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

Physical and Chemical Changes: In the realm of chemical change, the birth of one or more new substances, boasting unique physical and chemical properties, takes place. Consider the interaction between copper sulfate and iron, giving rise to ferrous sulfate and copper.

Distinguishingly, physical change manifests as a shift in color or state, sans the creation of novel substances. An illustration is the transition of water into steam upon boiling, where the change in state is the sole protagonist. Even though steam and water appear dissimilar, their reaction with a piece of sodium yields identical products, showcasing a mere change in state.

Observations Aiding Chemical Reaction Determination: The unfolding of a chemical reaction becomes apparent through various observations: a) Evolution of a gas b) Change in temperature c) Formation of a precipitate d) Change in color e) Change of state

Chemical Reaction Essence: Chemical reactions represent transformative journeys, where reactants metamorphose into products by forging or breaking bonds between different atoms. Defined by changes in electron locations within chemical bonds, these reactions occur at predictable rates, influenced by temperature and chemical concentration.

Diverse Types of Chemical Reactions: Taking into account various factors, chemical reactions find classification in multiple categories:

● Combination ● Decomposition ● Single Displacement ● Double displacement ● Redox ● Endothermic ● Exothermic ● Precipitation ● Neutralization

The exploration of these categories unveils the diverse nature of chemical transformations, from combining elements to their intricate rearrangements, reflecting the dynamic symphony inherent in the scientific world.

Chemical Reactions and Equations

Word Equation

A word equation serves as a verbal representation of a chemical reaction, expressing it in words rather than chemical formulas. It aids in identifying the reactants and products involved. This concise representation showcases the reactants on the left side, separated by a plus sign if there’s more than one, and the products on the right side, similarly separated.

For instance: Sodium + Chlorine → Sodium chloride This equation signifies: “Sodium reacts with chlorine to form sodium chloride.”

Symbols of Elements and Their Valencies

A symbol serves as the chemical code for an element, usually a one or two-letter atomic representation, often an abbreviation of its name. Valency, the combining capacity of an element, quantifies the electrons lost, gained, or shared during its interaction with another atom to form a molecule.

Writing Chemical Equations

The representation of a chemical reaction using symbols and chemical formulae of the reactants and products is termed a chemical equation. Various symbols denote states: “(s)” for solids, “(l)” for liquids, “(g)” for gases, “(aq)” for aqueous solutions, “(↑)” for produced gas, and “(↓)” for precipitate formed.

Balancing of a Chemical Reaction

Law of Conservation of Mass: This law asserts that in a chemical reaction, no atoms can be created or destroyed. The number of atoms for each element on the reactants side must equal the number on the products side. In simpler terms, the total mass of products equals the total mass of participating reactants.

Balanced Chemical Equation: An equation is balanced when the number of atoms of each element on the reactants side equals that on the products side.

Steps for Balancing Chemical Equations:

Represent the changes occurring in a chemical reaction using a chemical equation.

Maintain equilibrium on both sides of the arrow, ensuring the same number of each type of atom.

Introduce coefficients, numerical values added to the front of a chemical symbol or formula, indicating the number of atoms or molecules involved in the process.

Place coefficients strategically to balance the equation, ensuring an equal number of each type of atom on both sides.

Example: Zn + HCl → ZnCl2 + H2 The balanced equation is Zn + 2HCl → ZnCl2 + H2

Hit and Trial Method: In balancing an equation, adjust coefficients to make the number of atoms of each element the same on both sides of the chemical equation.

Short-Cut Technique for Balancing a Chemical Equation

Example: aCaCO3 + bH3PO4 → cCa3(PO4)2 + dH2CO3

Utilize a series of simultaneous equations 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

d = a = 3

b = 2c = 2

So, a = 3; b = 2; c = 1; d = 3.

The balanced equation is: 3CaCO3 + 2H3PO4 → Ca3(PO4)2 + 3H2CO3

For further insights into the Balancing of a Chemical Equation, visit [here](insert link).

Chemical Reactions and Equations II

Types of Chemical Reactions

Considering various factors, chemical reactions are categorized into multiple types. A few examples include:

- Combination

- Decomposition

- Single Displacement

- Double displacement

- Redox

- Endothermic

- Exothermic

- Precipitation

- Neutralization

- Combination Reaction:

In a combination reaction, two elements, one element and one compound, or two compounds combine to form a single product.

For instance: When quicklime or calcium oxide (CaO) reacts with water, slaked lime [Ca(OH)2] is formed. This reaction releases a substantial amount of heat, indicating its exothermic nature.

Decomposition Reaction

In a decomposition reaction, a single reactant disintegrates when subjected to heat, light, or electricity, yielding two or more products. Various types include:

a. Thermolytic Decomposition or Thermolysis:

Example: Thermal decomposition of HgO.

b. Photolytic Decomposition or Photolysis:

Example: Photolytic decomposition of H2O2.

c. Electrolytic Decomposition or Electrolysis:

Example: Electrolytic decomposition of H2O.

Displacement Reaction

In a displacement reaction, a more reactive element displaces a less reactive element from its compound or solution.

Double Displacement Reaction or Precipitation Reaction

An exchange of ions between reactants leads to new products. When two solutions containing soluble salts are combined, an insoluble compound called precipitate forms.

Example:

AgNO3(aqueous)+KCl(aqueous)→AgCl(precipitate)+KNO3(aqueous)

Redox Reaction

A redox reaction occurs when the oxidation states of the substrate change. Oxidation involves the loss of electrons, gain of oxygen, or loss of hydrogen, while reduction involves the gain of electrons, loss of oxygen, or gain of hydrogen. Both processes happen simultaneously.

Endothermic and Exothermic Reaction

Exothermic Reaction: Heat is released during the reaction. Most combination reactions fall under this category.

· Example: 2FeSO4(s)heat−−→Fe2O3(s)+SO2(g)+SO3(g)

· Example: CH4+2O2→CO2+2H2O+heat

· Endothermic Reaction: Heat is absorbed for the reaction to occur.

· Example: 6CO2+6H2O+Sunlight→C6H12O6+6O2 (Glucose)

· Most decomposition reactions are endothermic.

Effect of Oxidation Reaction in Everyday Life

In photosynthesis, an endothermic reaction fueled by sunlight, carbon dioxide, and water combine to produce glucose and oxygen. This process sustains life by providing energy for various organisms.

Reaction of Quicklime with Water – Exothermic Reaction

When quicklime or calcium oxide (CaO) reacts with water, it forms slaked lime [Ca(OH)2], and this reaction releases a substantial amount of heat. Therefore, this process is classified as an exothermic reaction. CaO+H2O→Ca(OH)2CaO+H2O→Ca(OH)2

Decomposition Reactions

In a decomposition reaction, a single reactant breaks down under the influence of heat, light, or electricity, yielding two or more products. There are different types of decomposition reactions:

a. Thermolytic Decomposition or Thermolysis:

Example: Thermal decomposition of HgO.

b. Photolytic Decomposition or Photolysis:

Example: Photolytic decomposition of H2O2.

c. Electrolytic Decomposition or Electrolysis:

Example: Electrolytic decomposition of H2O.

Displacement Reaction

In a displacement reaction, a more reactive element displaces a less reactive element from its compound or solution.

Double Displacement Reaction or Precipitation Reaction

An exchange of ions between reactants leads to the formation of new products. For instance, when two solutions containing soluble salts are combined, an insoluble compound, known as precipitate, forms.

Example: AgNO3(aqueous)+KCl(aqueous)→AgCl(precipitate)+KNO3(aqueous)AgNO3(aqueous)+KCl(aqueous)→AgCl(precipitate)+KNO3(aqueous)

Redox Reaction

In a redox reaction, the oxidation states of the substrates change. Oxidation involves the loss of electrons, gain of oxygen, or loss of hydrogen, while reduction involves the gain of electrons, loss of oxygen, or gain of hydrogen. Both processes occur simultaneously.

Endothermic and Exothermic Reactions

· Exothermic Reaction: Heat is released during the reaction. Most combination reactions fall under this category.

· Example: Al+Fe2O3→Al2O3+Fe+heatAl+Fe2O3→Al2O3+Fe+heat

· Example: CH4+2O2→CO2+2H2O+heatCH4+2O2→CO2+2H2O+heat

· Endothermic Reaction: Heat is absorbed for the reaction to occur.

· Example: 6CO2+6H2O+Sunlight→C6H12O6+6O26CO2+6H2O+Sunlight→C6H12O6+6O2 (Glucose)

· Most decomposition reactions are endothermic.

Effect of Oxidation Reaction in Everyday Life

In photosynthesis, an endothermic reaction fueled by sunlight, carbon dioxide, and water combine to produce glucose and oxygen. This process sustains life by providing energy for various organisms.

Corrosion

Corrosion is the gradual deterioration of a material, typically a metal, caused by the action of moisture, air, or chemicals in the surrounding environment.

Corrosion Processes:

Rusting: 4Fe(s)+3O2(from air)+H2O(moisture)→2Fe2O3⋅H2O (rust)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)Cu(s)+H2O(moisture)+CO2(from air)→CuCO3⋅Cu(OH)2 (green)

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

Rancidity: Rancidity refers to the oxidation of fats and oils in food stored for an extended period, leading to a foul smell and bad taste. Rancid food consumption can cause stomach infections.

Prevention Methods: (i) Use of air-tight containers (ii) Packaging with nitrogen (iii) Refrigeration (iv) Addition of antioxidants or preservatives

Frequently Asked Questions on Chemical Reactions and Equations:

Q1: Carbon reacts with oxygen to give carbon dioxide. This is an example of which type of reaction?

Answer: This is an example of a combination reaction since two reactants combine to form a single product.

Q2: Identify the type of chemical reaction taking place when silver chloride turns black on exposure to sunlight.

Answer: It is a decomposition reaction that occurs in the presence of sunlight, and hence it is a photochemical decomposition reaction.

Q3: In the electrolysis of water (acidified), what are the names of the gases that are evolved at the anode and cathode, respectively?

Answer: In the electrolysis of water (acidified), the gases evolved at the anode and cathode are oxygen and hydrogen, respectively. Hydrogen ions gain electrons from the cathode, forming hydrogen gas, while oxygen ions give electrons to the anode, producing oxygen gas.