Ch 04: Chemical Bonds & Reactions

⚗️ Chapter At a Glance

📘 Chemical Bond

Force of attraction between two or more atoms. Formed to achieve stability by completing 8 electrons in the outermost orbit (like inert gases) and to reduce potential energy.

Major Bond Types

Types of Chemical Reactions

Berzelius

Discoverer of Catalyst

Fe + Mo

Haber's Process Catalyst

📊 Bond Types — Quick Comparison

Bond Type	Formation	Example	Water Solubility	Conductor?
Ionic	Electron Transfer	NaCl, MgO	Soluble	Bad (solid); Good (aqueous)
Covalent	Electron Sharing	O₂, H₂O	Insoluble	Poor (exception: Graphite)
Hydrogen	N/O/F with H	H₂O, NH₃, DNA	—	—
Coordinate	Sharing by same atom	H₂SO₄, H₃O⁺	—	—

⚡ Ionic Bond (Electrovalent)

📘 Formation

By electron transfer from one atom to another. One atom loses e⁻ (becomes cation +) and other gains e⁻ (becomes anion −).

Na → gives 1 e⁻ → Na⁺; Cl → takes 1 e⁻ → Cl⁻ → NaCl
Mg → gives 2 e⁻; O → takes 2 e⁻ → MgO

Properties

Soluble in water
High melting point
Solid state → Bad conductor
Aqueous/molten state → Good conductor
Ionic nature

🔗 Covalent Bond

📘 Formation

By sharing of electron pairs between atoms (usually non-metals).

O₂ → Non-polar covalent (same atoms)
H₂O → Polar covalent (different atoms)

Properties

Insoluble in water
Soluble in organic solvents
Low melting & boiling point (Exception: Diamond)
Poor conductor (Exception: Graphite)

💧 Hydrogen Bond

📘 Formation

Bond formed between highly electronegative atoms (N, O, F) and a hydrogen atom.

Intermolecular H-Bond

Between different molecules. Examples: H₂O (water), NH₃ (ammonia). Responsible for high BP of water.

Intramolecular H-Bond

Between atoms of the same molecule. Examples: Protein, DNA, RNA, cellulose.

🔀 Coordinate Covalent Bond

📘 Formation

Sharing of an electron pair where both electrons come from the same atom (donor). Also called dative bond.

Example: H₂SO₄
Found in Hydronium ion (H₃O⁺)
One atom donates both electrons of the shared pair

🌐 Van der Waals' Force

Weak force of attraction or repulsion between molecules
Different from covalent and ionic bonds
Weaker than chemical bonds
Responsible for properties of noble gases in liquid/solid state

⚠️ Exam Trap — Bond Properties

Ionic bond: Solid = bad conductor; Aqueous = good conductor
Covalent bond exceptions: Diamond (high MP), Graphite (good conductor)
H-bond forms with N, O, F only (NOT Cl, S, etc.)
DNA double helix held by Hydrogen bonds
Water's high boiling point due to intermolecular H-bonds

⚡ Sigma (σ) and Pi (π) Bonds

—

Single Bond

1 Sigma (σ) bond

H–H (H₂)

Double Bond

1 σ + 1 π bond

O=O (O₂)

≡

Triple Bond

1 σ + 2 π bonds

N≡N (N₂)

Bond	Composition	Example	Pi bonds (π)
Single (–)	1 σ only	H₂, CH₄, H₂O	0
Double (=)	1 σ + 1 π	O₂, CO₂, C₂H₄	1
Triple (≡)	1 σ + 2 π	N₂, C₂H₂	2

⚠️ Exam Trap — Sigma & Pi

Every bond has at least one σ (Sigma) bond
Pi bonds only present in double and triple bonds
Triple bond = 1σ + 2π → 2 Pi bonds (NOT 3)
N₂ molecule has a triple bond → 1σ + 2π → very stable

🧬 Molecular Shapes & Structures

H₂O

Water

Angular / V-shaped

Polar Covalent Bond

NH₃

Ammonia

Pyramidal (Trigonal Pyramidal)

Polar Covalent Bond

CH₄

Methane

Tetrahedral

Non-polar Covalent Bond

CO₂

Carbon Dioxide

Linear

Non-polar Covalent Bond

⚠️ Exam Trap — Molecular Shapes

H₂O = Angular/V-shaped (NOT linear; O has 2 lone pairs)
NH₃ = Pyramidal (NOT tetrahedral; N has 1 lone pair)
CH₄ = Tetrahedral (no lone pairs on C)
CO₂ = Linear (no lone pairs; O=C=O)
H₂O is polar; CO₂ is non-polar even though it has polar bonds (symmetric)

🔢 Valency

Valency = combining ability of an atom (no. of electrons used/shared/transferred)
Fluorine (F) — valency always −1 (most electronegative; never positive valency)
IA and IIA groups: valency = group number (Frankland's rule)
17Cl: 2,8,7 → valency = −1 | 11Na: 2,8,1 → valency = +1
Fe, Cu, S, Hg, Sn — have variable valency (more than one valency)

Cross-Multiply Valencies → SubscriptsAlCl₃: Al(+3), Cl(−1) → Al₁Cl₃ → AlCl₃

🔢 Oxidation Numbers

📘 Definition

Charge present on an atom in a compound due to electrons. Can be positive, negative, zero, or fractional.

Element / Species	Oxidation Number	Notes
H (Hydrogen)	+1	−1 in metal hydrides (NaH)
O (Oxygen)	−2	−1 in peroxides (H₂O₂); +2 in OF₂
F (Fluorine)	−1	Always −1 (most electronegative)
Neutral molecule	0	Sum of oxidation numbers = 0
Na, K	+1	Alkali metals = always +1
Mg, Ca, Sr	+2	Alkaline earth metals = always +2

⚠️ Exam Trap — Oxidation Numbers

O = −2 (usually); −1 in H₂O₂; 0 in O₂
H = +1 (usually); −1 in metal hydrides (NaH, CaH₂)
F = always −1 (never positive — most electronegative element)
In neutral molecule, sum of all oxidation numbers = 0
In ions, sum = charge on the ion

⚗️ Types of Chemical Reactions

① Combination Reaction

A + B → C

2H₂ + O₂ → 2H₂O (Water formation)

② Decomposition Reaction

AB → A + B

CaCO₃ →(heat)→ CaO + CO₂↑

③ Displacement Reaction

AB + C → AC + B

Fe + CuSO₄ → FeSO₄ + Cu (Fe more reactive than Cu)

④ Double Displacement

AB + CD → AD + CB

Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl (ion exchange)

⑤ Exothermic Reaction

Reactants → Products + Energy (heat released)

CH₄ + 2O₂ → CO₂ + 2H₂O + Heat | Also: Combustion, Respiration

⑥ Endothermic Reaction

Reactants + Energy → Products (heat absorbed)

Digestion, Photosynthesis, Photochemical reactions

⑦ Reversible Reaction

A + B ⇌ AB

Proceeds in both forward and backward directions; reaches equilibrium

⑧ Irreversible Reaction

A + B → AB

Proceeds only in one direction; goes to completion

⑨ Redox Reaction

Oxidation + Reduction occur simultaneously

Fe + CuSO₄ → FeSO₄ + Cu (Fe oxidised → Fe²⁺; Cu²⁺ reduced → Cu)

⚠️ Exam Trap — Reactions

Exothermic = energy released; Endothermic = energy absorbed
Respiration = exothermic (releases energy); Photosynthesis = endothermic
Displacement: more reactive metal displaces less reactive from solution
Double displacement = ion exchange → often gives precipitate (↓)
CaCO₃ → CaO + CO₂ = Decomposition (NOT combination)

🔄 Oxidation & Reduction

🔴 Oxidation

Combination with O or electronegative element (F, Cl, N)
Loss of H or electropositive element
Loss of electrons (e⁻)
Oxidation number Increases ↑

e⁻ → away ↑ ON

🟢 Reduction

Combination with H or electropositive element (Na, Mg)
Loss of O or electronegative element
Gain of electrons (e⁻)
Oxidation number Decreases ↓

e⁻ → gained ↓ ON

Oxidising Agents (Oxidisers)

Substances that cause oxidation (get reduced themselves)

H₂O₂ (Hydrogen peroxide)
O₂ (Oxygen)
F₂ (Fluorine)
KMnO₄ (Potassium permanganate)

Reducing Agents (Reducers)

Substances that cause reduction (get oxidised themselves)

Li (Lithium)
Na (Sodium)
NaBH₄ (Sodium borohydride)
H₂ (Hydrogen gas)

📘 Memory Trick — OIL RIG

Oxidation Is Loss (of electrons) | Reduction Is Gain (of electrons)

⚠️ Exam Trap — Redox

Oxidising agent gets reduced; Reducing agent gets oxidised
Oxidation = loss of e⁻ = ON increases | Reduction = gain of e⁻ = ON decreases
In Fe + CuSO₄: Fe is oxidised (0 → +2); Cu²⁺ is reduced (+2 → 0)
H₂O₂ can be both oxidising AND reducing agent (amphoteric)

⚙️ Catalyst

📘 Definition

A substance that increases the rate of reaction without being consumed. Discoverer: Berzelius.

Homogeneous Catalysis

Reactant and catalyst are in the same physical state.

SO₂(g) + O₂(g) →(NO catalyst)→ SO₃(g)Both reactants and catalyst in gaseous state

Heterogeneous Catalysis

Reactant and catalyst are in different physical states.

N₂(g) + 3H₂(g) →(Fe solid)→ 2NH₃(g)Gaseous reactants, solid Fe catalyst — Haber's Process

Positive Catalyst

Increases reaction rate

Negative Catalyst

Decreases reaction rate

Discoverer

Berzelius

📋 Important Processes & Their Catalysts

Process	Catalyst Used
Ghee from vegetable oil (hydrogenation)	Nickel (Ni)
Contact method — Sulphuric acid (H₂SO₄)	Pt Powder, V₂O₅
Deacon process — Chlorine (Cl₂) formation	Cupric Chloride (CuCl₂)
Protein → Amino acids (digestion)	Erepsin Enzyme
Making vinegar from sugar	Mycoderma aceti
Conversion of milk into curd	Lactobacilli (Lactose)
Haber's Process — Ammonia (NH₃)	Fe (Iron) + Mo (Molybdenum) as promoter

⚠️ Exam Trap — Catalysts

Haber's Process: N₂ + 3H₂ → 2NH₃; catalyst = Fe + Mo (promoter)
Ghee from oil: catalyst = Nickel (Ni)
H₂SO₄ (Contact method): catalyst = V₂O₅ (most asked)
Curd from milk: catalyst = Lactobacilli (biological catalyst)
Catalyst = Berzelius discovered concept; enzyme = biological catalyst

🎯 High-Frequency BPSC/BSSC Exam Points

Chemical bond = force of attraction; formed to achieve 8 electrons in outer orbit
Ionic bond = electron transfer; NaCl, MgO; soluble in water; bad conductor in solid, good in aqueous
Covalent bond = electron sharing; O₂, H₂O; insoluble in water; poor conductor
Diamond = covalent bond but high melting point (exception)
Graphite = covalent but good conductor (exception)
H-bond: N, O, F with H; DNA held by H-bonds
Coordinate bond = in H₃O⁺, H₂SO₄
Single bond = 1σ | Double = 1σ+1π | Triple = 1σ+2π
H₂O = Angular (V-shape) | NH₃ = Pyramidal | CH₄ = Tetrahedral | CO₂ = Linear
F = valency always −1 | O = −2 (usually) | H = +1 (usually)
Combination: A+B→C | Decomposition: AB→A+B | Displacement: more reactive displaces less
Respiration = Exothermic | Photosynthesis = Endothermic
Oxidation = loss of e⁻, ON increases | Reduction = gain of e⁻, ON decreases
OIL RIG: Oxidation Is Loss | Reduction Is Gain
Catalyst discoverer = Berzelius
Haber's Process: N₂ + 3H₂ → 2NH₃; catalyst = Fe + Mo
H₂SO₄ (Contact): catalyst = V₂O₅
Ghee from oil: catalyst = Nickel (Ni)
Curd from milk: catalyst = Lactobacilli

📋 Quick Reference

Bond/Reaction	Key Fact
Ionic (Electrovalent)	e⁻ transfer; NaCl
Covalent	e⁻ sharing; O₂, H₂O
H-Bond	N/O/F with H; DNA
Coordinate	Same atom donates; H₃O⁺
Single bond	1σ only
Double bond	1σ + 1π
Triple bond	1σ + 2π
Exothermic	Energy released
Endothermic	Energy absorbed
Oxidation	Loss of e⁻; ON ↑
Reduction	Gain of e⁻; ON ↓

⚠️ Most Common Exam Traps

Ionic solid = bad conductor; ionic solution = good conductor
Diamond = covalent bond but hard & high MP
Graphite = covalent bond but conducts electricity
H₂O = V-shaped (NOT linear)
CO₂ = linear, non-polar (NOT bent like H₂O)
Triple bond = 1σ + 2π (NOT 3 sigma)
F = always −1; O = usually −2 but −1 in H₂O₂
Respiration = exothermic (NOT endothermic)
Reducing agent gets oxidised; oxidising agent gets reduced
Haber's = Fe catalyst (NOT V₂O₅); Contact (H₂SO₄) = V₂O₅

⚗️ Chemical Bonds & Reactions