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Chapter 1: Chemical Reactions and
Equations
1. Chemical Reactions and Chemical Equations
A chemical reaction is a process where the original substances lose their nature and identity to form entirely new chemical substances with completely different properties.
Characteristics of a Chemical Reaction
To determine whether a chemical reaction has taken place, we typically look for one or more of the following observable changes:
Change in State: E.g., Burning of candle wax (solid turns into liquid and gas). Change in Color: E.g., Rusting of iron changes gray metal into a reddish-brown powder. Evolution of a Gas: E.g., Zinc granules reacting with dilute sulfuric acid release Hydrogen gas. Change in Temperature: E.g., Quicklime reacting vigorously with water to release huge amounts of heat energy. Formation of a Precipitate: E.g., Mixing Potassium Iodide and Lead Nitrate forms an insoluble yellow precipitate of Lead Iodide.
Chemical Equation
A chemical equation is the symbolic representation of a chemical reaction using symbols and formulae of the substances involved.
Reactants: The starting substances that undergo chemical change (written on the left-hand side, LHS). Products: The new substances formed as a result of the reaction (written on the right-hand side, RHS).
2. Balancing Chemical Equations
According to the Law of Conservation of Mass , mass can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of the elements present in the products must
Complete Detailed Revision & Study Notes | Class 10 Science (Chemistry)
equal the total mass of the elements present in the reactants. Consequently, the number of atoms of each element must remain the same on both sides.
Hit-and-Trial Method Steps:
Step 1: Write down the skeletal framework of the chemical equation. Step 2: Count and list the number of atoms of each element on both sides (LHS and RHS). Step 3: Equate the atoms of the element with the maximum count first by putting balancing coefficients in front of the formula (never alter the subscripts inside a chemical formula). Step 4: Balance the remaining elements one by one. Step 5: Write down the physical states of reactants and products to make the equation more informative: Solid ( s ), Liquid ( l ), Gas ( g ), and Aqueous solution ( aq ).
Example of Balancing:
Unbalanced: Fe(s) + H₂O(g) → Fe₃O₄(s) + H₂(g)
Balanced: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
3. Types of Chemical Reactions
Chemical reactions can be broadly classified into five major categories based on how atoms rearrange:
I. Combination Reaction
A reaction in which a single product is formed from two or more reactants combined together.
A + B → AB
Burning of Coal: C(s) + O₂(g) → CO₂(g) Slaking of Lime: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat
Important Note: Calcium hydroxide [ Ca(OH)₂ , slaked lime] is applied to walls during whitewashing. It reacts slowly with carbon dioxide in the air over 2-3 days to form a thin, shiny layer of Calcium Carbonate [ CaCO₃ ], giving the walls a bright finish.
Observation: The vibrant blue color fades to green because Iron is more reactive than Copper and displaces it.
IV. Double Displacement Reaction
A chemical reaction in which two compounds react by an exchange of ions to form two entirely new compounds.
AB + CD → AD + CB
Example: Mixing Sodium Sulfate solution with Barium Chloride solution.
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ [White ppt] + 2NaCl(aq)
Observation: An insoluble white solid precipitate of Barium Sulfate forms immediately. This is also called a precipitation reaction.
V. Oxidation and Reduction (Redox) Reactions
Oxidation: The gain of oxygen or the loss of hydrogen from a substance during a reaction. Reduction: The loss of oxygen or the gain of hydrogen by a substance during a reaction. Redox Reaction: A reaction in which both oxidation and reduction occur simultaneously.
CuO + H₂ &xrightarrow;Δ Cu + H₂O
(Copper Oxide loses oxygen → Reduced | Hydrogen gains oxygen → Oxidized)
Oxidizing Agent: The substance that provides oxygen or removes hydrogen (it undergoes reduction itself). Reducing Agent: The substance that gains oxygen or provides hydrogen (it undergoes oxidation itself).
4. Classification Based on Energy/Heat Changes
Reaction Type Definition Key Examples
Exothermic Reactions
Reactions in which heat energy is released and emitted along with the products.
- Respiration process
- Burning of Natural Gas: CH₄ + 2O₂ → CO₂ + 2H₂O + Heat
Endothermic Reactions
Reactions in which energy is absorbed from the environment to carry out the chemical transformation.
- Photosynthesis in plants
- All types of Decomposition Reactions (Thermal, Electrolytic, Photolytic) **5. Effects of Oxidation Reactions in Everyday Life
- Corrosion**
The slow eating up or deterioration of metals due to the attack of atmospheric air, moisture, water vapor, or chemicals on their surfaces.
Examples: Rusting of iron (flaky reddish-brown layer), blackening coating on silver items, and green layer on copper structures. Prevention Methods: Painting, oiling/greasing, galvanization (coating with a layer of Zinc), chrome plating, or transforming metals into alloys.
2. Rancidity
The aerial oxidation of fats and oils present in food materials, changing their composition and leading to an unpleasant smell and foul taste.
Prevention Methods: Storing cooked food materials in strictly airtight containers. Flushing food packaging bags (like potato chips) with inert, unreactive Nitrogen gas to eliminate oxygen. Keeping food stored inside low-temperature refrigerators. Adding specialized chemical antioxidants like BHA (Butylated Hydroxyanisole) or BHT.
