Surface area and particle size effects

The rate and extent of chemical change • Rate of reaction

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Why does surface area change not alter activation energy?

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Surface area changes only affect collision frequency and exposure of reactive sites; activation energy depends on the energy barrier for each successful collision and remains unchanged.

Key concepts

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Definition of surface area and particle size

Surface area is the total area of the outer surfaces of a solid. Particle size describes the dimensions of individual pieces of a solid reactant, from large lumps to fine powder. Small particles give more external surface area for the same mass than large particles. Small particle size therefore means a greater area for collisions between reactant particles and molecules in the surrounding phase (gas or solution). The greater exposed area increases the chance of successful collisions per second.

How surface area affects rate (collision theory)

Reaction rate depends on the frequency of successful collisions between reacting particles. Increasing the exposed surface area of a solid increases the number of sites where collisions can occur. Cause: greater exposed area. Effect: more collisions per unit time and a higher reaction rate. The energy requirement for each collision does not change when surface area changes. Surface area affects only the number of collisions, not the activation energy or temperature dependence.

Surface area to volume ratio (SA:V) explained

Surface area to volume ratio is surface area divided by volume for a solid piece. For simple shapes, SA:V increases as size decreases. Example for a cube: surface area = 6s^2, volume = s^3, so SA:V = 6/s. Halving the side length doubles the SA:V. Cause: reduction in linear dimensions. Effect: increased SA:V, which leads to more surface exposed per unit of material and faster overall reaction for the same mass of solid.

Predicting effects when particle size changes

Smaller particle size causes higher SA:V and so increases reaction rate. Cause: breaking a lump into many small pieces increases total surface area while keeping mass constant. Effect: faster rate and shorter time to complete. Large lumps show lower SA:V and react more slowly. Predictive statement: when particle size decreases, expect a faster initial rate and quicker completion if surface area is the controlling factor.

Limiting factors and exceptions

Surface area only increases rate while it remains the rate-limiting factor. If temperature, concentration, pressure (for gases) or availability of a catalyst limit the rate, changes in surface area have little or no effect. Cause: another step limits overall rate. Effect: altering surface area produces negligible change. Physical factors such as surface coatings, passivation layers, or poor mixing can limit the effect of increased surface area. Small particles may agglomerate in solution, reducing effective surface area and reducing the expected rate increase.

Key notes

Important points to keep in mind

Surface area increases collision frequency; activation energy remains unchanged.

SA:V increases as particle size decreases; for cubes SA:V = 6/s, for spheres SA:V = 3/r.

Smaller pieces of the same mass react faster because total exposed area increases.

Surface area only affects rate while it remains the limiting factor.

Keep temperature, concentration and mixing constant when testing surface area effects.

Agglomeration, coatings or poor mixing can reduce effective surface area.

Predict changes by comparing SA:V before and after size changes.

Report observations (gas volume, colour change, temperature change) to indicate rate differences.