Welcome to Chemical Reactions & Equations — the heart of chemistry. While Module 1 taught you about individual atoms and elements, this module is all about what happens when substances interact, transform and create something new.
You will learn how to write and balance chemical equations, identify different types of reactions (combustion, neutralisation, decomposition and more), understand energy changes in reactions, work with rates of reaction, and explore acids, bases and salts. These topics form the largest chunk of the CSCA Chemistry paper.
By the end of this module you will be able to predict products, balance equations efficiently, and explain why some reactions are fast and others slow. Let's get mixing!
Chemical reactions make up roughly 30–35% of the CSCA Chemistry paper. Expect 2–3 calculation questions (reacting masses, rates), 1–2 explanation questions (collision theory, acid reactions), and 1 question requiring equation balancing. Redox identification and catalyst questions are common.
A chemical equation is a shorthand way of describing a chemical reaction. Reactants go on the left, products on the right, and an arrow (→) means "reacts to form".
Word Equations
Start simple: sodium + water → sodium hydroxide + hydrogen
Symbol Equations
Replace names with formulas: Na + H₂O → NaOH + H₂
Balancing
The law of conservation of mass says atoms cannot be created or destroyed. Every atom on the left must appear on the right. We balance by adding coefficients (big numbers in front).
Steps to Balance
Example: Na + H₂O → NaOH + H₂
Na: 1 left, 1 right ✓
H: 2 left, 3 right ✗ → multiply: 2Na + 2H₂O → 2NaOH + H₂
Na: 2=2 ✓, H: 4=4 ✓, O: 2=2 ✓
State Symbols
Show the physical state of each substance: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).
Final balanced equation: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
💡Balance equations by adjusting coefficients to ensure the same number of each type of atom on both sides. Never change the subscripts in formulas — only add coefficients.
📋 Key Formulas
Balanced equation: same atoms on both sides | State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved | Conservation of mass: total mass of reactants = total mass of products
📝 Worked Example 1
Example 1: Balance: Fe + O₂ → Fe₂O₃
Fe: 1 left, 2 right → put 4Fe left. O: 2 left, 3 right → put 3O₂. Recount: 4Fe + 3O₂ → 2Fe₂O₃. Fe: 4=4 ✓, O: 6=6 ✓.
📝 Worked Example 2
Example 2: Balance: CH₄ + O₂ → CO₂ + H₂O
C: 1=1 ✓. H: 4 left, 2 right → put 2H₂O. O: 2 left, 4 right → put 2O₂. CH₄ + 2O₂ → CO₂ + 2H₂O. C:1=1, H:4=4, O:4=4 ✓.
📝 Worked Example 3
Example 3: Balance: Al + HCl → AlCl₃ + H₂
Al: 1=1 ✓. Cl: 1 left, 3 right → 3HCl. H: 3 left, 2 right — not even. Try: 2Al + 6HCl → 2AlCl₃ + 3H₂. Al:2=2, H:6=6, Cl:6=6 ✓.
🧠If you struggle with balancing, try the "odd-even trick": if an element appears an odd number of times on one side and even on the other, double the odd side first.
🧠State symbols are often worth a separate mark — do not forget them if the question says "include state symbols".
⚠️Changing subscripts to balance: Writing H₃O instead of putting a coefficient in front of H₂O changes the substance entirely.
⚠️Forgetting to re-count after adjusting: Changing one coefficient may unbalance another element. Always do a final check.
🎯 Try This Yourself
Balance: Mg + HCl → MgCl₂ + H₂
Open and read all sections to complete this module