VCE Vault

Year 9 Science · Chemical Sciences · 18m read

Chemical Reactions and Equations

Ever wonder why a match bursts into flame or rust slowly eats through metal? These are chemical reactions, and learning to write equations is like learning chemistry's secret code — a universal way to show exactly what substances transform and how. You'll represent reactions using both words (simple language) and symbols (the compact chemistry shorthand), which helps you predict what happens in labs and understand real-world changes from cooking to corrosion. Mastering this skill is essential for VCE chemistry and builds the foundation for everything else you'll study.

Learning objective: Represent chemical reactions using word and symbol equations.

By the end of this lesson you can:

  • I write word equations for reactions.
  • I balance symbol equations.
  • I identify types of reactions.

Chemical reactions are partner swaps at a dance. Atoms swap partners — but no one leaves the dance floor. That's why equations must balance: the same atoms before and after.

Chemical equations and reaction types

WHAT IS A CHEMICAL REACTION?
• Old bonds broken, new bonds formed.
• REACTANTS → PRODUCTS.
• Mass is conserved (Law of Conservation of Mass).

### Word Equations
Written in plain language:
methane + oxygen → carbon dioxide + water

### Symbol Equations
Use chemical formulas:
CH₄ + 2O₂ → CO₂ + 2H₂O

### State Symbols
• (s) = solid
• (l) = liquid
• (g) = gas
• (aq) = aqueous (dissolved in water)

### BALANCING EQUATIONS
The Law of Conservation of Mass means: atoms cannot be created or destroyed.
COUNT must be equal on BOTH sides.

Steps to balance:
1. Write the unbalanced equation.
2. Count atoms of each element on each side.
3. Add COEFFICIENTS (numbers before formulas) to balance.
4. NEVER change the formulas (subscripts).
5. Check: count again.

Example: Fe + O₂ → Fe₂O₃
Step 1: Unbalanced: Fe + O₂ → Fe₂O₃
Step 2: Fe: 1 left, 2 right. O: 2 left, 3 right.
Step 3: Balance Fe: 4Fe + O₂ → 2Fe₂O₃ (now 4 Fe each side, but O: 2 left, 6 right)
Balance O: 4Fe + 3O₂ → 2Fe₂O₃
Step 4: Check: Fe: 4, 4 ✓. O: 6, 6 ✓.

### Types of Chemical Reactions

| Type | Pattern | Example |
|------|---------|---------|
| Synthesis (combination) | A + B → AB | 2H₂ + O₂ → 2H₂O |
| Decomposition | AB → A + B | 2H₂O → 2H₂ + O₂ |
| Combustion | fuel + O₂ → CO₂ + H₂O | CH₄ + 2O₂ → CO₂ + 2H₂O |
| Precipitation | ions in solution form a solid | Na⁺ + Cl⁻ → NaCl(s) |
| Acid-base neutralisation | acid + base → salt + water | HCl + NaOH → NaCl + H₂O |
| Displacement | A + BC → AC + B (if A more reactive) | Zn + CuSO₄ → ZnSO₄ + Cu |

### Evidence of Chemical Change
• Colour change.
• Gas produced (bubbles).
• Precipitate formed (solid appears).
• Temperature change (heat released or absorbed).
• New smell.

### Energy in Reactions
• EXOTHERMIC: releases heat to surroundings (combustion, neutralisation). Temperature increases.
• ENDOTHERMIC: absorbs heat from surroundings (photosynthesis, some decompositions). Temperature decreases.

### Australian context
The Pilbara region of Western Australia processes iron ore (Fe₂O₃) into iron using a blast furnace reaction. This is a reduction reaction central to Australia's steel industry:
Fe₂O₃ + 3CO → 2Fe + 3CO₂

Key Relationships

  • Conservation of mass: \text{mass of reactants} = \text{mass of products}
  • Complete combustion: \text{CₓHᵧ} + \text{O}_2 \to \text{CO}_2 + \text{H}_2\text{O}
  • Acid + base: \text{acid} + \text{base} \to \text{salt} + \text{H}_2\text{O}

Balancing watchlist

  • Change coefficients only: NEVER alter subscripts in a formula. H₂O is always H₂O.
  • Balance one at a time: Start with elements that appear in only one compound on each side.
  • Check everything: After balancing, count EVERY element on both sides.

Worked example (Medium)

Balance the equation: Mg + HCl → MgCl₂ + H₂

  1. Unbalanced. Mg + HCl → MgCl₂ + H₂
  2. Count. Mg: 1=1 ✓. H: 1 left, 2 right ✗. Cl: 1 left, 2 right ✗.
  3. Balance Cl and H. Put 2 in front of HCl: Mg + 2HCl → MgCl₂ + H₂
  4. Check. Mg: 1=1 ✓. H: 2=2 ✓. Cl: 2=2 ✓.

Answer: Mg + 2HCl → MgCl₂ + H₂

Worked example (Hard)

Balance: C₃H₈ + O₂ → CO₂ + H₂O (complete combustion of propane).

  1. Start with C. 3C on left, so 3CO₂ on right: C₃H₈ + O₂ → 3CO₂ + H₂O
  2. Balance H. 8H on left, so 4H₂O on right: C₃H₈ + O₂ → 3CO₂ + 4H₂O
  3. Balance O. Right side: 3×2 + 4×1 = 6 + 4 = 10 O atoms. Need 10/2 = 5 O₂: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
  4. Check. C: 3=3 ✓. H: 8=8 ✓. O: 10=10 ✓.

Answer: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Common mistakes

  • Changing subscripts to balance (writing H₃ instead of 3H) Subscripts define the chemical formula — changing them makes a different compound entirely. H₂O and H₃O are different substances. Fix: Only add COEFFICIENTS (big numbers in front of the formula). Never alter the subscripts within a formula.
  • Counting O₂ as "1 oxygen" in the atom count O₂ contains 2 oxygen atoms. A coefficient of 3 before O₂ gives 6 oxygen atoms (3 × 2). Fix: Always multiply the subscript by the coefficient: 3O₂ = 6 oxygen atoms.
  • Not recognising combustion as incomplete when CO (not CO₂) is produced COMPLETE combustion (excess O₂) produces CO₂ and H₂O. INCOMPLETE combustion (limited O₂) produces CO (poisonous) and/or C (soot). Fix: Check oxygen availability. If O₂ is limited: products include CO or C.

Practice questions

  1. Which reaction type is: 2H₂O → 2H₂ + O₂?

    1. Synthesis
    2. Decomposition
    3. Combustion
    4. Neutralisation

    (1 marks)

  2. Balance: Al + O₂ → Al₂O₃.

    (2 marks)

  3. What evidence would you observe if a reaction is exothermic?

    (2 marks)

  4. Write a balanced symbol equation for the combustion of methane (CH₄) in oxygen.

    (3 marks)

  5. A student adds zinc to copper sulfate solution and observes a colour change. Write a word equation and explain what type of reaction this is.

    (3 marks)

Going Deeper — The Reactivity Series

The REACTIVITY SERIES ranks metals from most to least reactive:

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

A more reactive metal will DISPLACE a less reactive metal from its compound.

Example: Iron displaces copper because Fe is more reactive than Cu.
Fe + CuSO₄ → FeSO₄ + Cu

However, copper cannot displace iron:
Cu + FeSO₄ → NO REACTION

Uses:
• Explains why gold and silver occur naturally (unreactive, don't form compounds easily).
• Explains why potassium is stored under oil (reacts violently with water and air).
• Guides which metal to use in galvanising (zinc coating on steel) — zinc is more reactive and corrodes preferentially, protecting the iron.

This is called SACRIFICIAL PROTECTION or cathodic protection, used on ships, pipelines and harbour structures in Australian coastal environments.

Summary

  • Chemical equations: reactants → products.
  • Law of Conservation of Mass: atom count must balance on both sides.
  • Balance by adjusting COEFFICIENTS only — never change subscripts.
  • Main reaction types: synthesis, decomposition, combustion, precipitation, acid-base, displacement.
  • Exothermic: heat released (temperature rises). Endothermic: heat absorbed (temperature falls).

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