What "rate" means and how to measure it
The rate of reaction is how fast reactants turn into products — how much change per unit time. You measure it by tracking anything that changes: the volume of gas given off, the loss in mass as gas escapes, or the time for a solution to turn cloudy or a colour to change.
A rate is fastest at the start — reactant concentration is highest — and tails off to zero as reactants are used up. On a graph of product against time, a steeper line means a faster reaction.
Collision theory
Reactions happen when particles collide — but not every collision works. A collision only reacts if the particles hit with at least a minimum energy, the activation energy, and with the right orientation. Anything that makes collisions more frequent or more energetic speeds the reaction up.
This one idea explains every rate factor below: they all work by increasing either the frequency of collisions or the proportion that have enough energy.
The four factors
Each factor is just collision theory in action:
| Factor | Effect on rate | Why (collision theory) |
|---|---|---|
| Higher temperature | faster | particles move faster → collide more often AND more have the activation energy |
| Higher concentration / pressure | faster | more particles in the same space → more frequent collisions |
| Larger surface area (smaller pieces) | faster | more particles exposed at the surface to be hit |
| Catalyst added | faster | provides a route with a lower activation energy, so more collisions succeed |
Catalysts
A catalyst speeds up a reaction without being used up — you get it all back at the end. It works by offering an alternative pathway with a lower activation energy, so a greater fraction of collisions have enough energy to react.
Catalysts are specific and hugely important industrially (iron in the Haber process, nickel in hydrogenating oils). Enzymes are biological catalysts, which is why temperature and pH affect how fast living reactions run.
Temperature does two things, not one
Raising temperature is the most powerful factor because it acts twice: particles collide more often AND a bigger proportion clear the activation-energy bar. Concentration and surface area only change collision frequency.
A worked example
Marble chips react with acid to give CO₂. Powdered marble reacts much faster than the same mass of large chips. Explain using collision theory.
- Powder has a far larger total surface area than a few big chips of the same mass.Breaking a solid into small pieces exposes much more of its surface.
- More of the marble is exposed to acid particles, so collisions between acid and marble happen more frequently.Reaction occurs at the solid surface; more surface = more collision sites.
- More frequent successful collisions per second means a faster rate.Rate depends on how often reacting collisions occur.
The powder has more surface area, so acid particles collide with it more frequently, giving more successful collisions per second and a faster rate.
Measuring a rate in the lab
To measure a rate you follow something that changes as the reaction proceeds. If a gas is made, collect it in a gas syringe or measure the loss in mass as it escapes. If a precipitate forms, time how long a mark takes to disappear through the cloudy mixture (the 'disappearing cross'). Plot the measured quantity against time.
On that graph the rate is the gradient: steep at the start when reactants are plentiful, flattening as they run out, and finally horizontal when the reaction stops. A steeper initial slope means a faster reaction.
The spread of collision energies (A Level)
At any temperature the molecules do not all have the same energy — they follow a Boltzmann distribution, a spread from slow to fast. Only the molecules to the right of the activation energy can react. A small rise in temperature shifts many molecules past the activation energy, which is why a modest heating produces a large increase in rate.
A catalyst works differently: it does not heat anything, but by lowering the activation energy it moves the threshold to the left, so a far greater fraction of the existing molecules already have enough energy to react.
Temperature vs catalyst
Temperature moves molecules to higher energies; a catalyst lowers the energy they need. Both increase the fraction of successful collisions — but only the catalyst is recovered unchanged at the end.