CBSE Class 10 Science Revision Notes Chapter 1

CBSE Class 10 Science Revision Notes Chapter 1 – Chemical Reactions and Equations

Science is one of the most interesting subjects in Class 10 loaded with interesting facts. Students should prepare the subject accurately to score well in the board examinations. Candidates interested in the medical and non-medical streams should give extra attention to the topics of science as it will build their concepts for future examinations. Below are the Class 10 Science Chapter 1 Notes given so that students can understand the concepts accurately. 

CBSE Class 10 Science Revision Notes for the Year 2022-23

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Introduction 

Change in the state, change in colour, the evolution of a gas, and change in temperature are all indicators that a chemical reaction has occurred. There are numerous different chemical reactions occurring all around us as we observe how the world evolves. In this chapter, we’ll learn about the numerous kinds of chemical reactions and how they’re represented symbolically.

Chemical change: 

The reaction leading to the formation of one or more compounds is known as chemical change. It is also known as a chemical reaction. 

Observations in a Chemical Reaction: 

In a chemical change, the following observations can be noticed:

• Formation of precipitate 
• Permanent change in state and colour 
• Change in temperature 
• Evolution of gas 
• Changes in energy 
• Change in mass 
• Formation of new substances 

The following are examples of chemical reactions:

• Ripening of fruit 
• Souring of milk 
• Heating of lead nitrate 
• Rusting of iron 
• Cooking of food 

Rusting iron is a chemical change. Following changes occur: 

• As the iron rusts, there is the formation of a new substance on its surface known as iron oxide. 
• The modification of the iron due to rusting is permanent. The rust layer can get peeled off in most cases.
• The mass of the object increases when rusting occurs on its surface. 
• An invisible shift in energy occurs during the rusting of iron. 

There are two types of reactions. They are as follows: 

Exothermic reactions: 

The reactions due to which the release of heat energy occurs are known as exothermic reactions. 

• Example: (1) C(s)+O2(g)→CO2(g)+393.5kJ

(2) N2(g)+3H2(g)→2NH3(g)+92.3kJ

The heat produced during the product formation is recorded here to prove the emission of heat. 

Endothermic reactions: 

The reaction involving the absorption of energy is known as an endothermic reaction. 

• Examples: (1) N2 (g)+O2(g)+180.5kJ→2NO(g)

(2) 2HgO(g)+180 kJ→ 2Hg(l)+O2(g)

Chemical formulae 

The symbolic representation of the number of atoms present on a single molecule in a compound is known as chemical formulae. 

• H20 

H20 is one molecule of water. It demonstrates that one molecule of water contains two hydrogen atoms and one oxygen atom. 

• H2SO4

This formula shows that one molecule of sulphuric acid contains 2 hydrogen atoms, 1 sulfur atom, and 4 oxygen atoms. 

Chemical equations: 

The representation of a chemical change in terms of symbols and formulae of reactants and products is known as chemical equations. 

• Example: 

KMnO4 +        HCl → KCl     +    MnCl2 + H2O  +    Cl2

Steps to writing a chemical equation 

Step 1: Write the skeleton of the equation properly. 

Step 2: Now start balancing the compound with the maximum number of atoms or type of atoms present. 

Step 3: You should balance the elements that appear once on each side of the arrow. Then balance others occurring more than once. 

Step 4: Elementary substances should be balanced last. 

Step 5: Now to make all the coefficients as whole numbers, multiply the entire equation by a suitable number if necessary. 

Short technique for balancing a chemical equation:

Let us now try the short technique for balancing a chemical equation.  

aKMnO4+ bHCl → cKCl + dMnCl2 + eH2O + fCl2

Now comparing the LHS and RHS, we get:

K: a = c

Mn: a = d

O: 4a = e

H: b = 2e

Cl: b = c + 2d + 2f

Let us take a = 2, then

c = d = a = 2

b = 2e = 8a = 16

 So, e = 8

So, 16 = 2 + 2(2) + 2(f)

f = 5

The required equation will be 2KMnO4+ 16HCl → 2KCl + 2MnCl2 + 8H2O + 5Cl2.

From the chemical reaction, one can get information about:

• The names of various reactants and products
• Formulae of reactants and products 
• The number of moles of reactants and products in relation to each other 
• Masses of reactants and products relative to each other 
• Volumes of gaseous reactants and products in relation to each other

Limitation of the reaction: 

To discuss the limitation of the reaction, let us take an example:

CaCO3(s)+ HCl(aq)→ CaCl2(s)+ H2O(l)+ CO2(g)

• It is not possible to predict the finishing point of a reaction through its chemical equation. 
• The equation does not give any information on the reaction time or its speed. 
• It does not give the substance concentration information. 
• The equations cannot specify the temperature, pressure, catalyst, humidity, etc, in specific cases.

• The changes in the colour have to be mentioned separately. 

PbO2 → 2PbO + Cl2 

Chocolate Yellow Colorless 

Brown

• The production or the absorption of heat has to be mentioned separately. 

C + O2 → CO2 + Heat 

2C + O2 → 2CO + Heat

• Some of the reactions are reversible. They can be written using ⟷ or  ⇄ symbols.
• Ionic equation balancing: A balanced ionic equation involves both the mass and charge balance. 

Chemical equations: 

Chemical equations can provide a wealth of quantitative data. One can use chemical equations to calculate the moles of products and reactants. 

Types of reactions: 

A] Combination or synthesis reaction 

The type of reaction in which two or more chemicals combine to generate a new compound. 

Types of combination reactions: 

• Combination of two elements to form a compound

2H2(g) → O2(g) +  2H2O(l)

Hydrogen   Oxygen    Water

• Combination reactions involving an element and a compound 

2CO(g)+ O2(g) → 2CO2(g)

• Combination reactions involving two compounds 

NH3(g) +              HCl(g) → NH4Cl(s) 

Ammonia      Hydrogen chloride      Ammonium chloride 

Colourless           Colourless                        White

B] Decomposition reactions 

• A reaction occurs when a complex is broken down into two or more components using heat or electricity. 
• Thermal decomposition occurs when a substance decomposes due to heat, whereas electrolytic decomposition occurs when a component decomposes due to electricity.
• The process of decomposing a substance with light is known as photolysis. 

Examples: 

Thermal decomposition 

• 2HgO(s) −→2Hg(l) + O2(g)↑ 

Mercuric oxide red  Mercury  oxygen 

                                     silvery liquid 

• 2Cu(NO3)2(s) −→2CuO(s) +     4NO2(g) + O2(g)

Electric decomposition 

• 2H2O(l) −→2H2(g) + O2(g) 

  Water         hydrogen  oxygen 

• PbBr2 −→Pb(s) +      Br2 

Lead           Lead        Bromide

bromide  

Activity series : 

It is a list in which the metals are arranged in decreasing chemical reactivity order. 

A substitution or displacement reaction: 

It occurs when the atoms of one element replace the atoms of another in a compound.

The electropositive elements that are abundant will displace the electronegative elements that are less abundant. 

Whereas the electronegative elements with a greater charge density will displace electronegative elements with a lower charge density. 

Double displacement reactions

The mutual exchange of ions in which two different chemicals react with molecules is known as double displacement reactions. 

 Types of displacement reactions: 

• Precipitation: In this reaction, a white material will be generated which is insoluble in water. 
• Neutralization: It is a form of double displacement reaction in which the reactants are base and acid with products such as salt and water. 

Oxidation – reduction reactions / redox reactions 

Oxidation is a reaction in which oxygen is added and hydrogen is removed from a particular substance. The reactions given below will provide more clarity.

• Addition of oxygen: 

2Mg(s) + O2 → 2MgO(s)

Magnesium   Oxygen      Magnesium oxide

• Removal of hydrogen 

2HI(g) +    2H2(g) → I2(g)

Hydroiodic    Hydrogen            Iodine 

Acid

• Removal of oxygen 

CuO(s) +    CO(g) → Cu(s) + CO2(g)

            Copper       Carbon Copper   Carbon dioxide

oxide      

• Addition of hydrogen 

H2(g) +    Cl2(g) → 2HCl(g)

Hydrogen    Chlorine          Hydrogen chloride

Redox reactions: 

It occurs when oxidation and reduction reactions occur at the same time. 

Example of:

• Redox reactions: 

Mg(s) + H2SO4(aq) → MgSO4(aq)+ H2(g)

Magnesium Sulphuric  Magnesium     Hydrogen  

                         acid        Sulphate

• Non-redox reactions: 

NaCl(aq) +  AgNO3(aq)→ AgCl(s)↓ +    NaNO3(aq)

Sodium        Silver             Silver        Sodium            

chloride        Nitrate Chloride    Nitrate

Oxidizing and reducing agents 

• Oxidizing agents: A substance that carries out oxidation
• Reducing agents: A substance that carries out reduction

Electronic concepts of oxidation and reduction

• Oxidation: The process involving the loss of electrons
• Reduction: The process involving the gain of electrons

The effects of oxidation reactions in everyday life: 

• Corrosion: 

Many chemically active metals can be affected due to oxygen, moisture, acids, etc. The process involving the change of a shiny coat that can sometimes turn reddish brown is known as corrosion, the rusting of iron is a common name used for this phenomenon. Oxidation does not adhere to the surface of metal properly, leading to weakening and metal breakdown. 

• Rancidity: The process of changes in the flavour of food and odour is known as rancidity. It occurs due to oxidation, which causes the fats and fatty acids to deteriorate. This is the most common cause of rancidity. 

CBSE Class 10 Science Chapter 1 – Chemical Reactions and Equations Revision Notes 

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