Class 10 Science Chapter 1 Chemical Reactions and Equations Notes

10 Class Science Chapter 1 Chemical Reactions and Equations Notes

TextbookNCERT
ClassClass 10
SubjectScience
Chapter Chapter 1
Chapter NameChemical Reactions and Equations
CategoryClass 10 Science Notes
MediumHindi

Class 10 Science Chapter 1 Chemical Reactions and Equations Notes. here we will be learn about Chemical reaction, chemical equation, types of chemical reactions, combination reaction, decomposition reaction, displacement reaction, oxidation and reduction reaction etc.

Class 10 Science Chapter 1 Chemical Reactions and Equations Notes

📚 Chapter = 1 📚
💠 Chemical Reactions and Equations💠

❇️ Chemical Reaction :-

🔹 The process in which new substances with new properties are formed from one or more substances is called Chemical Reaction.

🔹 Examples :- Digestion of food , Respiration , Rusting of iron , Burning of Magnesium ribbon , Formation of curd etc.

❇️ Characteristics of a Chemical Reaction :- 

  • The product that is formed as new compound with new chemical formula. 
  • These reactions required splitting and forming of chemical bonds. 
  • Properties of products formed during a chemical reaction are different from those of the reactants. 
  • It is difficult to alter a chemical reaction.
  • There are other types of energies that are electricity and light that is used in carrying out chemical changes. 
  • In all chemical reactions, the conversion from reactants to products is accompanied by different characteristics, that are also used in carrying out chemical changes. 

❇️ Identification of chemical reaction :-

🔹 These factors show that a chemical reaction has taken place :-

  • Change in state 
  • Change in colour 
  • Change in temperature 
  • Evolution of gas

❇️ Reactants :-

🔹 The substances which take part in chemical reaction are called Reactants. 

❇️ Products :-

🔹 The substances which are formed in a chemical reaction are called Products.

❇️ Chemical Equations :-

🔹 All chemical reactions are shown by chemical equations. A chemical equation is a shorthand representation of a chemical reaction using the symbols and formulae of substance involved in the chemical reaction. 

  • A chemical reaction can be represented by chemical equation. 
  • It involves uses of symbol of elements or chemical formula of reactant and product with mention of physical state. 
  • The necessary conditions such as temperature, pressure or any catalyst should be written on arrow between reactant and products.

❇️ Types of Chemical Equations :-

🔹 There are two types of chemical equation :-

  • Word equations
  • Symbol equation

🔶 (i) Word equations :- A word equation is that equation which links together the names of the reactants and products. For example, the word equation, when sodium metal react with water to give sodium hydroxide and hydrogen gas, it written as :- 

Sodium + Water → Sodium hydroxide + Hydrogen gas

🔹 In a word equation There are certain rules for writing word equation :-

  • On the left hand side the reactants are written with a positive sign (+) is put between them. 
  • On the right hand side the products are written with a plus sign (+) is put between them. 
  • The direction of the arrow shows the product formation direction. 
  • An arrow (→) is put between the reactants and products.

🔶 (ii) Symbol equation :- Symbol equation is short representation of a chemical reaction in terms of formulae and symbols of the element/substance that are involved in reaction. 

🔹 In a symbol equation, the symbols and formulae of the elements and compounds are written in place of their word names. 

🔹 For example :- Sodium metal react with Water to give sodium hydroxide and hydrogen gas 

Na + H₂O → NaOH + H₂

❇️ Unbalanced Chemical Equations :- 

🔹 In an unbalanced equation, the number of atoms of different elements that are taking part in a chemical reaction on both side of the equation are not equal.

🔹 For example :- in the equation given below, the number of Cl atoms on both sides of the equation and the number of hydrogen atoms are not equal, It is known as an unbalanced equations. 

Mg + 2HCl → MgCl₂ + H₂ 

🔹 An unbalanced equation is also known skeletal equation. 

❇️ Balanced Chemical Equation :-

🔹 In a balanced equation, the number of different elements on both sides of the equation are always equal. The balanced equation when magnesium reacts with HCl is written as :-

 Mg + 2HCI → MgCl₂ + H₂ 

❇️ Importance of balanced chemical equation :-

🔹 The balancing of a chemical equation is essential or necessary to fulfill the condition of “Law of conservation of mass“.

🔶 Law of conservation of Mass :- Matter can neither be created nor be destroyed in a chemical reaction.

❇️ Balancing of Chemical Equation ( Hit and Trial method ) :-

🔹 The balancing of a chemical equation is done with the help of Hit and Trial method. In this method, the coefficients before the symbols or formulae of the reactants and products are adjusted in such a way that the total no. of atoms of each element on both the side of the arrow are equal. 

🔹 This balancing is also known as mass balancing because the atoms of elements on both side are equal and their masses are also be equal.

❇️ The major steps that are involved in balancing a chemical equation are given :-

  • The chemical equations are written in the form a word equations. Reactants are kept on the left hand side and the products are on the right side. The reactants and products are separated by an arrow. 
  • Word equation is then converted into the symbol equation with symbols and formulae used for all the reactants and product are written. 
  • Then the atoms of different elements on both side of the equation are made equal. This is how balancing of equation is done. 
  • The formulae of the substance while balancing the equation should not be changed. 
  • Try to make the equations more informative if possible.

❇️ STEPWISE BALANCING (Hit and Trial) :- 

🔶 Step 1 :- Write chemical equation and draw boxes around each formula. 

Fe + H₂OFe₃O₄ + H₂

* Do not change anything inside the box. 

🔶 Step 2 :- Count the number of atoms of each element on both the sides of chemical equation.

ElementNo. of atoms at reactant sideNo. of atoms at product side
1. Fe13
2. H22
3. O14

🔶 Step 3 :- Equalise the number of atoms of element which has maximum number by putting in front of it.

Fe + 4H₂O → Fe₃O₄ + H₂

🔶 Step 4 :- Try to equalize all the atoms of elements on reactant and product side by adding coefficient in front of it. 

3Fe + 4H₂OFe₃O₄ + 4H₂

* Now all the atoms of elements are equal on both sides.

🔶 Step 5 :- Write the physical states of reactants and products.

3Fe ( s ) + 4H₂O ( g ) → Fe₃O₄ ( s ) + 4H₂( g )

🔶 Step 6 :- Write necessary conditions of temperature, pressure or catalyst on arrow above or below.

❇️ Writing State Symbols :- 

🔹 The physical states of the reactant and product species that are involved in the reaction are not written in symbol equation the equation is made more informative by mentioning the physical state with the use of certain specific symbols known as state symbols. These symbols are :-

  • (s) for solid state 
  • (l) for liquid state 
  • (g) for gaseous state 
  • (aq) for aqueous solution i.e., solution that prepared in water.

🔹 A gas evolution is shown by the symbol (↑) in a reaction i.e., with an arrow point towards upwards. Similarly the precipitate formation is shown by the symbol (↓) in a reaction i.e., with an arrow point towards downwards.

❇️ Importance of State Symbols :-

🔹 The state symbols are most important part for those chemical reactions that are either accompanied by the release of heat (exothermic) or by the absorption of heat (endothermic). For example :- 

2H₂(g) + O₂(g) → 2H₂O(l) + 572 kJ 

2H₂(g) + O₂(g) → 2H₂O(g) + 44 kJ

🔹 The above reactions are of exothermic as heat is released in these reactions. Thus, actual amounts of heat are different when water is in the liquid state i.e. H₂O(l) and when it is in the gaseous state.

❇️ Importance of Chemical Equation :-

  • The weight of reactant or products can be calculated by chemical equation. 
  • Total weight of reactants is equal to the total weight of products because matter is never destroyed. 
  • It gives information about the substance that are taking part and formed in the reaction.  
  • The information about the number of molecules of elements or compounds which are either taking part or formed in the chemical reaction. 
  • In a chemical equation with the help of product we can get information about the valency as well.

❇️ Some of Limitations of Chemical Equations :- 

  • It does not give information about the physical state of reactants and products. For example solid, liquid or gas. 
  • We does not get information about the reactants and products concentration. 
  • It does not give information about the speed of reaction and sense of timing. 
  • We does not get information about the favorable conditions of the reactions such as temperature, catalyst, pressure etc. 
  • The information whether heat is absorbed or released is not obtained during reaction.
  • The information about the necessary precautions that has to be taken for the completion of reaction is not obtained. 
  • The information if the reaction is reversible or irreversible does not obtained.

❇️ TYPES OF CHEMICAL REACTIONS :-

🔶 Combination Reactions :- The reaction in which two or more reactants combine to form a single product. Example :-

  • Burning of coal :- C (s) +0₂ (g) → CO₂ (g)
  • Formation of water :- 2H₂ (g) + O₂ (g) → 2H₂O (l)
  • ( Quick lime ) CaO (s) + H₂O (l) → Ca(OH)₂ (aq) ( Slaked lime )           

🔶 Exothermic Reactions :- Reaction in which heat is released along with formation of products. Example :-

  • Burning of natural gas :- CH₄ (g) + 0₂ (g) → CO₂ (g) + 2H₂O (g) + Heat
  • Respiration is also an exothermic reaction :- C₆H₁₂0₆ (aq) + 60₂ (g) → 6C0₂ (g) + 6H₂O (l) + energy

🔶 DECOMPOSITION REACTION :- The reaction in which a compound splits into two or more simple substances is called decomposition reaction.

A→B + C

🔹Type of DECOMPOSITION REACTION :-       

  • Thermal decomposition :- When decomposition is carried out by heating.
  • Electrolytic Decomposition :- When decomposition is carried out by passing electricity.
  • Photolytic Decomposition :- When decomposition is carried out presence of sunlight.

🔹 The decomposition reactions are mostly endothermic in nature. Energy in the form of heat, light or electricity is generally absorbed in these reactions.

🔶 Endothermic Reactions :- The reactions which require energy in the form of heat, light or electricity to break reactants are called endothermic reactions.

🔶 DISPLACEMENT REACTION :- The chemical reaction in which more reactive element displaces less reactive element from its salt solution.

       Fe (s) + CuSO₄ (aq) → FeSO₄ (aq) + Cu (s)

🔹 The iron nail becomes brownish in colour by deposition of Cu and blue CuSO, changes to dirty green colour due to formation of FeSO.

Zn + Cuso₄ → ZnSO₄ + Cu

  • Zn is more reactive than copper. 

🔶 DOUBLE DISPLACEMENT REACTION :- A reaction in which new compounds are formed by mutual exchange of ions between two compounds.

Na₂SO₂ (aq) (Sodium sulphate)  + BaCl₂ ( aq )  (Barium chloride) →    BaSO₄ (s)  (Barium sulphate) + 2Nacl (aq) (Sodium chloride)

🔹 white precipitate of BaSO₄ is formed, so it is also called precipitation reaction. 

Note :- All double displacement reactions are not precipitation reaction.

❇️ Oxidation :-

🔹 It is a chemical reaction in which a substance gains oxygen or loses hydrogen. As O, is an electronegative element and H, is an electropositive element. Thus, oxidation is defined as a reaction in which a substance gains an electronegative ion or loses an electropositive ion.

❇️ Reduction :-

🔹 The chemical reaction in which there is a gain of hydrogen or any electropositive ion or a loss of oxygen or electronegative ion.

❇️ Redox Reactions :-

🔹 Reduction is loss of electronegative element or ions. The oxidation and reduction occur simultaneously, i.e. there can be no oxidation without and equivalent reduction. In a reaction when one substance is oxidised the other is reduced and vice-versa. Those reactions in which oxidation and reduction take place simultaneously are known as redox reactions. 

  • reducing agent :- A substance, that brings about reduction, is known reducing agent. 
  • oxidizing agent :- A substance, that brings about oxidation, is known an oxidizing agent.

❇️ Effects of Oxidation in Daily Life 

🔶 1. Corrosion :-

🔹 When a metal is exposed to moisture, air, acid etc. for some time, a layer of hydrated oxide is formed which weakens the metal and hence metal is said to be corroded. 

🔹 Example :- Rusting of iron, black coating on silver and green coating on copper are examples of corrosion.

🔸 Measures to prevent corrosion :- Corrosion can be prevented by galvanization, electroplating or by putting paints. 

🔶 2) Rancidity :- The oxidation of fats and oils when exposed to air is known as rancidity. It leads to bad smell and bad taste of food. 

🔸 Methods to Prevent Rancidity :-

  • By adding antioxidants :- Manufacturer sometimes add certain food additives to the food materials. These are known as antioxidant and check their oxidation. 
  • Keeping food in air tight containers :- Food should be kept in air tight containers to prevent its oxidation. 
  • Refrigeration :- Refrigeration of food also retards rancidity as the temperature inside refrigerator is very low and direct contact with air or oxygen is avoided. 
  • Replacing air by nitrogen :- Chips manufacturers usually fills their bags with nitrogen gas (as it is unreactive gas) before packing so that they may not be oxidised.
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