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Chemical Reactions and Equations Class 10 Notes Science Chapter 1

In our daily lives, physical changes involve alterations in appearance or state without creating new substances, like melting ice. On the other hand, chemical changes result in the formation of entirely new substances, such as cooking food or rusting iron. Recognizing these changes helps us understand when a chemical reaction occurs.
Chemical Reactions and Equations Class 10 Notes Science Chapter 1

What is a Chemical Reaction?

A chemical reaction is a process in which one or more substances, the Reactants, are converted to one or more different substances, the Products.  
These are the following changes to determine that the chemical reaction has taken place:

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

Chemical Equations

Chemical Equation is representation of chemical reaction using symbols and formulae of the substances.  A chemical equation represents a chemical reaction. 

Chemical Reactions and Equations Class 10 Notes Science Chapter 1

The substances that undergo chemical change in the reaction, Magnesium and 0xygen, are the Reactants. The new substance, Magnesium Oxide, formed during the reaction, is the product.

Chemical equations can be made more concise and useful if we use chemical formulae instead of words. Example: The chemical formula for water is H2O, which indicates that it is made up of two hydrogen atoms and one oxygen atom.

Question for Chapter Notes: Chemical Reactions & Equations
Try yourself:
Which of the following is an example of a chemical change?
View Solution

1. Writing a Chemical Equation

Representation of a chemical reaction in terms of symbols and chemical formulae of the reactants and products is known as a chemical equation.

  • For solids, the symbol is “(s)”.
  • For liquids, it is “(l)”.
  • For gases, it is “(g)”.
  • For aqueous solutions, it is “(aq)”.
  • For gas produced in the reaction, it is represented by “(↑)”.
  • For precipitate formed in the reaction, it is represented by “(↓)”.

The reactants are on the left (LHS) of the arrow, while the products are on the right (RHS). A plus sign (+) links the different reactants and products together.

A balanced account of a chemical transaction is a complete chemical equation, which symbolically depicts the reactants, products, and their physical states.

2. Balanced Chemical Equations

(i) Law of Conservation of Mass
The Law of Conservation of Mass asserts that in a chemical reaction, atoms cannot be created or destroyed. Consequently, the total number of atoms for each element in the reactants must match those in the products, maintaining overall mass equality.

Chemical Reactions and Equations Class 10 Notes Science Chapter 1

(ii) Balanced chemical equation

A chemical equation is considered balanced when the number of atoms for each element on the reactant side is identical to the product side.

(iii) Steps for Balancing Chemical Equations

  • To balance chemical equations, coefficients (numeric values preceding chemical symbols or formulas) are utilized. These coefficients represent the number of atoms or molecules involved. Adjust these coefficients as necessary to ensure an equal number of each type of atom on both sides of the equation.
  • For instance, in the equation Zn + HCl → ZnCl2 + H2, balancing involves adding coefficients to achieve equilibrium: Zn + 2HCl → ZnCl2 + H2. This balancing process often involves trial and error adjustments to ensure the equality of atoms on both sides.

Here are the steps to write a balanced chemical equation: Fe + H2O → Fe3O+ H2

Step 1:  Write the number of atoms of elements present in reactants and in products in a table as shown here.

Chemical Reactions and Equations Class 10 Notes Science Chapter 1

Step 2:  Balance the atom which is maximum in number on either side of a chemical equation.
Start by balancing the number of oxygen atoms, which is the maximum on the right-hand side (RHS).
To balance oxygen, multiply the number of oxygen atoms on the left-hand side (LHS) by 4.
Fe + 4 × H2O → Fe3O4 + H2

Step 3: Balancing oxygen created an imbalance in the number of hydrogen atoms. There are now eight hydrogen atoms on the LHS and two on the RHS.
To balance hydrogen, multiply the number of hydrogen atoms on the RHS by 4.
Fe + 4 × H2O → Fe3O4 + 4 × H2

Step 4: There is only one iron atom on the LHS and three on the RHS.
To balance iron, multiply the number of iron atoms on the LHS by 3.
3 × Fe + 4 × H2O → Fe3O4 + 4 × H2

The equation is now balanced, and the same number of atoms of each element is present on both sides.

Chemical Reactions and Equations Class 10 Notes Science Chapter 1

After balancing, the above equation can be written as follows:
3Fe + 4H2O  Fe3O4 + 4H2 

  • To make a chemical equation more informative, it is important to include the physical state of the substances involved.
  • The symbol (g) is used to represent a gaseous state, (l) for liquid state, (s) for solid state, and (aq) for an aqueous solution.
  • The conditions under which the reaction takes place are also important to include in the chemical equation.
  • These conditions can be written above and/or below the arrow in the chemical equation.
  • Including this information can help provide a clearer understanding of the chemical reaction taking place.
  • Example : Chemical Reactions and Equations Class 10 Notes Science Chapter 1
  • In this equation, (g) indicates that carbon dioxide is a gas, (l) indicates that water is a liquid, and (aq) indicates that glucose is dissolved in water. Sunlight and chlorophyll are specifically mentioned as the sources of energy used in the reaction.  This highlights the importance of these specific components in the photosynthesis process.

Question for Chapter Notes: Chemical Reactions & Equations
Try yourself:What is the balanced chemical equation for the reaction between hydrogen gas and oxygen gas to form water?
View Solution

(iv) Short-Cut Technique for Balancing a Chemical Equation

aCaCO3 + bH3PO4 → cCa3(PO4)2 + dH2CO3
Set up a series of simultaneous equations, one for each element.
Ca: a=3c
C: a=d
O: 3a+4b=8c+3d
H: 3b=2d
P: b=2c
Let’s set c=1
Then a=3 and
d = a = 3
b = 2c = 2
So a=3; b=2; c=1; d=3
The balanced equation is
3CaCO+ 2H3PO4 → Ca3 (PO4)2 + 3H2CO3

Types of Chemical Reactions

Considering various factors, chemical reactions are classified into several categories.

1. Combination Reaction  


It occurs when two or more reactants combine to form a single product.  or we say, such a reaction in which a single product is formed from two or more reactants is known as a combination reaction.
Example:   
(i) Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.
CaO(s) + H2O(l) → Ca(OH)2 (aq) + Heat
(ii)  A solution of slaked lime produced by the reaction is used for white-washing walls. Calcium hydroxide reacts slowly with the carbon dioxide in the air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of whitewashing and gives a shiny finish to the walls. It is interesting to note that the chemical formula for marble is also CaCO3.
Chemical Reactions and Equations Class 10 Notes Science Chapter 1

(iii) Burning of Coal: When carbon is burnt in oxygen (air), carbon dioxide is formed. In this reaction, carbon is combined with oxygen.
 C(s) + O2 (g) → CO(g) 

2. Decomposition Reaction


A single reactant breaks down when exposed to heat, light, or electricity, resulting in the formation of two or more products. There are three types of decomposition reactions:
a. Thermolytic decomposition, where heat is required for the reaction. An example is the thermal decomposition of HgO.
b. Photolytic decomposition, where light is needed for the reaction. An example is the photolytic decomposition of H2O2.
Photolytic DecompositionPhotolytic Decomposition

c. Electrolytic decomposition, where electricity is necessary for the reaction. An example is the electrolytic decomposition of H2O.

3. Displacement Reaction


Displacement reaction is a chemical reaction in which a more reactive element displaces a less reactive element from its compound. Both metals and non-metals take part in displacement reactions.

Some Important Reactions as:
(i) Reaction of iron nails with copper sulphate solution.Fe (s) + CuSO4(aq) →  FeSO4(g) + Cu (s)
(ii)Reaction of lead with copper chloride solution.
Pb (s) + CuCl2(aq) →  PbCl2 (aq) + Cu (s)

Note:  Zinc and lead are more reactive elements than copper. They displace copper from its compounds. 

Question for Chapter Notes: Chemical Reactions & Equations
Try yourself:Which of the following is an example of a displacement reaction?
View Solution

4. Double Displacement Reaction  


A double displacement reaction is a type of chemical reaction where two compounds react, and the positive ions (cation) and the negative ions (anion) of the two reactants switch places, forming two new compounds or products.

Example: When the solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride.
Na2(SO)4 (aq) + BaCl2 (aq) →  BaSO4 (s) + NaCl (aq)

Note: Double Displacement Reaction, in which precipitate is formed, is also known as precipitation reaction. Neutralisation reactions are also examples of double displacement reactions


Chemical Reactions and Equations Class 10 Notes Science Chapter 1

5. Endothermic and Exothermic Reaction

(i) Exothermic Reaction:  An exothermic reaction is a type of chemical reaction in which energy is released from the reaction system into the surroundings, usually in the form of heat, light, or sound. 

Example: Formation of Carbon dioxide
The chemical reaction can be depicted as:  
C(s) + O2(g) → CO2(g) + Heat

(ii) Endothermic Reaction: An endothermic reaction is a type of chemical reaction in which energy is absorbed from the surroundings in the form of heat, light, or electricity.  

Example: Photosynthesis
Plants absorb heat energy from sunlight to convert carbon dioxide and water into glucose and oxygen. 

Reaction as:   6CO2 + 6H2O → C6H12O6 + 6O2

Chemical Reactions and Equations Class 10 Notes Science Chapter 1

Question for Chapter Notes: Chemical Reactions & Equations
Try yourself:Select the appropriate state symbols of the products given as X and Y in the following chemical equation by choosing the correct option from table given below:Zn(s) + H2SO4(l) → ZnSO4(X) + H2(Y)
View Solution

6. Redox Reaction

A redox reaction happens when there is a change in the oxidation states of the substances involved. Oxidation is the loss of electrons or an increase in the oxidation state of a substance or its atoms, while reduction is the gain of electrons or a decrease in the oxidation state of a substance or its atoms.

Chemical Reactions and Equations Class 10 Notes Science Chapter 1

(i) Oxidation is the process in which an atom, ion, or molecule loses one or more electrons. During oxidation, the oxidation state of the atom or molecule increases, since it becomes more positive or less negative.   

Some examples of oxidation include the reaction of iron with oxygen to form rust and the reaction of glucose with oxygen in cellular respiration.

(ii) Reduction is the opposite process of oxidation, in which an atom, ion, or molecule gains one or more electrons.
Some examples of reduction include the reaction of silver ions with electrons to form silver metal, and the reaction of hydrogen ions with electrons to form hydrogen gas.

Question for Chapter Notes: Chemical Reactions & Equations
Try yourself:Which of the following statements is true about oxidation and reduction reactions?
View Solution

Redox Reaction, Oxidized Agent, Reducing Agent:
  • Oxidation and reduction always occur together in a chemical reaction and are often referred to as redox reactions.
  •  In a redox reaction, one substance is oxidized while another substance is reduced. The substance that is oxidized is called the reducing agent.
  • It causes the reduction of another substance by donating electrons. The substance that is reduced is called the oxidizing agent. It causes the oxidation of another substance by accepting electrons.  
  • Example: In the reaction between zinc metal and hydrochloric acid, zinc is oxidized to form zinc ions, while hydrogen ions are reduced to form hydrogen gas:
    Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
  • In this reaction, zinc is the reducing agent and hydrogen ions are the oxidizing agent.

Have You Observed the Effects of Oxidation Reaction in Everyday Life? 

1. Corrosion

  • Numerous metals, prone to chemical activity, easily react with moisture, oxygen, and acids. Iron, for example, starts as shiny but eventually rusts, forming a reddish-brown powder due to oxidation.
  • Metals, like iron, face corrosion as their oxides don't firmly attach, causing flaking, structural weakening, and breakdown. The process of metal deterioration due to environmental substances is called corrosion.
  • Copper and silver items tarnish when exposed to air and water. Copper develops a green oxide layer, while silver acquires a black oxide covering. These oxide formations act as barriers, limiting further exposure and reducing corrosion.
  • Rusting:

    4Fe(s) + 3O2(from air) + xH2O(moisture) → 2Fe2O3.xH2O(rust)
    Corrosion of copper:
    Cu(s) + H2O(moisture) + CO2(from air) → CuCO3.Cu(OH)2(green)

    Corrosion of silver:
    Ag(s) + H2S (from air) → Ag2S(black) + H2(g)
    RustingRusting

2. Rancidity

  • The taste and odour of food materials containing fat and oil change when they are left exposed to air for a long time. This is called rancidity. It is caused due to oxidation of fat and oil present in food material.
  • Prevention:
    (i) Seal food in air-tight containers.
    (ii) Employ nitrogen packaging.
    (iii) Store in refrigeration.
    (iv) Add antioxidants or preservatives.
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FAQs on Chemical Reactions and Equations Class 10 Notes Science Chapter 1

1. What is a chemical reaction?
Ans. A chemical reaction is a process in which one or more substances (reactants) are converted into new substances (products) with different chemical properties. This process involves the breaking and forming of chemical bonds.
2. What are chemical equations?
Ans. Chemical equations are symbolic representations of chemical reactions. They use chemical formulas and symbols to show the reactants and products involved in the reaction, as well as the stoichiometry (relative amounts) of each substance.
3. What are the different types of chemical reactions?
Ans. There are several types of chemical reactions, including: - Combination reactions: Two or more substances combine to form a single product. - Decomposition reactions: A single compound breaks down into two or more simpler substances. - Displacement reactions: An element replaces another element in a compound. - Double displacement reactions: Two compounds exchange ions to form new compounds. - Redox reactions: Involve the transfer of electrons between reactants, also known as oxidation-reduction reactions.
4. Have you observed the effects of oxidation reactions in everyday life?
Ans. Yes, oxidation reactions are commonly observed in everyday life. Some examples include: - Rusting of iron: Iron reacts with oxygen in the presence of moisture to form rust. - Burning of fuels: Combustion reactions involve the rapid oxidation of fuel, releasing heat and producing carbon dioxide and water. - Oxidation of food: When food is exposed to air, it undergoes oxidation, leading to spoilage and the formation of off-flavors. - Tarnishing of silver: Silver objects, when exposed to air, undergo oxidation, leading to tarnish formation. - Fading of colored substances: Many dyes and pigments undergo oxidation when exposed to light, resulting in color fading.
5. What are some frequently asked questions about chemical reactions and equations?
Ans. Some frequently asked questions about chemical reactions and equations include: - How do you balance a chemical equation? - What is the difference between a physical change and a chemical change? - Can a chemical equation be reversed? - What is the law of conservation of mass, and how does it relate to chemical reactions? - How do you determine the type of chemical reaction based on the reactants and products?
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