Effect of Surface Area on Rate

Collision Theory Recap

  • Reactions only happen when particles collide with enough energy.

  • Rate of reaction depends on the frequency of successful collisions.

  • Frequency = number of successful collisions per second.

Effect of Surface Area

  • Only the particles on the surface of a solid can collide with particles in solution (or gas).

  • Larger solid blocks → fewer surface particles exposed → slower reaction.

  • Smaller pieces / powdered solids → greater surface area-to-volume ratio (SA:V) → more exposed particles → more collisions per second → faster reaction.

Practical Example: Marble Chips + Hydrochloric Acid

Reaction:

Calcium carbonate (s) + Hydrochloric acid (aq) → Carbon dioxide (g) + Calcium chloride (aq) + Water

Method 1: Measuring Gas Volume

  • Conical flask with marble chips + HCl.

  • Delivery tube into water trough + measuring cylinder (or gas syringe).

  • Record volume of CO₂ produced over time.

  • Repeat with different sizes of marble chips (large vs. powdered).

⚠ Issue: Measuring cylinder can be inaccurate if bubbles form too quickly. → Gas syringe is more precise.

Method 2: Mass Loss

  • Conical flask on a balance.

  • Reactants: marble chips + HCl.

  • As CO₂ gas escapes, mass decreases.

  • Plot mass lost over time.

  • Use cotton wool plug in flask:

    • Allows gas to escape.

    • Stops acid splashing out (which would give anomalous results).

Key Points for Exams

  • Increasing surface area = faster reaction (more frequent collisions).

  • Smaller particles → higher SA:V ratio.

  • Graphs:

    • Steeper curve = faster reaction.

    • Reaction with powdered chips finishes quicker (but both produce the same final amount of product if the same mass of CaCO₃ is used).

✅ Conclusion:
The rate of reaction increases as the surface area of solid reactants increases, due to more collisions per second.