Chemical Reactions
Chemical reactions involve the breaking of bonds in reactants and formation of new bonds in products, conserving mass and atoms throughout.
A. Types of Chemical Reactions
Eight Reaction Types
| Type | Description | Example |
|---|---|---|
| Combination (Synthesis) | Two or more reactants combine to form ONE product: A + B → AB | 2H₂ + O₂ → 2H₂O · 2Na + Cl₂ → 2NaCl |
| Decomposition | ONE reactant breaks into two or more products: AB → A + B | 2H₂O → 2H₂ + O₂ · CaCO₃ → CaO + CO₂ |
| Single Displacement | One element replaces another in a compound: A + BC → AC + B | Zn + CuSO₄ → ZnSO₄ + Cu · Fe + 2HCl → FeCl₂ + H₂↑ |
| Double Displacement | Ions of two compounds swap partners: AB + CD → AD + CB | NaCl + AgNO₃ → AgCl↓ + NaNO₃ |
| Combustion | Substance burns in O₂, releasing heat and light. Hydrocarbon + O₂ → CO₂ + H₂O | CH₄ + 2O₂ → CO₂ + 2H₂O · C + O₂ → CO₂ |
| Redox | Simultaneous oxidation and reduction — electrons transferred between species | 2Na + Cl₂ → 2NaCl · Fe + CuSO₄ → FeSO₄ + Cu |
| Neutralisation | Acid + Base → Salt + Water (a special double displacement) | HCl + NaOH → NaCl + H₂O |
| Precipitation | Reaction produces an insoluble solid (precipitate) that falls out of solution (shown by ↓) | BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl |
⚡ Symbols to Know ↓ = precipitate (insoluble solid forms). ↑ = gas evolved. (aq) = aqueous solution. (s) = solid. (l) = liquid. (g) = gas. Combustion always produces CO₂ + H₂O (complete combustion).
B. Redox Reactions — OIL RIG
Oxidation & Reduction
OIL — Oxidation Is Loss
Loss of electrons
- Loss of electrons (e⁻)
- Gain of oxygen
- Loss of hydrogen
- Oxidation state increases
RIG — Reduction Is Gain
Gain of electrons
- Gain of electrons (e⁻)
- Loss of oxygen
- Gain of hydrogen
- Oxidation state decreases
| Agent | What it does TO others | What happens TO IT |
|---|---|---|
| Oxidising Agent | Causes oxidation of others (takes electrons) | Gets REDUCED itself (gains electrons) |
| Reducing Agent | Causes reduction of others (gives electrons) | Gets OXIDISED itself (loses electrons) |
Classic Redox Example
Zn + CuSO₄ → ZnSO₄ + Cu
Zn loses 2e⁻ → oxidised (Zn is the reducing agent). Cu²⁺ gains 2e⁻ → reduced (Cu²⁺ is the oxidising agent).
⚡ MCQ Tip OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons). The oxidising agent is itself reduced. The reducing agent is itself oxidised. Rusting = oxidation of iron. Electrolysis: cathode = reduction, anode = oxidation (AN OX — RED CAT).
C. Balancing Chemical Equations
Rules for Balancing
- Law of Conservation of Mass: atoms are neither created nor destroyed — total atoms on left = total on right
- Only coefficients (numbers in front of formulae) can be changed — NEVER subscripts inside formulae
- Balance atoms one element at a time — leave H and O till last
- Check all elements are balanced and coefficients are in the lowest whole-number ratio
| Symbol | Meaning |
|---|---|
| ↓ | Precipitate — insoluble solid formed |
| ↑ | Gas evolved / escapes |
| (aq) | Aqueous — dissolved in water |
| (s) | Solid |
| (l) | Liquid |
| (g) | Gas |
Quick MCQ Revision
| Fact | Answer |
|---|---|
| OIL RIG | Oxidation Is Loss, Reduction Is Gain (of electrons) |
| Oxidising agent | Gains electrons, causes oxidation in others — itself REDUCED |
| Reducing agent | Loses electrons, causes reduction in others — itself OXIDISED |
| Combustion products | Always CO₂ + H₂O (complete combustion of hydrocarbons) |
| Precipitate symbol | ↓ (insoluble solid falls out of solution) |
| Gas evolved symbol | ↑ |
| Combination reaction | A + B → AB (two reactants make one product) |
| Decomposition reaction | AB → A + B (one reactant makes two products) |
| Law of Conservation of Mass | Atoms are neither created nor destroyed — balance equations with coefficients |