THP Coaching Classes
Chemistry Notes | Mohit Sir | Academic Session 2025-26
CHEMICAL REACTION
A chemical reaction is a process in which one or more substances (reactants) are converted into new substances (products) with new chemical and physical properties. This involves breaking of old bonds and formation of new bonds.
CHARACTERISTICS OF CHEMICAL REACTIONS
You can recognize a chemical reaction has taken place if you observe:
- Change in state (solid → liquid, liquid → gas, etc.)
- Change in color
- Evolution of a gas
- Change in temperature (heat evolved or absorbed)
- Formation of a precipitate (solid formed in solution)
CHEMICAL EQUATIONS
A chemical equation is the symbolic representation of a chemical reaction where reactants are written on the left, and products on the right, separated by an arrow (→).
Example (Unbalanced):
Mg (s) + O₂ (g) → MgO (s)
Balanced Equation:
2Mg (s) + O₂ (g) → 2MgO (s)
TYPES OF REPRESENTATION
- Word Equation: Magnesium + Oxygen → Magnesium oxide
- Skeletal Equation: Mg + O₂ → MgO (unbalanced)
- Balanced Chemical Equation: 2Mg + O₂ → 2MgO
SMART TRICK TO BALANCE CHEMICAL EQUATIONS EASILY
USE THE RULE: MEC-H-O
Balance elements in this specific order:
- M → Metals
- E → Non-metals (like Cl, S, P)
- C → Carbon
- H → Hydrogen
- O → Oxygen (Always Last)
STEPS TO BALANCE AN EQUATION:
- Write the unbalanced equation.
- List elements on both sides (LHS & RHS).
- Apply the MEC-H-O order to balance atoms one by one.
- Use fractional coefficients temporarily if needed.
- Multiply all terms to eliminate fractions (if any).
- Check total atoms of each element on both sides.
EXAMPLE 1: Balancing C₃H₈ + O₂ → CO₂ + H₂O
Step 1: Balance Carbon (C)
3 C → Put 3CO₂ on RHS
Step 2: Balance Hydrogen (H)
8 H → Put 4H₂O on RHS
Step 3: Balance Oxygen (O)
RHS: 3CO₂ = 6 O, 4H₂O = 4 O → Total = 10 O → Put 5O₂
Final Balanced Equation: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
TYPES OF CHEMICAL REACTIONS
1. COMBINATION REACTION
A reaction in which two or more reactants combine to form a single product.
A + B → AB
Features:
- Only one product is formed.
- Often exothermic (releases heat).
- May involve elements or compounds.
EXAMPLES:
Magnesium + Oxygen → Magnesium Oxide
2Mg(s) + O₂(g) → 2MgO(s)
- Magnesium ribbon burns with a dazzling flame.
- White ash (MgO) is formed.
Calcium Oxide + Water → Slaked Lime
CaO(s) + H₂O(l) → Ca(OH)₂(aq)
- Vigorous reaction.
- Heat is released (exothermic).
- Used in whitewashing walls.
2. DECOMPOSITION REACTION
A reaction in which a single compound breaks down into two or more simpler substances (elements or compounds).
AB → A + B
| Type |
Energy Used |
Example |
| Thermal |
Heat |
Decomposition of CaCO₃ |
| Electrolytic |
Electricity |
Electrolysis of water |
| Photolytic |
Sunlight (UV rays) |
Decomposition of AgCl |
EXAMPLES:
Thermal Decomposition:
CaCO₃(s) → CaO(s) + CO₂(g)
- Lime stone decomposes into quicklime and carbon dioxide.
Electrolytic Decomposition (Electrolysis):
2H₂O(l) → 2H₂(g) + O₂(g)
- Water is broken into hydrogen and oxygen.
- H₂ and O₂ are collected in 2:1 ratio.
3. DISPLACEMENT REACTION
A reaction in which a more reactive element displaces a less reactive element from its compound.
A + BC → AC + B
Examples:
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
- Iron is more reactive than copper.
- Solution turns green, copper metal deposits.
REACTIVITY SERIES – MOST TO LEAST REACTIVE
MNEMONIC: "Please Stop Calling Me A Careless Zebra Instead Try Learning How Copper Saves Gold"
(From wild to chill — reactivity goes downhill!)
| Mnemonic Word |
Element |
Reactivity |
| P |
Potassium |
Most Reactive |
| S |
Sodium |
|
| C |
Calcium |
|
| M |
Magnesium |
|
| A |
Aluminium |
|
| G |
Carbon |
|
| Z |
Zinc |
|
| I |
Iron |
|
| T |
Tin |
|
| L |
Lead |
|
| H |
Hydrogen |
Reference Point |
| C |
Copper |
|
| S |
Silver |
|
| G |
Gold |
Least Reactive |
QUICK TIP:
- Elements above Hydrogen in the series can displace H₂ gas from acids.
- Higher in the list = More Reactive
- Lower in the list = Less Reactive
4. DOUBLE DISPLACEMENT REACTION
A reaction in which ions are exchanged between two compounds to form new compounds.
AB + CD → AD + CB
- Often results in a precipitate (insoluble solid).
- Also called precipitation reactions.
EXAMPLES:
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)
- White precipitate of BaSO₄ is formed.
5. REDOX REACTIONS (REDUCTION + OXIDATION)
A redox reaction is a type of chemical reaction in which oxidation and reduction occur simultaneously.
| Process |
In Terms of Electrons |
In Terms of Oxygen |
In Terms of Hydrogen |
| Oxidation |
Loss of electrons |
Gain of oxygen |
Loss of hydrogen |
| Reduction |
Gain of electrons |
Loss of oxygen |
Gain of hydrogen |
ZnO + C → Zn + CO
- ZnO is reduced to Zn → (it loses oxygen).
- C is oxidized to CO → (it gains oxygen).
This is a Redox Reaction because both oxidation and reduction are happening together.
EFFECTS OF OXIDATION IN EVERYDAY LIFE
1. CORROSION:
What is Corrosion?
Corrosion is a slow destruction of metals due to reaction with oxygen, moisture, acids, or other substances in the environment.
Rusting of Iron:
4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O (Hydrated iron oxide)
- Forms reddish-brown rust on iron.
- Rust is porous and does not protect the metal underneath.
| Prevention Method |
How it works |
| Painting |
Prevents contact with air/water |
| Greasing/Oiling |
Protective layer for metals |
| Galvanization |
Coating with zinc to prevent rust |
| Alloying |
Improves resistance (e.g., stainless steel) |
2. RANCIDITY:
What is Rancidity?
Rancidity is the spoiling of food, especially oily or fatty food, due to oxidation.
- Oxygen reacts with oils/fats, producing unpleasant smell and taste.
- It is a kind of slow oxidation reaction.
- Example: Chips or fried food smell bad when kept open for long.
THP Coaching Classes
Chemistry Notes | Mohit Sir