Year 10 Science · Chemical Sciences · 10m read

Chemical Reactions in Detail

Ever wonder why a candle flame needs oxygen to keep burning, or how your body transforms food into energy? Chemical reactions are the transformations where substances break apart and recombine to form completely new materials. You'll learn to identify different reaction types, write the equations that describe them, and prove that matter never disappears—it just changes form. Mastering this is essential for VCE chemistry and explains how everything from cooking to medicine actually works.

Learning objective: Describe reaction types, write balanced equations and apply conservation of mass.

By the end of this lesson you can:

I balance chemical equations.
I classify reactions.
I apply conservation of mass.

Reactions are partner swaps. Atoms rearrange partners, but no atoms are created or lost — so equations must balance.

Reactions and equations

CHEMICAL EQUATION: reactants → products.

BALANCING EQUATIONS:
• Law of conservation: atoms can't be created or destroyed.
• Same number of each atom on both sides.
• Adjust COEFFICIENTS (not subscripts).
• e.g., 2H₂ + O₂ → 2H₂O.

REACTION TYPES:
1. SYNTHESIS (combination): A + B → AB.
2H₂ + O₂ → 2H₂O.
2. DECOMPOSITION: AB → A + B.
2H₂O₂ → 2H₂O + O₂.
3. DISPLACEMENT (single): A + BC → AC + B.
Zn + 2HCl → ZnCl₂ + H₂.
4. DOUBLE DISPLACEMENT: AB + CD → AD + CB.
NaOH + HCl → NaCl + H₂O.
5. COMBUSTION: fuel + O₂ → CO₂ + H₂O (+ heat).
6. NEUTRALISATION: acid + base → salt + water.

CONSERVATION OF MASS:
• Total mass of reactants = total mass of products.
• Count every atom carefully.

STATES OF MATTER:
• (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).

IONIC EQUATIONS:
• Show only the IONS involved in the reaction.
• Ignore "spectator ions".

ENERGY IN REACTIONS:
• EXOTHERMIC: release heat (ΔH negative).
• ENDOTHERMIC: absorb heat (ΔH positive).

RATE FACTORS:
• Temperature, concentration, surface area, catalyst.

Balance like a pro

Metals first: Balance non-H, non-O atoms first.
H, O last: Usually easiest to handle at the end.
Never change formulas: Only coefficients.

Quick reference

| Reaction type | Example |
|---|---|
| Combustion | CH₄ + 2O₂ → CO₂ + 2H₂O |
| Neutralisation | Acid + base → salt + water |
| Precipitation | Two solutions → insoluble solid |
| Acid + metal | → salt + hydrogen |
| Synthesis/decomposition | Combine / break apart |

Worked example (Medium)

Balance: Fe + O₂ → Fe₂O₃.

Balance Fe. Need 2 Fe on left → 2Fe + O₂ → Fe₂O₃.
Balance O. Right has 3 O, left has 2. LCM = 6. Need 3 O₂ on left and 2 Fe₂O₃ on right.
Adjust. So need 4 Fe: 4Fe + 3O₂ → 2Fe₂O₃.
Check. Fe: 4 = 4. O: 6 = 6. Balanced!

Answer: 4Fe + 3O₂ → 2Fe₂O₃.

Chemical Reaction Formulas

Conservation of mass: \text{mass}_{\text{reactants}} = \text{mass}_{\text{products}}
Moles: n = \dfrac{m}{M} (n = moles, m = mass (g), M = molar mass (g/mol))

Worked example (Hard)

Balance: $\text{Fe}_2\text{O}_3 + \text{CO} \to \text{Fe} + \text{CO}_2$ and identify the reaction type.

Identify. Iron oxide + carbon monoxide → iron + carbon dioxide. Used in blast furnaces for iron production.
Balance Fe. Fe₂O₃ has 2 Fe → need 2Fe on right: Fe₂O₃ + CO → 2Fe + CO₂.
Balance O and C. Try 3CO: Fe₂O₃ + 3CO → 2Fe + 3CO₂. Check: Fe 2=2, O 3+3=6=6, C 3=3. Balanced!
Reaction type. This is a REDOX reaction: iron is REDUCED (loses oxygen), carbon in CO is OXIDISED to CO₂.

Answer: Fe₂O₃ + 3CO → 2Fe + 3CO₂. Type: Redox (reduction of iron ore).

Common mistakes

Changing subscripts to balance an equation instead of coefficients Subscripts define the chemical formula. Changing H₂O to H₃O creates a completely different compound. Fix: Only change COEFFICIENTS (large numbers in front of formulas). Never change subscripts within a formula.
Confusing complete and incomplete combustion products Complete combustion (excess O₂) produces CO₂ and H₂O. Incomplete (limited O₂) produces CO (carbon monoxide, toxic) and/or C (soot). Fix: Check O₂ availability. Excess O₂ → complete combustion (CO₂ + H₂O). Limited O₂ → incomplete (CO or C + H₂O).
Forgetting that mole ratio from balanced equation gives mass ratios The balanced equation tells you the MOLE ratio of reactants and products, which leads to mass calculations. Fix: C + O₂ → CO₂: 1 mol C (12 g) + 1 mol O₂ (32 g) → 1 mol CO₂ (44 g). Total mass conserved: 12 + 32 = 44 g.

Practice questions

An exothermic reaction:
1. Absorbs heat.
2. Releases heat to surroundings.
3. Has no energy change.
4. Produces only oxygen.
(1 marks)
Balance: $\text{C}_3\text{H}_8 + \text{O}_2 \to \text{CO}_2 + \text{H}_2\text{O}$ (combustion of propane).
(3 marks)
Magnesium reacts with dilute hydrochloric acid. Write the balanced equation with state symbols and classify the reaction type.
(2 marks)
12 g of carbon burns completely in oxygen. (a) Write the balanced equation. (b) Calculate the mass of CO₂ produced. (Relative atomic masses: C=12, O=16)
(3 marks)

Going Deeper — Rates of Reaction

RATE OF REACTION = speed at which products form or reactants are consumed.

Factors that INCREASE reaction rate:
1. TEMPERATURE: more energy → more frequent and energetic collisions → more exceed the activation energy (Ea).
2. CONCENTRATION (or pressure for gases): more particles → more collisions per second.
3. SURFACE AREA: smaller particle size → more surface exposed → more collision sites.
4. CATALYST: provides an alternative lower-energy pathway. Not consumed in the reaction.

COLLISION THEORY: for a reaction to occur, particles must collide with:
• Sufficient energy (≥ activation energy Ea).
• Correct orientation.

Industrial examples:
• Haber process (ammonia fertiliser): iron catalyst, high pressure (200 atm), 400–500°C.
• Catalytic converters in cars: platinum/rhodium catalysts convert CO and NOx to CO₂ and N₂.
• Enzyme catalysis in biology: biological catalysts that lower Ea for metabolic reactions.

Summary

Conservation of mass: total atoms must balance on both sides.
Balance equations by changing COEFFICIENTS only. Never change subscripts.
Reaction types: synthesis, decomposition, combustion, acid-base, displacement, redox.
Exothermic: heat released (temperature rises). Endothermic: heat absorbed (temperature falls).
Rate factors: temperature, concentration, surface area, catalyst (all increase rate when increased).