Surface Area & Rate of Reaction
Cambridge IGCSE Chemistry (0620) · Core & Supplement
🧟 THE ZOMBIE SIEGE ANALOGY
Imagine your soldiers (marble chip atoms) packed in a tight 5×5 formation. The zombies (acid particles) can only attack the soldiers on the outside perimeter. The inner soldiers are completely safe — they're surrounded by teammates!
But if you crush the marble into powder, every single soldier is now exposed. The zombies attack from ALL sides simultaneously. The battle ends in a fraction of the time. This IS surface area.
🪖 LUMP — Soldiers in Formation
Yellow border = exposed soldiers attacked by acid
💥 POWDER — Soldiers Scattered
ALL soldiers exposed — reaction MUCH faster!
🔬 The Science Behind the Siege
According to Collision Theory, a chemical reaction only occurs when reactant particles collide with sufficient energy (≥ activation energy). Breaking a marble chip into powder exposes internal atoms that were previously surrounded on all sides by other atoms — these atoms couldn't be attacked by acid. Now they can. More exposed surface → more frequent collisions → higher rate of reaction.
⚠️ EXAMINER TRAP
Never write "more surface area means more energy." The surface area affects frequency of collisions, NOT the energy per collision. The activation energy stays the same!
📋 SYLLABUS SUMMARY CARD
Core (All Students)
Powders react faster than lumps
Increasing surface area increases rate
Frequency of collisions increases
Supplement (Extended)
Breaking lump exposes internal atoms
Only surface atoms can collide with reactant
Industrial applications: explosion hazards
🏆 GOLDEN MARK-SCHEME PHRASE:
"Greater surface area → more frequent collisions between reactant particles per unit time → higher rate of reaction."
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