Collision theory and activation energy: predict and explain the effects of changes in the size of pieces of a reacting solid in terms of surface area to volume ratio

The idea

For a solid reactant, only particles at the exposed surface can collide with particles of the other reactant, so the rate depends on how much surface is available. Breaking a solid into smaller pieces increases its total surface area without changing its mass or volume, so the surface area to volume ratio rises. A greater ratio means more solid particles are exposed to collisions per unit of volume, so collisions occur more frequently and the rate of reaction increases. For example, 10 g of marble chips in excess acid reacts slowly, but the same 10 g powdered exposes far more surface and fizzes faster, although the same total volume of gas is eventually produced. The ratio is calculated by dividing surface area by volume, so halving particle size roughly doubles the ratio for similarly shaped pieces.

What earns the marks

• Only particles at the surface of a solid can collide with particles of the other reactant, so reaction occurs at the surface.
• Breaking a solid into smaller pieces increases its total surface area while mass and volume stay the same, so the surface area to volume ratio increases.
• A higher surface area to volume ratio exposes more solid particles to collisions per unit volume, increasing collision frequency.
• More frequent collisions between reactant particles increase the rate of reaction, so the reaction finishes sooner.
• The total amount of product formed eventually is unchanged because the same mass of solid reacts; only the rate changes.
• Comparing equal masses of large chips and powder in the same volume of reactant solution allows a fair test of the effect.

Common mistakes and how to put them right

• Thinking that powder has more mass or reacts to give more product.

  Put it right: The mass is unchanged, so the same total volume of gas or amount of product forms, but faster.

• Saying smaller pieces have more surface area without linking it to volume.

  Put it right: The key idea is the surface area to volume ratio, which increases as pieces get smaller.

• Believing smaller pieces are more reactive because their particles have more energy.

  Put it right: Particle energy and activation energy are unchanged; the increase is in collision frequency at the exposed surface.

Exam tips

• Name the variable you change (size of pieces) and the variable you measure (rate, for example gas volume per unit time or loss in mass per unit time).
• Use the phrase surface area to volume ratio and explain that it increases when a solid is broken into smaller pieces.
• Link the ratio to collision frequency and then to rate, and state that the total product is unchanged for the same mass of solid.