Chemical Reactions and Equations Blog 4

Dear Learners,

I hope you all are following the instruction given in previous blogs. I also presume that you all are well aware of writing and balancing of chemical equations.

I feel you all have not only taken the notes but also completed the home assignment in your respective chemistry note book.

In the previous blog, we have discussed writing and balancing chemical equation. I must help you to recall

(i) Chemical reactions are balanced to follow law of conservation of mass.

(ii) Follow the rules to balance the chemical equation.

Now let us discuss the very important topic- TYPES OF CHEMICAL REACTIONS

 Types of reaction- Combination, Decomposition and Single Displacement reactions will be discussed in todays blog.

Happy Learning!

At the end of this topic, learners will be able to 

(i) define combination reactions, decomposition reactions, displacement reactions.

(ii) write the reactions of above stated chemical reactions.

(iii) apply the reactions in your real life.

(iv) give correct answer based on activities given in your text book.

(iv) write the correct answer in your board exams.

Key Instructions:

Green lines are meant to be written in note book.

Home Assignment need to be done in note book.

So, Let us begin our topic TYPE OF CHEMICAL REACTION

As we all aware that during chemical reaction atoms of one element do not change into those of another element. Nor do atoms disappear from the mixture or appear from elsewhere. Actually, chemical reactions involves the breaking and making of bonds between atoms to produce new substances.

NOTE: You all will study about types of bonds formed between atoms in Chapter 3 and 4.

Let us study here first types of chemical reactions.

The first reaction is

1. Combination Reaction: When two or more substances (element or compounds) combine to form a single product, the reactions are called combination reactions.

Example of Combination Reaction happening between two elements

           

  (i) Burning of coal                                         C   +   O₂   à CO₂

  (ii) Formation of water from H₂ and O₂          2H₂   +   O₂   à 2H₂O

 (iii) Formation of Ammonia gas                 N₂   +    3H₂      à 2NH₃

Example of Combination Reaction happening between two compounds

Refer Activity 1.4 NCERT Page 6 Chapter 1

(i) Reaction between Quick Lime (CaO) and water    

                    CaO     + H₂O   à    Ca(OH)₂  + heat

In this reaction, a single product is formed from two or more reactant is known as a combination reactions. Dear learners, you should note here that large amount of heat is also released in this reaction which will be discussed in next topic.

Now let us take few more examples of combination reactions

Example 1: Hydrogen combines with chlorine to form hydrogen chloride.
                 H₂   +    Cl₂   à 2HCl
Example 2: Sodium metal burns in chlorine to form sodium chloride.
                  2Na    +    Cl2    à 2NaCl
Example 3: iron is heated with Sulphur, it forms iron sulphide.
                     Fe   +   S   à FeS
Example 4: Ammonia reacts with hydrogen chloride to form ammonium chloride.
                NH₃   +     HCl     à  NH₄Cl               
Example 5: Carbon monoxide reacts with oxygen to form carbon dioxide.
            2CO    +    O₂  à 2CO₂
Example 6: Sulphur dioxide reacts with oxygen to produce Sulphur trioxide.
              2SO₂    +   O₂   à   2SO₃

I hope you all are now well aware of combination reactions. So now, let us move to next reaction.

https://www.youtube.com/watch?v=-0DCopjuCwg

https://www.youtube.com/watch?v=-0DCopjuCwg


2. Decomposition Reaction

Those reaction in which a compound splits up into two or more simpler substances are known as decomposition. The decomposition reactions are carried out by applying heat, light or electricity which provide energy to break a compound into two or more simpler compounds. When a decomposition reaction is carried out by heating, it is called ‘thermal decomposition reaction’. When a decomposition reaction is carried out by passing electricity, it is called ‘electrical decomposition reaction.  Decomposition reaction can also be carried by light energy.

Thermal Decomposition:
Example 1: when calcium carbonate is heated, it decomposes to give calcium oxide and carbon dioxide.
                    CaCO₃   à  CaO     +   CO₂
Decomposition of calcium carbonate to calcium oxide and carbon dioxide on heating is thermal decomposition. Calcium oxide (CaO) is called lime or quick lime. It has used in manufacture of cement.
Example 2:  when potassium chlorate is heated in the presence of manganese dioxide catalyst, it decomposes to give potassium chloride and oxygen.
             2KClO3  à  2KCl    + 2O₂
Example 3: Refer activity 1.5 when ferrous sulphate is heated strongly, it decomposes to form ferric oxide, Sulphur dioxide and Sulphur trioxide.
           2FeSO₄  à Fe₂O₃    +    SO₂    +   SO₃
Example 4:Refer activity 1.6  when lead nitrate is heated strongly, it breaks down to forms lead monoxide, nitrogen dioxide and oxygen.
    2Pb(NO₃)₂    à  2PbO    +  4NO₂    +   O₂

      Colourless           yellow       brown fumes

 Electrical Decomposition

Example 1: refer activity 1.7 when electric current is passed through acidified water, it decomposes to give hydrogen gas and oxygen gas.
                                        2H₂O  à   2H₂  +  O₂

Example 2: when electric current is passed through molten sodium chloride, it decomposes to give sodium metal and chlorine gas.
                    

                   2NaCl    à     2Na    +     Cl₂

Example 3: when electric current is passed through molten aluminium oxide, it decompose to give aluminium metal and oxygen.
                  2Al₂O₃    à  4Al    + 3O₂

Photo Decomposition
Example 1: Refer activity 1.8 when silver chloride is exposed to light, it decomposes to form silver metal and chlorine gas.
     2AgCl   à  2Ag    + Cl2
Example 2: Silver bromide decompose into silver and bromine gas.
     2AgBr     à    2Ag   +   Br₂

https://www.youtube.com/watch?v=ZDfxr6CK5PE

Uses of Decomposition Reaction:
The decomposition reaction carried out by electricity are used to extract several metals form their naturally occurring compounds like chlorides or oxides. When the fused molten metal chloride or metal oxide is decomposed by passing electricity, then metal is produced at the cathode (negative electrode).  To be discussed in detail in chapter 3.
Decomposition Reaction in our body: the digestion of food in the body is decomposition reaction. When we eat food which contains starch it decompose into simple sugar, protein decomposes into amino acids.

3. Displacement Reaction:

The reactions in which one element takes the place of another element in a compound are known as displacement Reaction. A more reactive element displace a less reactive element from its compound.

Refer Activity 1.9 Page 10 Chapter 1 NCERT
In the reaction, iron nail become brownish in colour and the blue colour of copper sulphate fades. The following chemical reaction takes place in this activity-
        Fe      +   CuSO4    à    FeSO4     +     Cu
In this reaction, iron has displaced or removed another element, copper, from copper sulphate solution. This reaction is known as displacement reaction.
Other examples of displacement reactions are
Example 1: zinc metal is placed in copper sulphate solution, then zinc sulphate solution and copper are obtained.
               Zn     +     CuSO₄       à     ZnSO₄     +      Cu
Example 3: Lead metal is placed in a solution of copper chloride then lead chloride and copper metal is formed.

                       CuCl₂      +      Pb       à      PbCl₂    + Cu
Zinc and lead are more reactive elements than copper. They displace copper from its compounds.
Example 4: Copper reacts with silver nitrate then copper nitrate and silver metal is formed.
                  2AgNO₃      +  Cu   à Cu(NO₃)₂      +    2Ag
Example 2: Magnesium metal is placed in copper sulphate solution then magnesium sulphate solution and copper metal is formed.            
       Mg       +      CuSO₄     à      MgSO₄      +  Cu

https://www.youtube.com/watch?v=r2u3J0dDsfU

Home Assignments

1. Complete your notebook.

 2. A colourless lead salt, when heated, produces a yellow residue and brown fumes.

      i. Name the lead salt.

      ii. Name the brown fumes.

      iii. Write a chemical equation of the reaction involved.

   3.  What happens when silver chloride is exposed to sunlight? Write a chemical equation for this reaction. Also give one use of such a reaction.

    

    4. Why are decomposition reactions called the opposite of combination reactions? Explain with equations of these reactions.

    

    5. When a green iron salt is heated strongly, its colour finally changes to black and odour of burning Sulphur is given out.

a) Name the iron salt.

b)      Name the type of reaction that takes place during the heating of iron salt.

c) Write a chemical equation of the reaction involved.