BaSO₄   +   2AlCl₃
 

3. Sodium  +  Water  →  Sodium hydroxide  +  Hydrogen

        2Na   +   2H₂O   →   2NaOH   +   H₂

 1. Solutions of barium chloride and sodium sulphate in water react to give insoluble barium suphate and the solution of sodium chloride 

                   BaCl₂ (aq)   +   Na₂SO₄ (aq)    →    BaSO₄ (S)   +   NaCl (aq)

 2. Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solutions and water.

                   NaOH (aq)   +   HCl (aq)    →    NaCl (aq)   +   H₂O

(i)  Substance 'X' is calcium oxide commonly known as Quick Lime.

     Its formula : CaO

(ii) Calcium oxide reacts with water and form calcium hydroxide or slaked lime.

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

     Calcium oxide               Water              Calcium hydroxide

Electrolysis of water is carried out in this activity. In which electric current is passed through water with sulphuric acid.

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

The gas collected over cathode is Hydrogen and the gas collected over anode is Oxygen.

Volume of H₂ gas is double than that of  O₂ gas collected over anode. Through a balanced chemical equation, its clear that  two molecules of hydrogen and one molecule of oxygen evolved.

       Fe (s)        +         CuSO₄ (aq)          ----→          FeSO₄ (aq)                +          Cu(s)

       Iron                  Copper Sulphate                      Ferrous Suplhate                  Copper

Displacement reaction occurs. As Iron is more reactive than copper, iron displace copper from copper sulphate and forms ferrous sulphate, which is light green colour compared to dark blue colour solution of copper sulphate.

     Pb(NO₃)₂ (aq)      +       2KI (aq)               →          PbI₂ (s)           +          2KNO₃ (aq)
     Lead Nitrate             Potassium Iodide                  Lead Iodide                Potassium Nitrate

When the aqueous solution of lead Nitrate is mixed with potassium Iodide, we obtain a precipitate of lead iodide and a solution of potassium nitrate. In double ddisplacement reaction their is exchange of ions between the reactant. In this reaction, the cation Pb²⁺ and K⁺ exchange their anions NO₃^- and I^- to give new substance PbI₂ and KNO₃.

(i)  4Na(s)   +   O₂(g)    →    2Na₂O(s)

In this reaction sodium is oxidised because oxygen combines to sodium form sodium oxide and oxygen is reduced because it is converted into sodium oxide.

(ii)   CuO(s)   +   H₂(g)    →    Cu(s)   +   H₂O(l)

In above reaction, copper oxide is reduced to form copper because copperoxide loses oxygen and form copper where as Hydrogen gets oxidised by gaining oxygen from copper oxide and form water molecule.

Explanation:

(a) Lead is getting reduced is incorrect as lead oxide is getting reduced.

(b) Carbon dioxide is getting oxidised is incorrect answer as carbon is getting oxidised.

(c) Carbon is getting oxidised is correct answer.

(d) Lead oxide is getting reduced is correct answer.

Correct answer is (d).

In this example displacement reaction occurs, because Fe in Fe₂O₃ has been displaced by Al. Aluminium being more reactive than Iron, it displaces iron to form Aluminium oxide Al₂O₃.

Correct answe is (a).

Explanation :       Fe      +      2HCl        →        FeCl₂      +      H₂
                           Iron         Hydrochloric          Ferrous         Hydrogen
                                               acid                  chloride             gas
                                                                        or Iron
                                                                        chloride

A balanced chemical equation is an equation in which the number of each element are equal on the reactant side as well as in product side.

for example:                  BaCl₂  +  K₂SO₄  →   BaSO₄   +   2KCl
                                         reactant side               product side

Chemical equation should be balanced because only a balanced equation tells us the relative quantities of different reactants and product involved in the reaction.

(a) Hydrogen gas combines with nitrogen to form ammonia

         H₂                   +      N₂               ---------→              NH₃

Hydrogen gas          nitrogen gas                          Ammonia

       3H₂                    +       N₂              -----------→              3NH₃
   Hydrogen  gas            NItrogen gas                                Ammonia
        

(b)   Hydrogen sulphide gas burns in air to give water and sulpur dioxide

        H₂S(g)                                +           O₂(g)                    ----------------→        H₂O(l)     +     SO₂(g)
      Hydrogen sulphide gas           Oxygen (from air)                                      water          Sulpur dioxide

       2H₂S(g)                               +          3O₂(g)                    --------------→         2H₂O(l)     +     2SO₂(g)

(c)    Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate

        BaCl₂(aq)               +            Al₂(SO₄)₃(aq)                --------→         AlCl₃(aq)                        +                  BaSO₄(s)

      Barium chloride                Aluminium sulphate                       Aluminium chloride                      Barium sulphate

        3BaCl₂(aq)               +            Al₂(SO₄)₃(aq)                -------→          2AlCl₃(aq)                     +                   3BaSO₄(s)

     
(d)  Potassium metal reacts with water to give potassium hydroxide and hydrogen gas

       K(s)           +     H₂O(l)       -----→              KOH(aq)                   +             H₂(g)

     Potassium         Water              potassium hydroxide                 hydrogen gas

       2K(s)           +  2H₂O(l)   ------- →       2KOH(q)                         +               H₂(g)

(a)   2HNO₃  +  Ca(OH)₂  →  Ca(NO₃)₂  +  2H₂O

(b)   2NaOH  +  H₂SO₄   →   Na₂SO₄  +  2H₂O

(c)   NaCl  +  AgNO₃   →   AgCl  +  NaNO₃

(d)   BaCl₂  +  H₂SO₄   →   BaSO₄  +  2HCl

(a)   Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

                   Ca(OH)₂  +  CO₂   →   CaCO₃   +   H₂O

(b)  Zinc + Silver nitrate → Zinc nitrate + Silver

               Zn  +  AgNO₃   →   Zn(NO₃)₂  +  Ag

(c)   Aluminium + Copper chloride → Aluminium chloride + Copper

                      2Al  +  3CuCl₂   →   2AlCl₃  +  3Cu

(d)   Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

             BaCl₂  +  K₂SO₄   →   BaSO₄  +  2KCl

(a)  2KBr  +  BaI₂  →  2KI  +  BaBr₂ 
Type of reaction: Double displacement reaction. Because the cation K⁺ and Ba²⁺ have exchanged their anions Br^- and SO₄^2- .

(b)  ZnCO₃   →   ZnO  +  CO₂
Type of reaction: Decomposition reaction. Because in this reaction single reactant break down into two product

(c)  H₂  +  Cl₂   →   2HCl
Type of reaction: Combination. Because in this reaction two reactant combine into single product.

(d)  Mg  +  2HCl   →   MgCl₂  +  H₂
Type of reaction : disclacement reaction because H in HCl has been displaced by Mg.

Exothermic reaction: A reaction in which energy is released in the form of heat or light.

Eg.:  Burning of Natural gas (methane)

         CH₄(g)  +  O₂(g)  →  CO₂(g)  +  H₂O(l)  +  Heat

Endothermic reaction: A reaction in which energy is absorbed from the surroundings and cooling is produced.

Eg.:  Decomposition of calcium carbonate

        CaCO₃(s)                  -------------→      CaO(s)           +             CO₂(g)
      Calcium  Carbonate                         Calcium  oxide                Carbon dioxide
           or  Lime stone                             or quick lime
                               

Respiration is a process in which we inhale oxygen from the atmosphere which oxidises glucose in our body to produce carbon dioxide, water and heat is evolved.

               C₆H₁₂O₆(aq)  +  6O₂(g)  ----------> 6CO₂ + 6H ₂O(l)  +  Heat

Hence, heat is evolved during respiration it is considered as an exothermic reaction.

In decomposition reaction, a single reactant breaks down into two or more simpler product, while in combination reaction two or more reactant combine to form a single product. Therefore decomposition and combination reaction are opposite.

Example of decomposition :

(i)    2FeSO₄(s)                    --^Heat→    Fe₂O₃(s)      +    SO₂(g)    +    SO₃(g)
       Ferrous  sulphate                         Ferric oxide
       

(ii)   CaCO₃(s)    --^Heat→    CaO(s)    +    Co₂(g)
       Lime stone                  Quick lime

Example of combination :

(i)   CaO(s)    +    CO₂(g)                 →      CaCO₃(s)
     Quick lime     Carbon dioxide               Lime stone
                           

(ii)   Burning of coal →

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

Decompositive reaction in presence of Heat:

        2Pb(NO₃)₂ (s)     -- ^Heat →     2PbO (s)     +              4NO₂(g)       +     O₂(g)
         Lead Nitrate                         Leadoxide           Nitrogen dioxide         Oxygen
                                                                                  

Decompositive reaction in presence of Light:

                       2AgCl                  - ^Light→     2Ag     +     Cl₂ gas
        Silver Chloride (white)                         (grey)            
       

Decompositive reaction by the use of electricity:

        2H₂O (l)    -- ^Electricity→     2H₂ (g)            +     O₂ (g)

        water                                 hydrogen gas        oxygen gas

Displacement Reaction :  In this reaction more reactive element displaces the less reactive element

                                             AB     +     C     →     AC     +     B

eg.: 

(i)   Fe  +  CuSO₄  →  FeSO₄  +  Cu
(ii)   Pb  +  CuCl₂  →  PbCl₂  +  Cu

Double Displacement Reaction :  In this reaction exchange of ions takes place

                                             AB     +     CD     →     AC     +     BD

eg.: 

(i)   Na₂SO₄  +  BaCl₂  →  2NaCl  +  BaSO₄
(ii)   HCl  +  AgNO₃  →  AgCl  +  HNO₃

Reaction occurs as:

    2AgNO₃     +     Cu     →     Cu(NO₃)₂     +     2Ag
      Silver            Copper          Copper             Silver
      nitrate                                  nitrate

In a chemical reaction a insoluble substance (precipitate) is formed is called precipitation reaction.

eg.:

(i)   AgNO₃     +     NaCl     →     AgCl     +     NaNO₃
       Silver            Sodium           Silver          Sodium
      nitrate           Chloride         Chloride        nitrate

(ii)    Na₂SO₄     +     BaCl₂     →     BaSO₄     +     2NaCl
        Sodium            Barium             Barium           Sodium
        Sulphate         Chloride           Sulphate         Chloride
                                                          (ppt)

(a) Oxidation: Reaction in which a substance gains oxygen or addition of oxygen or removal of hydrogen in a reaction is called oxidation.

Eg.: 

       (i)   2Cu  +  O₂   →   2CuO
       (ii)  4Al  +  3O₂   →   2Al₂O₃

(b) Reduction: Reaction in which a substance looses oxygen or removal of oxygen or gain of hydrogen in a reaction is called reduction.HEg.:

       (i)   CuO  +  H₂   →   Cu  +  H₂O
            oxygen is removed from copper oxide so reduction of CuO occurs.

       (ii)  H₂S  +  Cl₂   →   2HCl  +  S
             gain of hydrogen by chlorine gas, so chlorine is reducted to form HCl.

The brown coloured element 'X' is copper. On heating in air it forms copper oxide which is black in colour.

            2Cu(s)    +    O₂(g)      →      2CuO(s)
            Copper        Oxygen             Copper oxide
            (Brown                                   (Black)
              color)

Iron articles is painted to prevent from rusting, so that iron article is do not come in contact with atmosphere oxygen and moisture. This  will prevented it from rusting.

As nitrogen is inert gas it doesnot react with oil and fat. Nitrogen gas prevent oil and fat containing food items from rancidity. Food items get rancid means it smells and tastes bad due to oxidation with the atmosphere oxygen. 

(a)  Corrosion: Metal are reacted with atmosphere oxygen, carbon dioxide and moisture and converted into metal oxides and carbonate. This phenomenon is called corrosion.

Eg.: 

        (i)   Black coating on silver.

             4Ag + 2H₂S + O₂ -----------> 2Ag₂S + 2H₂O
        (ii)  Green coating on copper. 

              Cu + CO₂ + H₂O  -----------> CuCO₃. Cu (OH)₂
(b)   Rancidity: Oil and fat containing items when comes in contact with atmosphere oxygen they get oxidised and start smelling and tasting bad. The food item have gone rancid. This phenomeon is called rancidity.

Eg.:

        (i)   Smell and taste on food items get change after some times.