CBSE Class 10 Science Chapter 1: Chemical Reactions And Equations Notes

Key Points to Remember

• A chemical reaction involves the breaking and making of chemical bonds, resulting in new substances with different properties.
• Chemical equations represent chemical reactions symbolically using chemical formulae of reactants and products.
• Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of reactants must equal the total mass of products, necessitating balancing chemical equations.
• Balancing a chemical equation ensures that the number of atoms of each element is the same on both sides of the equation (reactants and products).
• Main types of chemical reactions include: Combination, Decomposition, Displacement, Double Displacement, and Redox.
• Oxidation is the gain of oxygen or loss of hydrogen/electrons; Reduction is the loss of oxygen or gain of hydrogen/electrons.
• Redox reactions involve both oxidation and reduction occurring simultaneously.
• Corrosion (e.g., rusting of iron) and Rancidity (oxidation of fats/oils) are examples of oxidation reactions with significant daily life impact.

Key Definitions

Chemical Reaction

A process that involves rearrangement of the atomic structure of a substance, resulting in the formation of new substances with different chemical properties.

Reactants

The substances that take part in a chemical reaction and undergo chemical change.

Products

The new substances formed as a result of a chemical reaction.

Chemical Equation

A symbolic representation of a chemical reaction using chemical formulae of reactants and products, showing their physical states and reaction conditions.

Balanced Chemical Equation

A chemical equation where the number of atoms of each element is equal on both the reactant and product sides, satisfying the Law of Conservation of Mass.

Oxidation

The process involving the gain of oxygen, loss of hydrogen, or loss of electrons by a substance during a chemical reaction.

Reduction

The process involving the loss of oxygen, gain of hydrogen, or gain of electrons by a substance during a chemical reaction.

Redox Reaction

A chemical reaction in which oxidation and reduction occur simultaneously.

Corrosion

The gradual deterioration of metals due to their reaction with air, moisture, or chemicals present in the environment (e.g., rusting of iron).

Rancidity

The spoilage of food items containing fats and oils, caused by their oxidation when exposed to air, leading to unpleasant smell and taste.

Understanding Chemical Equations and the Need for Balancing

A chemical equation is a shorthand notation to represent a chemical reaction. It provides qualitative and quantitative information about the reaction. The substances that react are called reactants, and they are written on the left side of the arrow. The new substances formed are called products, and they are written on the right side. An arrow → separates reactants from products and indicates the direction of the reaction. Physical states of reactants and products (solid (s), liquid (l), gas (g), aqueous (aq)) are often included for more information.

For example, the reaction of hydrogen gas with oxygen gas to form water can be written as: H₂(g) + O₂(g) → H₂O(l).

However, this equation is unbalanced. An unbalanced equation violates the Law of Conservation of Mass, which states that matter can neither be created nor destroyed in a chemical reaction. This means the total mass of the reactants must be equal to the total mass of the products. Consequently, the number of atoms of each element must be the same on both sides of the equation. To satisfy this law, we must balance the chemical equation by adjusting the stoichiometric coefficients (the numbers placed in front of chemical formulae) in the equation. This ensures that the number of atoms of each element is conserved throughout the reaction. Balancing equations is a critical skill for Class 10 students and forms the basis for understanding quantitative aspects of chemistry.

Steps to Balance a Chemical Equation (Hit and Trial Method)

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Types of Chemical Reactions

Chemical reactions are broadly classified based on the rearrangement of atoms. Understanding these types helps predict products and reaction behavior:

1. Combination Reaction: Two or more reactants combine to form a single product. It's often exothermic (releases heat).

• General form: A + B → AB
• Example: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat (Formation of slaked lime)

1. Decomposition Reaction: A single reactant breaks down into two or more simpler products. These reactions typically require energy (heat, light, or electricity) and are often endothermic (absorb heat).

• General form: AB → A + B
• Example: CaCO₃(s) + Heat → CaO(s) + CO₂(g) (Thermal decomposition of limestone)

1. Displacement Reaction: A more reactive element displaces a less reactive element from its compound. Usually occurs in solutions.

• General form: A + BC → AC + B
• Example: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) (Iron displaces copper)

1. Double Displacement Reaction: Two different atoms or groups of atoms exchange places between two compounds to form two new compounds. These often form a precipitate or water.

• General form: AB + CD → AD + CB
• Example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq) (Precipitation reaction)

1. Redox Reaction: Reactions involving simultaneous oxidation and reduction. Oxidation is loss of electrons/gain of oxygen/loss of hydrogen. Reduction is gain of electrons/loss of oxygen/gain of hydrogen. The substance that gets oxidized is the reducing agent, and the substance that gets reduced is the oxidizing agent.

• Example: CuO(s) + H₂(g) → Cu(s) + H₂O(l)
• CuO is reduced to Cu (loss of oxygen).
• H₂ is oxidized to H₂O (gain of oxygen).

Everyday Effects of Redox Reactions: Corrosion and Rancidity

Oxidation reactions are not just confined to laboratory settings; they have significant impacts on our daily lives. Two common examples are Corrosion and Rancidity.

Corrosion is the process by which metals are gradually eaten up by the action of air, moisture, or a chemical (like an acid) on their surface. It is essentially an oxidation reaction where the metal reacts with oxygen and water in the atmosphere. The most common example is the rusting of iron, where iron reacts with oxygen and moisture to form hydrated iron(III) oxide (rust), a reddish-brown flaky substance. 4Fe(s) + 3O₂(g) + 2xH₂O(l) → 2Fe₂O₃.xH₂O(s) (Rust). Corrosion weakens structures and objects made of metal, leading to significant economic losses. Methods to prevent corrosion include painting, oiling, greasing, galvanizing (coating with zinc), chrome plating, anodizing, and making alloys.

Rancidity refers to the undesirable change in the taste and smell of food items, especially those containing fats and oils, when they are left exposed to air for a long time. This occurs due to the oxidation of fats and oils present in the food. The unsaturated fatty acids react with atmospheric oxygen, forming compounds that give off foul smells and tastes. To prevent rancidity, methods such as adding antioxidants (substances that prevent oxidation), packaging food in nitrogen gas (e.g., potato chips), refrigerating food, and storing food in airtight containers are employed.

Exam Tips for Chemical Reactions and Equations

1. Practice Balancing Equations: This is a guaranteed question. Practice various types, including those with polyatomic ions. Always double-check your atom count on both sides.
2. Memorize Reaction Types & Examples: Be ready to identify the type of reaction from a given equation or write an example for a specific type. Pay attention to precipitation in double displacement and energy changes (exothermic/endothermic).
3. Understand Redox Clearly: Distinguish between oxidation and reduction, oxidizing agent, and reducing agent. Remember: 'OIL RIG' (Oxidation Is Loss, Reduction Is Gain of electrons/oxygen/hydrogen rules).
4. Daily Life Applications: Questions on corrosion and rancidity, their causes, and prevention methods are common. Provide clear, concise explanations.
5. Physical States & Conditions: If asked to write a complete chemical equation, include physical states (s), (l), (g), (aq) and reaction conditions (e.g., heat Δ, pressure, catalyst) above/below the arrow, if given.

Quick Revision Check

• Q: Why is it essential to balance a chemical equation? A: It is essential to balance a chemical equation to satisfy the Law of Conservation of Mass, which states that atoms cannot be created or destroyed in a chemical reaction. Therefore, the number of atoms of each element must be equal on both sides of the equation.
• Q: Identify the type of reaction: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). A: This is a Displacement Reaction, as zinc (a more reactive metal) displaces copper from copper sulphate solution.
• Q: What happens when quicklime is added to water? Write the equation and mention the type of reaction. A: When quicklime (Calcium Oxide, CaO) is added to water, it reacts vigorously to form slaked lime (Calcium Hydroxide, Ca(OH)₂), releasing a large amount of heat. The equation is CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat. This is a Combination Reaction.
• Q: Define rancidity and suggest one method to prevent it. A: Rancidity is the unpleasant change in taste and smell of food containing fats and oils due to their oxidation when exposed to air. One method to prevent it is by adding antioxidants to food or by packaging food in an inert gas like nitrogen.

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