Class 10: Chemical Reactions and Equations Notes

CHAPTER 1:

CHEMICAL REACTIONS AND EQUATIONS

Chemical Change: It is a change in which one or more new substances are formed. In a chemical change –    

1. New substances are formed
2. Change in mass
3. Changes in involved energy
4.  Some permanent change may also takes place

Chemical Reaction: Chemical reactions are the chemical changes that results in the formation of a new substance. Following observations helps us to determine whether a chemical reaction has taken place-

1. Change in state
2. Change in colour
3.  Evolution of gas
4. Change in temperature

What happens when magnesium ribbon is burnt?

When magnesium ribbon burns in air, it burns with a dazzling white flame and changes into a white powder. This powder is magnesium oxide.

What will you observe when lead nitrate solution is added in potassium iodide solution?

When both the solution mixes, the lead particles and iodide particles combine and create two new compounds, a yellow solid called lead iodide and white solid called potassium nitrate.

Pb(NO₃)₂   + 2KI → PbI₂     +   2KNO₃

What happens when zinc granules are added in the dilute hydrochloric acid or sulphuric acid?

By the action of dilute sulphuric acid on zinc, hydrogen gas is released with the formation of zinc chloride.

 How can we determine that whether a chemical reaction takes place or not?

Following observations helps us to determine whether a chemical reaction has taken place-

1. ·         Change in state
2. ·         Change in colour
3. ·         Evolution of gas
4. ·         Change in temperature

Chemical Equations: The description of a chemical reaction in a sentence form is quite long. It can be written in a shorter form, the simplest way to do this is to write it in the form of a word equation. This word equation is known as chemical equation.

Reactant: The substances that undergo chemical changes in the reaction are the reactants.

Product: The new substance that is formed during the reaction is known as product.

Balanced Chemical Equations: A balanced chemical equation is the one in which the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in reactants.

How to balance chemical equation:

Let us try to balance following equation –

Fe     +      H₂O       →        Fe₃O₄      +     H₂

(i)Count the number of atoms of different elements

Element	No. of atoms in reactant	No. of atom in product
Fe	1	3
H	2	2
O	1	4

(ii)Start balancing with the compound that contains maximum no. of atoms, it may be reactant or product. We select Fe₃O₄ and element oxygen in it.

Element	No. of atoms in reactant	No. of atom in product
Fe	1	3
H	2	2
O	1  X  4 = 4	4

Fe     +     4H₂O       →        Fe₃O₄      +     H₂

(iii)Now let’s balance hydrogen atoms

Element	No. of atoms in reactant	No. of atom in product
Fe	1	3
H	8	2 X 4 = 8
O	4	4

Fe     +     4H₂O       →        Fe₃O₄      +     4H₂

(iv)Now we have to balance Fe

Element	No. of atoms in reactant	No. of atom in product
Fe	1 X 3 = 3	3
H	8	8
O	4	4

 

Types of chemical reactions:

(1)    COMBINATION REACTION: The chemical reaction in which a single product is formed from two or more reactants is known as a combination reaction.

For example

(i) C + O₂ → CO₂

                          (ii) 2H₂ + O₂ →  H₂O

                        (iii) CaO + H₂O   →  Ca(OH)₂   + Heat

                                                                                                             

(2)    DECOMPOSITION REACTION: The chemical reaction in which a single reactant breaks down to give simple products is known as decomposition reaction.

For example

(i)                  2FeSO₄   → Fe₂O₃   +   SO₂(g)   + SO₃(g)

(ii)                +

v  Decomposition of calcium carbonate to calcium oxide and carbon dioxide on heating is an important decomposition reaction used in various industries. Calcium oxide is called lime or quick lime, it is used in the manufacture of cement.

v  When decomposition is carried out by heating, it is called thermal decomposition. The reactions given in examples above are thermal decomposition reactions.

(3)    DISPLACEMENT REACTION: The chemical reaction in which an element of a compound displaces or remove another element is known as displacement reaction.

For example

(i)                  Fe  +  CuSO₄     →   FeSO₄   +   Cu

(ii)                Zn   +  CuSO₄   →    ZnSO4  +  Cu

(iii)               Pb  + CuCl₂     →    PbCl₂    + Cu

       

(4)    DOUBLE DISPLACEMENT REACTION: The chemical reaction in which there is an exchange of ions between the reactants are called double displacement reaction.

For example

(i)                  Na₂SO₄    +   BaCl₂   →   BaSO₄     +    2NaCl

In this reaction SO₄^2- and Ba²⁺ ions exchange between the reactants.

(5)    OXIDATION AND REDUCTION: The chemical reaction in which if a substance gains oxygen or loses hydrogen during a reaction, it is oxidised and if a substance loses oxygen or gains hydrogen during a reaction, it is reduced.

For example

(i)                  Oxidation :  2Cu  +   O₂   →   2CuO

(ii)                Reduction : CuO  +  H₂    →   Cu   +    H₂O

v  REDOX REACTION: When one reactant gets oxidised while the other gets reduced during a reaction. Such reactions are called oxidation reduction reactions or redox reactions.

           

                                                       

What will happen by the reaction of calcium oxide with water?

By the reaction of calcium oxide with water, formation of slaked lime occurs.

CaO  +  H₂O  →  Ca(OH)₂  {SLAKED LIME}

v  Slaked lime is used for white washing the walls. Calcium hydroxide or slaked lime reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of white washing and gives a shiny finish to the walls.

Ca(OH)₂  +  CO₂  →  CaCO₃  +  H₂0

EXOTHERMIC REACTION: Reaction in which heat is released along with the formation of products are called exothermic reaction.

ENDOTHERMIC REACTION: Reaction in which heat is absorbed for the formation of products are called endothermic reaction.

Is respiration an endothermic reaction or exothermic reaction?

Respiration is an exothermic reaction because in the process of respiration, energy is released. As we need energy to stay alive, we get this energy from the food we eat. During digestion, food is broken down into simpler substances. For example, the food that contain carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy.

C₆H₁₂O₆  +   6O₂  →  6CO₂  +   6H₂O  +  ENERGY

RUSTING OF IRON: Iron articles are shiny when new, but get coated with a reddish brown powder when left alone for some time. This process is known as rusting of iron.

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.

For example: black coating on silver, green coating on copper.

RANCIDITY: Food turns rancid when the fats and oils within them get oxidised and the taste and smell of the food changes. Oxidation of fats generates highly reactive molecule in the ramcid food which is responsible for the unpleasant odour and flavour.

PREVENTION FROM CORROSION:

1.  Painting the metal surface.
2.  Powder coating of the surfaces.
3. Oiling the metal.
4. Galvanization: it is the process of applying a protective zinc coating to iron or steel articles, to prevent them from rusting. 

PREVENTION FROM RANCIDITY:

1. Adding antioxidants (substances which prevent oxidation) to food.
2. Storing food in airtight containers to slow the process of rancidification.
3. Refrigerating food also helps to slow down rancidification.
4. Replacing oxygen in the containers with another gas. For example, to prevent chips from turning rancid, chips manufacturers flush the bags with nitrogen gas.