Chemical Reactions and Equations Revision Notes CBSE Class 10
Chemical Reactions
Processes that involve the rearrangement of the atomic structure of substances, leading to the formation of new substances with different properties.
Characteristics: Often accompanied by observable changes such as change in color, change in state, evolution of gas, change in temperature, or formation of a precipitate.
Reactants: The substances that 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 formulas of reactants and products, with an arrow indicating the direction of the reaction.
Format: Reactants are written on the left side of the arrow and products on the right side. Plus signs (+) separate multiple reactants or products.
State Symbols: (s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous solution.
Conditions: Temperature, pressure, catalyst, etc., are often indicated above or below the arrow.
Balanced Chemical Equation
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.
Balancing: Involves ensuring that the number of atoms of each element is the same on both sides of the chemical equation. This is done by adjusting the stoichiometric coefficients (numbers in front of the chemical formulas), not by changing the subscripts within the formulas.
Significance: Ensures adherence to the Law of Conservation of Mass and provides quantitative information about the reaction.
Types of Chemical Reactions
Combination Reaction
Two or more reactants combine to form a single product.
General Form: A + B → AB
Example: CaO(s) + H2O(l) → Ca(OH)2(aq) (Slaking of lime)
Decomposition Reaction
A single reactant breaks down into two or more simpler products.
General Form: AB → A + B
Conditions: Often requires energy in the form of heat (thermal decomposition), light (photolytic decomposition), or electricity (electrolytic decomposition).
Example: CaCO3(s) -Heat→ CaO(s) + CO2(g) (Decomposition of limestone)
Displacement Reaction
A more reactive element displaces a less reactive element from its compound.
General Form: A + BC → AC + B
Reactivity Series: Understanding the reactivity series of metals is crucial to predict these reactions (e.g., K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au).
Example: Fe(s)+CuSO4(aq) → FeSO4(aq) + Cu(s) (Iron displaces copper)
Double Displacement Reaction
Two compounds react by an exchange of ions to form two new compounds.
General Form: AB + CD → AD + CB
Common Outcomes: Often result in the formation of a precipitate (precipitation reaction) or water (neutralization reaction).
Example: BaCl2(aq) + Na2SO4(aq) → BaSO4(s)↓ + 2NaCl(aq) (Formation of barium sulfate precipitate)
Precipitation Reaction
A reaction in which an insoluble solid (precipitate) is formed when two aqueous solutions are mixed.
Characteristic: The precipitate settles down from the solution.
Example: (See Double Displacement example above)
Endothermic and Exothermic Reactions
Exothermic Reactions:
Reactions that release energy, usually in the form of heat, to the surroundings.
Temperature Change: The temperature of the surroundings increases.
Example: Burning of natural gas (CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) + Energy)
Endothermic Reactions:
Reactions that absorb energy, usually in the form of heat, from the surroundings.
Temperature Change: The temperature of the surroundings decreases.
Example: Photosynthesis
(6CO2(g) + 6H2O(l) -Sunlight→ C6H12O6(aq) + 6O2(g))
Oxidation and Reduction (Redox Reactions)
Oxidation:
- Gain of oxygen.
- Loss of hydrogen.
- Loss of electrons (electron transfer concept - introduced briefly).
Reduction:
- Loss of oxygen.
- Gain of hydrogen.
- Gain of electrons (electron transfer concept - introduced briefly).
Redox Reactions:
Reactions where oxidation and reduction occur simultaneously.
Oxidizing Agent: The substance that causes oxidation (itself gets reduced).
Reducing Agent: The substance that causes reduction (itself gets oxidized).
Example: CuO(s) + H2(g) -Heat Cu(s) + H2O(l)
CuO is reduced (loses oxygen).
H2 is oxidized (gains oxygen).
CuO is the oxidizing agent.
H2 is the reducing agent.
Effects of Oxidation in Everyday Life
Corrosion:
The slow eating up of metals by the action of air, moisture, or a chemical on their surface.
Example: Rusting of iron (Fe2O3⋅xH2O)
Prevention: Painting, oiling, greasing, galvanizing, chrome plating, anodizing, alloying.
Rancidity:
The oxidation of fats and oils in food when exposed to air, leading to unpleasant smell and taste.
Prevention: Adding antioxidants, packaging in nitrogen gas, refrigeration, storing in airtight containers.