Difficulty: Intermediate | Prerequisites: understanding of elements, ions, the periodic table, and basic algebra for balancing.
Chemical reactions are the core of what chemistry does: atoms rearrange to form new substances. This topic covers how to write and balance chemical equations, the five major reaction types (synthesis, combustion, decomposition, single-replacement, double-replacement), oxidation-reduction (redox) reactions, molecular geometry, and the mole concept. These are the tools you need to predict what happens when substances interact, and to do the quantitative calculations that exams love.
Chemical equations must be balanced to satisfy conservation of mass. There are five main reaction types, each with a predictable pattern. Single-replacement reactions are always redox reactions. Molecular shape depends on the number of bonded atoms and lone pairs around the central atom. The mole (6.02 x 10²³ particles) is the bridge between atoms and grams.
Synthesis reaction
Two or more substances combine to form one compound. General form: A + B → AB.
Combustion reaction
A substance reacts with oxygen and releases energy as heat and light. Organic combustion typically produces CO₂ and H₂O.
Decomposition reaction
One compound breaks down into two or more simpler substances. Requires an energy source. General form: AB → A + B.
Single-replacement reaction
One element replaces another in a compound. General form: A + BX → AX + B. Always a redox reaction.
Double-replacement reaction
Two compounds exchange ions. General form: AX + BY → AY + BX. Not a redox reaction.
Oxidation
Loss of electrons. Remembered as LEO (Losing Electrons is Oxidation). Metals typically undergo oxidation.
Reduction
Gain of electrons. Remembered as GER (Gaining Electrons is Reduction). Nonmetals typically undergo reduction.
REDOX reaction
Any reaction in which oxidation numbers change. Practically every reaction type except double-replacement.
Precipitate
An insoluble solid formed during a reaction in aqueous solution.
Net ionic equation
An equation showing only the ions that participate in the reaction (not the spectator ions).
Spectator ions
Ions present in solution that do not participate in the reaction.
Empirical formula
The simplest whole-number ratio of elements in a compound. E.g. CH₂O for glucose.
Molecular formula
A multiple of the empirical formula showing the actual number of atoms. E.g. C₆H₁₂O₆ for glucose.
Mole
6.02 x 10²³ particles (Avogadro's number). The bridge between counting atoms and weighing them.
Molar mass
The mass of one mole of a substance, in grams per mole. Numerically equal to the atomic or molecular mass from the periodic table.
Hybridisation
The mixing of atomic orbitals to form new hybrid orbitals for bonding. Types: sp, sp², sp³, sp³d, sp³d².n VSEPR (molecular shape)
The arrangement of atoms around a central atom, determined by the number of bonded pairs and lone pairs. Shapes include linear, trigonal planar, tetrahedral, trigonal bipyramidal and octahedral.
Word equations: reactant 1 + reactant 2 → product. E.g. iron(s) + chlorine(g) → iron(III) chloride(s).
Skeleton equations: use element symbols instead of words. E.g. C(s) + S(s) → CS₂(l).
Balanced equations: adjust coefficients so atom counts match on both sides (law of conservation of mass). Steps: write the skeleton equation, count atoms on each side, adjust coefficients, reduce to lowest whole-number ratio.
State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous.
Type | General Form | What happens | Products |
|---|---|---|---|
Synthesis | A + B → AB | Two or more substances combine | One compound |
Combustion | Substance + O₂ → | Oxygen reacts, releases heat and light | CO₂ + H₂O (for organic) |
Decomposition | AB → A + B | One compound breaks apart (needs energy) | Two or more substances |
Single-replacement | A + BX → AX + B | One element swaps into a compound | New compound + displaced element |
Double-replacement | AX + BY → AY + BX | Two compounds exchange ions | Two new compounds |
A + BX → AX + B. The more reactive element (A) replaces the less reactive one (B).
If A is less reactive than B, the reaction does not occur (NR, no reaction).
For nonmetals, the more reactive halogen replaces the less reactive halogen in a compound.
All single-replacement reactions are redox reactions.
AX + BY → AY + BX. A and B are cations; X and Y are anions.
Always soluble: anything in Group 1, anything with ammonium (NH₄⁺), anything with nitrates, chlorates or perchlorates.
Insoluble: any halide paired with silver, zinc or mercury.
Most hydroxides are precipitates, except those of alkali metals and barium.
To find the net ionic equation, remove all spectator ions (the ones that stay dissolved on both sides) and keep only the precipitate and its source ions.
Acid + Base → Salt + Water.
The net ionic equation for every neutralisation reaction is: OH⁻ + H⁺ → H₂O.
LEO GER: Losing Electrons is Oxidation, Gaining Electrons is Reduction.
The oxidation number of any pure element is 0.
Example: Mg + O₂ → MgO. Mg goes from 0 to +2 (oxidation). O goes from 0 to −2 (reduction).
Electrons on the right side of a half-equation indicate oxidation. Electrons on the left indicate reduction.
Practically every reaction type is redox, except double-replacement.
Bonded pairs | Lone pairs | Molecular shape | Bond angle | Hybridisation |
|---|---|---|---|---|
6 | 0 | Octahedral | 90° | sp³d² |
5 | 0 | Trigonal bipyramidal | 90° / 120° | sp³d |
4 | 0 | Tetrahedral | 109.5° | sp³ |
3 | 1 | Trigonal pyramidal | 107.3° | sp³ |
3 | 0 | Trigonal planar | 120° | sp² |
2 | 2 | Bent (3D) | 104.5° | sp³ |
2 | 1 | Bent (2D) | < 120° | sp² |
2 | 0 | Linear | 180° | sp |
1 mole = 6.02 × 10²³ particles (Avogadro's number).
1 mole of any gas at STP occupies 22.4 L.
1 mole of a substance has a mass equal to its molar mass (in grams).
The conversion chain: grams ↔ moles ↔ particles (or litres at STP).
Empirical formula: simplest whole-number ratio. E.g. CH₂O.
Molecular formula: the actual count. E.g. C₆H₁₂O₆ (which is six times CH₂O).
To find the empirical formula from percent composition, convert percentages to grams, then to moles, then divide by the smallest mole value.
Temperature conversion: °C + 273 = K
Percent error: (actual − expected) / expected × 100
Percent yield: actual yield / theoretical yield × 100
Density: mass / volume (SI unit: kg/m³)
Atomic number = number of protons = number of electrons (in a neutral atom)
Number of neutrons = mass number − atomic number
Percent by mass: mass of element / mass of compound × 100
Molar volume at STP: 22.4 L/mol
Avogadro's number: 6.02 × 10²³ particles/mol
Mole conversions: 1 mol = molar mass in grams = 22.4 L at STP = 6.02 × 10²³ particles
Students often balance equations by changing subscripts instead of coefficients. Subscripts define the compound; only coefficients may be adjusted.
Combustion reactions are not always synthesis reactions. CH₄ + 2O₂ → CO₂ + 2H₂O is combustion but not synthesis because it produces two products.
Students forget that double-replacement reactions are not redox. No oxidation numbers change because ions simply swap partners.
"Percent error" and "percent yield" use different formulas. Percent error uses (actual − expected)/expected. Percent yield uses actual/theoretical. They are not interchangeable.
⚠️ Be able to identify all five reaction types from a given equation and predict products.
⚠️ Balancing equations appears on nearly every exam. Practise until it is automatic.
⚠️ Know the solubility rules for double-replacement reactions (Group 1, ammonium, nitrates = always soluble; silver/zinc/mercury halides = insoluble).
⚠️ Be able to write net ionic equations by removing spectator ions.
⚠️ Molecular geometry: memorise the table of shapes, bond angles and hybridisations. Expect a question asking for the shape given a Lewis structure.
⚠️ Mole conversions (grams ↔ moles ↔ particles or litres) are guaranteed exam material.
True or false: In a single-replacement reaction, if A is less reactive than B, the reaction proceeds normally. (False, it does not occur: NR.)
Fill in the blank: The net ionic equation for all neutralisation reactions is ____. (OH⁻ + H⁺ → H₂O)
True or false: A molecule with 4 bonded pairs and 0 lone pairs on the central atom is trigonal pyramidal. (False, it is tetrahedral.)
Fill in the blank: 1 mole of any gas at STP occupies ____ litres. (22.4)
True or false: Double-replacement reactions are redox reactions. (False.)
Q: Classify this reaction and predict the products: 2Na + Cl₂ → ?
A: Synthesis reaction. Two elements combine to form one compound: 2Na + Cl₂ → 2NaCl.
Q: In the reaction Zn + CuSO₄ → ZnSO₄ + Cu, identify what is oxidised and what is reduced.
A: Zinc is oxidised (goes from 0 to +2, loses electrons). Copper is reduced (goes from +2 to 0, gains electrons).
Q: Will this single-replacement reaction occur: Cu + NaCl → ? Explain.
A: No (NR). Copper is less reactive than sodium on the activity series, so it cannot replace sodium from the compound.
Q: What is the molecular geometry of a molecule with 3 bonded pairs and 1 lone pair on the central atom?
A: Trigonal pyramidal, with a bond angle of approximately 107.3° and sp³ hybridisation.
Q: How many molecules are in 2.0 moles of water?
A: 2.0 × 6.02 × 10²³ = 1.204 × 10²⁴ molecules.
Reaction types connect directly to periodic trends: the activity series used in single-replacement reactions is built on ionization energy and reactivity trends. Molecular geometry links back to orbital hybridisation from the Atomic Structure notes. Stoichiometry and mole calculations underpin all of quantitative chemistry, including limiting reagents, solution chemistry and gas laws in later chapters.
chemical reactions, balanced equation, synthesis, combustion, decomposition, single-replacement, double-replacement, redox, oxidation, reduction, LEO GER, activity series, solubility rules, precipitate, net ionic equation, spectator ions, neutralisation, molecular geometry, VSEPR, hybridisation, sp, sp2, sp3, tetrahedral, trigonal planar, bent, linear, octahedral, mole, Avogadro's number, molar mass, molar volume, STP, percent yield, percent error, density, empirical formula, molecular formula, percent composition