Chemical Reactions and Equations Class 10 Notes

WHAT IS CHEMICAL REACTION

The process in which the original substance loses their identities, nature and properties; and produce a new substance having different physical and chemical properties is called chemical reaction.

Q. Name the factors/components of chemical reaction.

Chemical reaction has two components:

a)      Reactants ­– The substance which participate in the chemical reaction are called reactants.

b)      Product – The new substance produced in a chemical reaction is called product.

 

 

 

CHARACTERISTIC PROPERTIES OF CHEMICAL REACTION

Q. How to determine a chemical reaction?

To determine a chemical reaction, the different changes are as follows:

        i.          Change in state

       ii.          Change in colour

     iii.          Change in temperature

     iv.          Evolution of gas

       v.          Formation of precipitate

 

ACTIVITY 1.1

1.      Why should we clean a magnesium ribbon with sand paper before burning in air?

Ans. It is so because to remove the oxide layer of magnesium oxide, so that the magnesium ribbon will burn easily.

 

2.      Why should we keep our eyes as far as possible from the dazzling white flame of the magnesium ribbon?

Ans. It is so because the dazzling white flame of magnesium ribbon contains/produce ultra violet radiations that may damage our eyes easily.

 

3.      What do you observe when magnesium ribbon burns?

Ans. The observation are as follows:

        i.          The magnesium ribbon burns with a dazzling white flame.

       ii.          It changes into white ash powder which is also known as magnesium oxide.

 

4.      What happens when magnesium oxide reacts with water? Also write its chemical equation.

Ans. When magnesium oxide reacts with water, it produces magnesium hydroxide.

MgO + H₂O →  Mg (OH)₂

Magnesium Oxide + Water → Magnesium Hydroxide

 

5.      What is the nature of magnesium oxide and magnesium hydroxide?

Ans. Magnesium oxide and magnesium hydroxide are basic in nature.

 

 

 

ACTIVITY 1.2

6.      What happens when lead nitrate reacts with potassium iodite solution? Write its balanced equation.

Ans. Lead nitrate when reacts with potassium iodite gives lead iodite (yellow precipitate) and potassium nitrate.

Pb (NO₃)₂ + 2KI → PbI₂ + KNO₃

 

ACTIVITY 1.3

7.      What happens when zinc granules react with hydrochloric acid and sulphuric acid. Write its balanced chemical equation.

Ans. When zinc granules react with HCL and H₂SO₄, it will produce zinc salt and hydrogen gas.

Zn + 2HCl → ZnCl₂ + H₂ ↑

Zn + H₂SO₄ → ZnSo₄ + H₂ ↑ 

 

CHEMICAL EQUATIONS

8.      What is a chemical equation?

Ans. The short hand representation of chemical reaction with the help symbols and formula is called chemical equation.

For ex.-

Hydrogen reacts with oxygen to form water.

2H₂ + O₂ → 2H₂O

 

Chemical equation is classified into two types:

A.      UNBALANCED / SKELETAL EQUATION

The chemical equation in which the number of atoms in reactant and product side is not equal.

B.      BALANCED EQUATION

The chemical equation in which the number of atoms in both reactant and product side are same.

 

9.      Why should we balance a chemical equation?

Ans. We should balance a chemical equation to satisfy the law of conservation of mass.

 

 

BALANCING A CHEMICAL EQUATION

There are two ways to balance a chemical equation-

i.                  The Traditional method

ii.                 The Algebric method

To know the detailed steps and explanation, click here.

 

 

 

TYPES OF CHEMICAL REACTION

There are 4 types of chemical reactions. They are:

 

1.    COMBINATION REACTION

When two or more substance combine to form a single product, the reaction is called combination reaction.

Ex.- C + O₂ → CO₂

2H₂ + O₂ → 2H₂O

 

v  EXOTHERMIC REACTION

The reaction in which heat is released along with the formation of products.

a)      Burning of natural gas

CH₄ + 2O₂ → CO₂ + 2H₂O + heat

b)      Respiration process

C₆H₁₂O₆ + 6O₂ + H₂ → 6CO₂ + 12H₂O

c)       The decomposition of vegetable matter into composes

d)      Quick lime – CaO

CaO + H₂O → Ca (OH)₂ + heat

 

·        Most of the combination reactions are examples of endothermic reaction.

 

 

10.   What happens when a solution of slaked lime is used for white washing the wall?

Calcium hydroxide (slaked lime) react slowly with carbon dioxide in air to form a thin layer of calcium carbonate on the wall. Because of the formation of calcium carbonate the wall gives a shinning finish.

Ca (OH)₂ + CO₂ → CaCO₃ + H₂O

 

2.    DECOMPOSITION REACTION

The reaction in which the single reactant breaks down into two or more simpler product is called decomposition reaction.

For ex.-           2H₂O →_electrolysis→ 2H₂ + O₂

CaCO₃ →_heat→ CaO + CO₂

 

·        They are of 3 types depending on how they react

 

i. THERMAL DECOMPOSITION REACTION

The decomposition reaction which is carried out by heating is called thermal decomposition reaction.

Ex.-

2FeSO₄ →_heat→ Fe₂O₃ + SO₂ + SO₃

Ferrous Sulphate (Greenish) when heated gives Ferric Oxide (Reddish brown) + Sulphur dioxide + Sulphur trioxide

 

2Pb (NO₃)₂ →_heat→ 2PbO + 4NO₂ + O₂

Lead Nitrate (colourless) when heated gives Lead oxide (yellowish) + Nitrogen dioxide (brown gas) + Oxygen

 

 

11.   What do you observe when hydrated ferrous sulphate is heated?

Ans. The observations are :

        i.          The green colour of the ferrous sulphate crystal changes into reddish brown colour, ferric oxide.

       ii.          The colourless gas is evolved.

     iii.          The smell of Sulphur odor is coming.

     iv.          The water droplets are visible on the upper part of the test tube.

 

12.   What do you observe when lead nitrate is heated?

Ans. The observations are :

        i.          The colourless salts of lead nitrate changes into yellow colour substance i.e lead oxide.

       ii.          Brown fumes of nitrogen dioxide is seen

     iii.          Colourless and odourless gass of oxygen is released.

 

 

II. ELECTROLYTIC DECOMPOSITION REACTION

When a decomposition reaction is carried out by passing electricity is called electrolysis.

Ex.

2H₂O →_electricity→ 2H₂ + O₂

 

ACTIVITY 1.7

13.   Why is the amount of gas collected in one of the test tubes is double to the amount of gas collected in other?

Ans. The amount of gas collected in one test tube is double to the amount in other because water is in ratio 2:1. Hence, two parts Hydrogen and one part oxygen is collected.

 

14.   Why should we add few drops of sulphuric acid to the water before doing the electrolysis of water?

Ans. It is so because to make the water a good conductor.

 

15.   Name the gases that is collected at anode and cathode.

        i.          At anode the gas – Oxygen is collected.

       ii.          At cathode the gas – Hydrogen is collected.

 

III. PHOTOLYTIC DECOMPOSITION REACTION

When a decomposition reaction is carried out by sunlight, it is called photolysis.

Ex.

2AgCl →_sunlight→ 2Ag + Cl₂ (g)

2AgBr →_sunlight→ 2Ag + Br₂ (g)

 

v  ENDOTHERMIC REACTION

Reaction in which energy is absorbed are known as endothermic reaction.

Ex.

Ba(OH)₂ + 2NH₄Cl → BaCl₂ + 2NH₃ + 2H₂O

Barium Hydroxide when reacts with Ammonium chloride gives Barium chloride + Ammonia + Water

 

·        Most of the decomposition reactions are examples of endothermic reaction.

 

 

 

3.    DISPLACEMENT REACTION

The chemical reaction in which a highly reactive metal displaces the less reactive metal in an aqueous solution is called displacement reaction.

Fe + CuSO₄ → FeSO₄ + Cu

Iron when reacts with copper sulphate (blue) gives Iron sulphate (green) + Copper (reddish brown)

 

16.   What do you observe when Iron is dipped in copper sulphate solution?

Ans. The observations are as follows:

        i.          The blue colour solution of copper sulphate changes into green colour solution of ferrous sulphate.

       ii.          The reddish brown colour copper metal gets deposited on the surface of iron.

 

 

 

4.    DOUBLE DISPLACEMENT REACTION

The chemical reaction in which there is an exchange of ions between the reactants are called double displacement reaction.

Na₂⁺SO₄^- → NaCl +BaSO₄

Sodium Sulphate when reacts with Barium Chloride gives Barium Sulphate + Sodium Chloride (white ppt.)

 

HCl + NaOH → NaCl +H₂O

Hydrochloric acid when reacts with Sodium hydroxide gives Sodium chloride + water

 

 

·        They are classified into two types:

 

I.             PRECIPITATION REACTION

The chemical rection in which an insoluble substance precipitates is called precipitation reaction.

Pb (NO₃)_2­ + 2KI → PbI₂ + KNO₃

Lead Nitrate when reacts with Potassium Iodite gives Lead Iodite (yellow ppt.) + Potassium Nitrate

 

II.            NEUTRALISATION REACTION

The chemical reaction in which acid and base react to give salt and water is called neutralization reaction.

HCl +NaOH → NaCl + H₂O

Hydrochloric acid when reacts with Sodium hydroxide gives Sodium chloride + water

 

 

5.    OXIDATION REACTION

The chemical reaction in which both oxidation and reduction takes place simultaneously, is called redox reaction.

For Ex.-

ZnO + C → Zn + CO

Zinc oxide when reacts with Coke gives Zinc + Carbon monoxide

2Cu + O₂ → 2CuO

CuO + H₂ → Cu + H₂O

H₂S + Cl₂ → S + 2HCl

 

·        The two terms used are-

 

I.             OXIDATION REACTION

The reaction in which addition of oxygen, removal of hydrogen and loosing of electrons is called oxidation reaction.

For Ex.-

C + O₂ → CO₂

2Mg + O₂ → 2MgO

 

II.            REDUCTION REACTION

The chemical reaction in which addition of hydrogen, removal of oxygen and gaining of electrons is called reduction reaction.

 

v  OXIDISING AGENT

The substance which helps other substance to get oxidized but itself gets reduced is called oxidizing agent.

 

v  REDUCING AGENT

The substance which helps other substance to get reduced but itself gets oxidized is called reducing agent.

 

For ex.-

H₂S + Cl₂ → 2HCl + S

Reducing agent – H₂S

Oxidising Agent – Cl₂

 

 

ADVANTAGES OF OXIDATION REACTION IN EVERYDAY LIFE

        i.          Respiration process

       ii.          Burning of fuels like wood, coal, petroleum, etc.

     iii.          Preventing some of the metals from corrosion as the oxide layer protects the metals from corrosion.

Ex. Zinc, Aluminium, lead, etc.

 

CORROSION

When a metal is attacked by substances such as moistures, acids, bases, atmospheric gases, etc, is said to corrode. This process is called corrosion.

 

I.             Corrosion of Iron (Fe)

Corrosion of iron is also called as rusting of iron. It is commonly known as rust.

·        The factors responsible for rusting of iron is Moisture.

The chemical equation is-

Fe + H₂O + O₂ → Fe₂O₃ . xH₂O

Here, Fe₂O₃ . xH₂O refers basic hydrated Ferric Oxide, commonly known as rust which is Reddish Brown in colour.

 

II.            Corrosion of Copper (Cu)

The factors responsible for corrosion of copper are Moisture, CO₂ and oxygen.

The chemical equation is-

Cu + H₂O + CO₂ + O₂ → CuCO₃ . Cu (OH)₂

 

III.          CORROSION OF SILVER

Factor responsible for corrosion of silver is Hydrogen Sulphide.

Ag + H₂S → Ag₂S + H₂

Here, Ag₂S is Silver Sulphide which is Black in colour.

 

 

METHODS TO PREVENT CORROSION

        i.          Painting

       ii.          Greasing

     iii.          Oiling

     iv.          Electroplating

       v.          Galvanization (the process of applying a thin layer of zinc coating on the surface of iron or steel to prevent rusting)

 

 

RANCIDITY

When fats and oils are oxidized, they become rancid and their smell and taste changes. This phenomenon is known as rancidity.

 

METHODS TO PREVENT RANCIDITY

        i.          By adding anti-oxidant

Ex. BHA (Butylated Hydroxy Anisole) and BHT (Butylated Hydroxy Toluene)

       ii.          Vacuum Packing/ Air Tight Packing

     iii.          Packaging the food items in nitrogen gas

     iv.          Refrigeration

       v.          Storing food items away from direct sunlight.

 

 

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