Cheatsheet — Chemical Reactions & Equations

⚗️ Types of Chemical Reactions — Overview

Type	General Form	Key Feature	Example
Combination	A + B → AB	One product formed	2Mg + O₂ → 2MgO
Decomposition	AB → A + B	One reactant breaks down	CaCO₃ → CaO + CO₂
Single Displacement	A + BC → AC + B	More reactive displaces less	Fe + CuSO₄ → FeSO₄ + Cu
Double Displacement	AB + CD → AD + CB	Exchange of ions; ppt. formed	Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl
Oxidation	X → X gains O / loses H	Loss of electrons (OIL)	2Mg → 2MgO (Mg is oxidised)
Reduction	X → X loses O / gains H	Gain of electrons (RIG)	CuO → Cu (Cu²⁺ is reduced)

🔗 Combination Reactions

Two or more substances combine to form a single product. Usually exothermic.

2Mg(s) + O₂(g) → 2MgO(s) Exo
Bright white dazzling flame; white MgO powder

CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat Exo
Quicklime + water → slaked lime (slaking)

C(s) + O₂(g) → CO₂(g) Exo

2SO₂(g) + O₂(g) → 2SO₃(g)
Contact process — intermediate for H₂SO₄

Quick check: Count the products — if only ONE, it's combination!

💥 Decomposition Reactions

Single compound breaks into simpler substances. Usually endothermic — requires energy input.

Thermal Decomposition Endo

CaCO₃(s) → CaO(s) + CO₂(g)
Used in cement industry; CO₂ turns lime water milky

2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g)
Brown fumes of NO₂; yellow PbO residue

2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g)
Green → brown; pungent smell

Electrolytic Decomposition Endo

2H₂O(l) → 2H₂(g) + O₂(g)
H₂ at cathode, O₂ at anode. Ratio H₂:O₂ = 2:1

Photodecomposition Light

2AgCl(s) → 2Ag(s) + Cl₂(g)
White AgCl turns grey in sunlight

2AgBr(s) → 2Ag(s) + Br₂(g)
Used in black-and-white photography

↔️ Displacement Reactions

Single displacement: More reactive element displaces less reactive from solution.

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Blue → pale green; reddish copper deposits on nail

Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
Zn > Cu in reactivity series

Double displacement / Precipitation:

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄↓ + 2NaCl(aq)
White precipitate of BaSO₄

Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂↓ + 2KNO₃(aq)
Yellow precipitate of PbI₂

Reactivity Series (High → Low)

        K > Na > Ca > Mg > Al > Zn > Fe > Ni > Sn > Pb > H > Cu > Hg > Ag > Au

        Metals above H displace H₂ from dilute acids

🔥 Oxidation & Reduction (REDOX)

	Oxidation	Reduction
Oxygen	Gains O₂	Loses O₂
Hydrogen	Loses H	Gains H
Electrons	Loses e⁻ (OIL)	Gains e⁻ (RIG)

CuO(s) + H₂(g) → Cu(s) + H₂O(l)
CuO is reduced (loses O) → Black to red-brown
H₂ is oxidised (gains O) — reducing agent

2Mg(s) + O₂(g) → 2MgO(s)
Mg is oxidised (OA = O₂); O₂ is reducing agent? No — Mg is RA, O₂ is OA

OIL RIG: Oxidation Is Loss · Reduction Is Gain (of electrons)

⚖️ Balancing Chemical Equations

Based on the Law of Conservation of Mass — atoms are neither created nor destroyed.

Hit-and-Trial Method Steps

1. Write the skeletal (word/symbol) equation
2. Count atoms on each side
3. Add coefficients (never change subscripts!)
4. Balance metals first, then non-metals, H last, O last
5. Verify total atoms are equal on both sides

Key Balancing Examples

H₂ + O₂ → H₂O
→ 2H₂ + O₂ → 2H₂O

Fe + H₂O → Fe₃O₄ + H₂
→ 3Fe + 4H₂O → Fe₃O₄ + 4H₂

HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
→ 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O

Remember: State symbols (s), (l), (g), (aq) must be added for a complete balanced equation.

🔍 Signs That a Chemical Reaction Has Occurred

🌡️ Temperature change 🎨 Change of colour 💨 Gas evolved ☁️ Precipitate formed 💡 Light emitted 👃 Odour produced

Endothermic: Absorbs energy from surroundings — solution feels cold.
Exothermic: Releases energy — solution feels warm; light/flame may appear.

🦠 Corrosion & Rancidity

Corrosion

Slow oxidation of metals exposed to moist air → surface damage.

4Fe(s) + 3O₂(g) + xH₂O → 2Fe₂O₃·xH₂O
Rusting — reddish-brown hydrated iron(III) oxide

2Cu + O₂ + CO₂ + H₂O → Cu(OH)₂·CuCO₃
Green patina on copper (verdigris)

Prevention of Rusting	How it Works
Painting / Oiling	Physical barrier to moisture + O₂
Galvanisation (Zn coat)	Zn is more reactive — sacrificial
Alloying (stainless steel)	Cr + Ni added — forms protective oxide
Electroplating	Coat with Tin, Chromium, Silver

Rancidity

Oxidation of fats & oils in food → unpleasant smell and taste.

Prevention: Antioxidants (BHA/BHT) · Nitrogen flushing (chip packets) · Refrigeration · Vacuum packaging · Dark-coloured airtight containers

🧠 Memory Tricks & Mnemonics

OIL RIG

Oxidation Is Loss
Reduction Is Gain
(of electrons)

Reactivity Series Mnemonic

K Na Ca Mg Al Zn Fe Ni Sn Pb H Cu Hg Ag Au
"King Needs Clean Milk, All Zebras Feel Nauseous — Some Pretty Heavy Creatures Have Angry Attitudes"

Types of Decomposition

Thermal = Temperature (Heat)
Electrolytic = Electricity
Photolytic = Photons (Light)
TEP = Three Energy types!

Balancing — Quick Rule

Balance in order:
1. Metals → 2. Non-metals → 3. Hydrogen → 4. Oxygen
Change ONLY coefficients, never subscripts!

Exo vs Endo

Exothermic = Energy EXITs → reaction is hot → spontaneous
Endothermic = Energy ENTERs → reaction cools surroundings

Precipitation Quick Test

If a reaction makes an insoluble ionic product — it precipitates!
BaSO₄ (white ↓), PbI₂ (yellow ↓), AgCl (white ↓)