NCERT Solutions for Class 10 Science Chapter 1 Chemical Reactions and Equations

Introduction

Every day in our life, we go through different kinds of physical and chemical changes. A physical change is a change that can be reversed like the melting of wax, freezing of water, etc. and chemical change is a change that cannot be reversed like digestion of food, rusting of iron, etc. All chemical changes are associated with chemical reactions and they are represented by equations. In this segment, we will study the different kinds of chemical changes represented by chemical reactions and the chemical equations. 

You can Find the Solutions of All the Science Chapters below.

NCERT Solutions for Class 10 Science

• Chapter 1 - Chemical Reactions and Equations

• Chapter 2 -  Acids, Bases and Salts

• Chapter 3 - Metals and Non-metals

• Chapter 4 - Carbon and Its Compounds

• Chapter 5 - Periodic Classification of Elements

• Chapter 6 - Life Processes

• Chapter 7 - Control and Coordination

• Chapter 8 - How do Organisms Reproduce?

• Chapter 9 - Heredity and Evolution

• Chapter 10 - Light Reflection and Refraction

• Chapter 11 - Human Eye and Colourful World

• Chapter 12 - Electricity

• Chapter 13 - Magnetic Effects of Electric Current

• Chapter 14 - Sources of Energy

• Chapter 15 - Our Environment

• Chapter 16 - Management of Natural Resources

Chemical Reactions 

A chemical reaction is a change in which one more substance(s) are reactants(s) react (s) to form new compound substance(s) with entirely different properties.

Reactants are substances like molecules, ions, atoms that react. They undergo a chemical change in the reaction and form a new substance as a result of the reaction. This new substance formed is called a product. 

         Ex:         2Na(s)      +      2H2O (l)       →  2NaoH(aq)    +      H2(g) ↑

        Sodium         Water       Sodium hydroxide     Hydrogen

| ____________ | |_____________|

In the above chemical reaction, sodium and water are the reactants and the products formed as a result of the reaction are Sodium hydroxide and Hydrogen.

Identification of Chemical Reaction

The following observations can be identified as changes during a chemical reaction:

(i) Change in state (ii) Change in the colour

(iii) Evolution of a gas (iv) Change in temperature

(v) Formation of a precipitate

Chemical Equations

A chemical equation is a shorthand form for a chemical change represented by symbols. The reactants and the products obtained as a resultant of a chemical change are represented by symbols and formulas. 

Ex: Methane + Oxygen → Carbon dioxide + Water

  |_________|                   |_____________|

      Reactants                   Products

This equation is called the word equation. The word equation can be changed into a chemical equation by writing the formulae and the symbols of the substance instead of their names. 

Ex: CH4(g)       +     2O2(g) →     CO2(g)        +      2H2O(l)

  Methane     Oxygen     Carbon dioxide    Water

Writing a Chemical Equation

A chemical equation shows a change of reactants to final products through an arrow symbol () that is placed between them.

The reactants are written with a plus sign (+) between them on the left-hand side of the arrow. Similarly, the final products are written on the right-hand side with a plus sign (+) between them. 

The arrowhead pointing towards the products showing the direction of the reaction.

Ex:        NH4OH  +  HCL   ------------------->  NH4Cl            +  H2O

    |________________| Arrow indicates     |_______________|

            Reactants     to yield         Products

Represents Direction

In the above reaction, the number of Ammonium Hydroxide and Hydrogen Chloride is reacting to form the products Ammonium Chloride and Water. If you observe, in the above equation the number of atoms in the reactants and the products are not the same on both sides. This kind of unbalanced equation is called a skeletal chemical equation.

Balanced Chemical Equations

When the total number of atoms of each element is equal on both sides of the equation then the equation is known as a balanced chemical equation. 

The balancing of a chemical equation is based on the law of conservation of mass. According to the law of conservation of mass, ‘mass can neither be created nor be destroyed during a chemical reaction.’ 

The method of hit and trial can be used for balancing a chemical equation. In this method, we keep making trials to balance the equation by using the smallest whole number of coefficients. The number of atoms of each element never changes, before and after a chemical reaction in this method. 

Balancing of a Chemical Equation

There are many steps involved in balancing a chemical equation. 

1. The first step is to write an unbalanced equation and enclose the formulae in brackets.

Ex:     (Na)     +    (H2O)       →   (NaOH)                          +    (H2)

          Sodium      Water     Sodium Hydroxide         Hydrogen

2. The second step is to make a list of the number of atoms of different elements as present in an unbalanced equation.

3. Next step is to balance the first element. From the above table, it is known that atoms in Hydrogen are not balanced. So, first we need to balance it.

Thus, the balanced equation is (Na) + 2(H2O) → 2(NaOH) + (H2)

4. Then balance the second element. We examine the obtained equation and select the next unbalanced element. In the above chemical equation, Na is still not balanced. To balance the number of atoms in Na,

Thus, after the balancing the Na, we get the final chemical equation:

2Na + 2H2O → 2NaOH + H2

5. Further, we have to balance the other elements. If we further examine the reaction, no element is found to be unbalanced. This method that we used is called the hit and trial method.

6. Finally, we need to check the accuracy of the equation. To check where the equation is correct, we further tabulate the number of atoms of each element separately.

The above table clearly reveals that the obtained equation is a balanced equation.

Example: Balance the following equation C2H6 + O2 →  CO2 + H2O

Sol.   Step 1: Enclose all the symbols or formulae in brackets.

(C2H6 )+ (O2 ) →  (CO2 )+ (H2O)

    Step 2: Count the number of atoms in each element on both the sides of the equation.

Step 3: From the above table, it is known that hydrogen has the maximum number of atoms. So to balance hydrogen atoms, multiply H2O by 3.

(C2H6 )+ (O2 ) →  (CO2 )+ 3(H2O)

Step 4: Count further the number of atoms in each element on both sides.

Now, to balance the atoms in carbon, multiply CO2 molecules by 2. 

(C2H6 )+ (O2 ) →  2(CO2 )+ 3(H2O)

There are 7 O-atoms on RHS. To make 7 O-atoms at LHS, we have to write 72 before O2 but we can use only whole number to balance the equation, so we write 72before  O2 and multiply the whole equation by 2.

[ C2H6 +  7/2 O2  →  2CO2 + 3H2O ] x 2

Thus, after removing the brackets we get the final and correct chemical equation

2C2H6 + 7O2  →  4CO2 + 6H2O

Making a Chemical Equation More Informative

The facts that remain unexplained in a chemical equation are 

1. The physical state of the substances like the changes in the colour during the reaction.

2. The reaction conditions.

3. The changes in the evolution or the absorption of energy.

4. Completion of the reaction. 

These Limitations Can Be Overcome by Adding the Following Information 

1. The physical states of the reactants and products can be represented by using the symbols, (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous solution, alongwith their respective formulae. Liberation of gas and precipitation can be represented by and respectively. 

2. The  circumstances under which reaction takes place like temperature, pressure, catalyst, etc. can be mentioned in the equation.

3. Evolution of heat or absorption of heat can be indicated by writing [+Heat] on the right hand side of the left hand side of the equation, respectively. 

Types of Chemical Reactions

The chemical reactions depend upon the type of chemical changes taking place and they are classified as:

1. Combination Reaction

When two or more reactants are combined to form a single substance in a chemical reaction, then the reaction is called a combination reaction.

Ex: Calcium Oxide reacts very fast and strongly with water to form calcium hydroxide (slaked lime). The reaction given below is highly exothermic, because a lot of heat is liberated during the reaction.

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

        Calcium oxide Water         Calcium

                            (Quick Lime)         Hydroxide

    (slaked lime)

2. Decomposition Reaction

This is a reaction in which a single reactant breaks down to form two or more products. This reaction is opposite to combination reaction. On the basis of the energy form required for the reaction, this reaction is classified into three types:

1. Thermal decomposition: This reaction uses the heat energy for decomposition of the reactant.

Ex: Ferrous Sulphate ( FeSO4.7H2O) when heated loses water of crystallisation and forms dehydrated FeSO4 and gives ferric oxide, sulphur dioxide and sulphur trioxide on decomposition.                  

heat

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

    Ferrous Sulphate            Ferric oxide   Sulpur dioxide   Sulphur trioxide

2. Electrolysis: These reactions use the electrical energy for the decomposition of the reactant molecules.

Ex: When electric current is passed through water, it decomposes to give water and hydrogen.   

                               Electric current

2H2O (l) →    2H2 (g)    +     O2 (g)

Water                 Hydrogen    Oxygen

3. Photolysis or photochemical decomposition: These reactions use light energy for the purpose of decomposition.

Ex: Silver chloride decomposes to produce silver metal and chlorine gas when exposed to sunlight.                    

                                             sunlight

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

                        Silver bromide       Silver       Bromine

  (Pale yellow)   (Greyish white)   (yellowish green)

3. Exothermic and Endothermic Reactions

A reaction can be exothermic or endothermic depending on whether heat is liberated or absorbed during a reaction.

1. Exothermic reactions: The reactions in which the products formed along with the liberation of heat, are called exothermic reactions or combustion reactions. Respiration is an exothermic process. The decomposed vegetable matter that turns into compost is also an exothermic process.

Ex: Burning of magnesium ribbon

2Mg (s) + O2 (g) → 2MgO (s)  +  Heat

Magnesium   Oxygen     Magnesium oxide

2. Endothermic reactions: The reactions which take place by the absorption of heat/ energy (either in the form of light or electricity), are called endothermic reactions. Photosynthesis is an endothermic process. All decomposition reactions are endothermic reactions because the reactions require some form of energy like heat, light or electricity. 

          sunlight

6CO2 (aq) + 12H2O (l) →  C6H12O6 + 6O2 + 6H2O

                                                        Chlorophyll   Glucose

4. Displacement Reaction

When a more reactive element takes the place of  a less reactive element from its compound, it is called displacement reaction. This reaction is of two kinds:

1. Single Displacement Reaction: This is a type of chemical reaction where an element reacts with a compound and displaces another element in that compound is called single displacement.

Ex: Zinc is more reactive than copper and displaces Cu from CuSO4 solution and forms a new product zinc sulphate and copper metal.

 Zn         +       CuSO4  →                ZnSO4                 +       Cu 

Zinc metal    Copper sulphate       Zinc Sulphate   Copper metal

(Grey)          solution (blue)          solution (colourless)    (Brown)

2. Double Displacement Reaction: The reaction in which two different ions or groups of atoms in the molecules of a reactant are displaced by each other is called a double displacement reaction. In this reaction, a precipitate is produced so It is also called a precipitation reaction.

Ex: When sodium sulphate is added to barium chloride, a curdy white precipitate of barium sulphate and a solution of sodium chloride are formed as end products. 

Na2SO4 (aq)  + BaCl2 (aq) →  BaSo4 + 2NaCl (aq)

Sodium Sulphate      Barium Chloride  Barium Sulphate   Sodium Chloride 

Note: The reaction in which acid or acidic oxide react with a base or basic oxide to form salt and water is called neutralization reactions.

2NaOH      +       H2SO4                          →                Na2SO4 +        H2O

Sodium Hydroxide  Sulphuric Acid Sodium sulphate  Water

5. Oxidation and Reduction Reactions Oxidation

This reaction is a process in which oxygen is added to a substance or removed from a substance or a substance loses an electron(s). If a substance gains oxygen during a reaction then the substance is said to be oxidized. If a Oxygen is lost from a substance during a reaction, it is said to be reduced. 

The substance that oxidizes the other substance is called an Oxidizing agent. The substance  that brings a reduction in another substance is called a Reduction agent.

i) 2Cu   +   O2 →   2CuO

Copper  Oxygen   Copper oxide               

ii) 2H2S            +               O2      →  2S + 2H2O

Hydrogen sulphide   Oxygen  Sulphur   Hydrogen  oxide

iii) Zn →  Zn2+ + 2e-

       Zinc

Redox Reactions

When a reaction has both oxidation and reduction  taking place simultaneously then the reaction is called a redox reaction. 

1. In the reaction below, the copper (II) oxide is losing oxygen and is being reduced while oxygen is added to hydrogen and water is formed.

            Reduction

                                            ________________

              |                 |

                                               CuO       +   H2 →      Cu  +       H2O

        Copper  Hydrogen   Copper      Water

  Oxide       |_________________|

                  Oxidation

2. In the reaction below, HCL is oxidised to HCL is oxidized to Cl2 whereas, MnO2 is reduced to MnCl2

       Reduction

  ________________ 

| |

MnO2 (s) + 4HCL → MnCl2 (aq) + Cl2 (g) + 2H2O

        (Conc.)

Oxidation

|_________________|

Effects of Oxidation Reactions in Everyday Life

Corrosion

The phenomenon due to which the upper surface of the metals get eaten away when they come in contact with air, water and chemicals present in the atmosphere, then it is called corrosion.

Ex: When iron substances are new then they shine but get coated with a reddish brown powder when left for sometime. 

Rusting is basically the process of corrosion of iron. The rusting of iron is a redox reaction. Painting, galvanizing, and electroplating are few methods to prevent corrosion. 

Corrosion can cause damage to the body of a car, bridge, iron railing, ships because they are made up of iron. 

Rancidity

In this process,  the oil and fat which are volatile in nature present in the food materials slowly get oxidized resulting in the change of smell and taste in them. 

The methods to prevent rancidity are:

1. Keep food in air-tight containers to prevent air from getting in.

2. Refrigeration of cooked food. 

3. Packing of certain food items in packets containing nitrogen gas.

4. Avoid keeping food under the direct sunlight. 

5. Rancidity can also be prevented by adding antioxidants like BHA and BHT.