Cambridge O Level Chemistry · Syllabus 5070 · Chemical Reactions
Surface Area
What is Surface Area?
The total area of a solid that is exposed to the surrounding reactant. Breaking a fixed mass of solid into smaller pieces exposes surfaces that were previously buried inside, so the surface area rises while the mass stays the same, and the reaction rate rises with it.
This definition is part of the Chemical Reactions chapter in Cambridge O Level Chemistry.
Surface Area in context
Chemical reactions are changes in which new substances form as bonds break and re-form between particles, and this chapter answers three questions about them. Rate asks how fast a reaction happens, and is governed by collision theory: particles must collide with combined energy at least equal to the activation energy, \(E_\mathrm{a}\), and factors such as concentration, pressure, surface area, temperature and catalysts all work by changing how often successful collisions occur. Equilibrium asks how far a reversible reaction goes before the forward and reverse rates become equal. Redox asks which species lost electrons and which gained them, tracked through oxidation numbers.
Common mistakes with Surface Area
- 5. “Grinding the solid increases its concentration.” Why temptingBoth changes make a reaction faster, so the words get swapped. RepairSolids do not have a concentration; concentration is defined for a solute in a solution. Grinding increases the surface area, exposing more particles. TransferState what is unchanged when a lump is ground to powder. The mass, the number of particles present and the final amount of product.
Examiner tips on Surface Area
- Say “surface area”, never “concentration”. A solid does not have a concentration; concentration is defined for a solute in a solution. “Grinding the solid increases its concentration” names the wrong quantity, so the sentence is wrong however sound the reasoning around it is.
Questions students ask about Surface Area
Why doesn't grinding a solid increase its concentration?
Concentration is defined only for a solute dissolved in a solution, and a solid reactant is not in solution. Grinding a lump into powder increases its surface area instead, exposing particles that were previously buried inside the lump, which raises collision frequency at the surface without changing the mass or the amount of product formed.