Part 1: Collision theory and factors affecting rate

Build a particle explanation for concentration, pressure, surface area and temperature effects, then distinguish a faster reaction from a larger final amount of product.

A rate tells us how quickly composition changes

Rate measures change per unit time. Concentration change is commonly measured in mol dm⁻³ s⁻¹; gas-volume change may be followed in cm³ s⁻¹ and mass change in g s⁻¹. These are related ways of monitoring a reaction, but they are not interchangeable units. State which species and measured property the rate describes.

Reacting particles need to collide. A collision is successful only if it has sufficient energy for the reaction pathway and a suitable orientation. Activation energy Ea is the minimum collision energy needed for reaction by that pathway. Molecules often separate unchanged after a collision because the barrier was not crossed or the necessary parts of the molecules did not meet.

Even an exothermic reaction can have a high activation barrier and proceed slowly at room temperature. Reaction enthalpy compares endpoints; activation energy concerns reaching a reactive configuration. A fuel can therefore store chemical energy without immediately burning until a suitable initiation raises the rate.

Concentration changes collision frequency

Increasing reactant concentration places more reacting particles in each unit volume. With other conditions fixed, collisions between those species generally occur more frequently, so there can be more successful collisions per second. At the same temperature the energy distribution is unchanged: the explanation is not that each particle has gained energy.

For acid reacting with a solid carbonate, more acid particles are available near the exposed surface. If surface area is held constant, encounters with reactive sites become more frequent. More concentrated acid may also provide more total acid moles if volume is unchanged, so distinguish a change in initial rate from a change in which reagent is limiting.

Collision theory gives a qualitative explanation. It does not establish that doubling concentration always doubles the measured rate; actual rate dependence is determined experimentally and is developed in Topic 16. A valid AS answer connects particles per unit volume to collisions per second without inventing an unsupported numerical law.

Compression raises reacting gas concentrations

For a fixed amount of gas at constant temperature, reducing its volume increases pressure and the number of reacting particles per unit volume. Collisions become more frequent. The temperature condition matters: heating a sealed rigid container also raises pressure, but through a different change in particle energies rather than through compression.

Adding an inert gas at constant volume does not increase the concentration of the original reacting gases, even though total pressure rises. The simple compression argument therefore cannot be applied merely because a pressure gauge reads a larger value. Identify what changed physically before applying collision theory.

For predominantly liquid or solid systems, ordinary pressure changes have a much smaller effect on concentration because these phases are much less compressible. An answer about squeezing gas particles closer together needs gaseous reactants in the context.

Only accessible solid surfaces can meet the other reactant

A liquid or gas reacts initially at the exposed surface of a solid. Crushing a fixed mass into smaller pieces increases total accessible surface area, giving more sites at which reactant particles can collide. It does not add more moles of solid and does not lower the reaction's activation energy.

Equal masses of fine and coarse CaCO₃ can generate the same total CO₂ if the same carbonate amount reacts completely and acid is in excess. The powder produces a steeper gas-volume curve and reaches the same plateau sooner. A greater final gas volume requires a difference in reacting amount or conditions, not merely faster reaction.

To test particle-size effects, keep carbonate mass and purity, acid concentration and volume, temperature, mixing and collection apparatus constant. Use defined size ranges rather than claiming that any two visually different samples isolate surface area perfectly. Powder can also cause rapid frothing, so select manageable quantities.

Temperature changes both collision frequency and success fraction

Heating increases average kinetic energy and particle speeds, giving more collisions per second. More importantly, a larger fraction of collisions has energy at least Ea, often producing a much larger increase in reaction rate than the collision-frequency change alone. Not every particle gains the same energy, and not every collision becomes successful.

Ea for an unchanged pathway is not lowered by heating. Part 2 shows the energy distribution shifting relative to the same threshold. At a lower temperature the energetic fraction is smaller but not necessarily zero; collisions continually redistribute energy, so molecules are not permanently divided into a reactive group and an unreactive group.

A reaction time may fall from 80.0 s to 40.0 s when heated. If both times correspond to the same extent of reaction, the approximate average rate doubles. This observed factor does not prove a universal rule that a particular temperature increase always doubles every reaction's rate.

Match each explanation to its cause

A useful explanation forms a chain: changed condition → changed particle behaviour → changed frequency of successful reaction events. 'More successful collisions' on its own names the outcome without explaining why.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official Edexcel mark allocations.

Sources