Factors Affecting Rates: Concentration, Temperature, Area
The rate of a chemical reaction increases with higher reactant concentration, higher temperature, larger surface area of solids and the presence of a catalyst, because each of these increases the number or the success of collisions between particles. In brief: as a rule of thumb, the rate of many reactions doubles for each 10 °C rise in temperature, so heating a mixture from 20 °C to 60 °C could make the reaction about 16 times faster.
Understanding what controls the rate lets us speed up useful reactions, such as the production of ammonia, and slow down harmful ones, such as the spoilage of food. This article describes each factor with its molecular explanation and a worked example, gives a summary table and explains the history of the temperature rule.
Which factors affect reaction rates?
According to collision theory, a reaction takes place when particles collide with enough energy and with the right orientation. Anything that increases the frequency of collisions or the fraction of collisions that are successful will speed up the reaction. The main factors are:
- the concentration of the reactants (or pressure for gases),
- the temperature,
- the surface area of solid reactants,
- the presence of a catalyst,
- the nature of the reactants and, in some cases, light.
How does concentration affect the rate?
At a higher concentration, there are more particles in a given volume, so they collide more often. In general, the rate is proportional to the concentration of each reactant raised to some power, as given by the rate law. For gases, an increase in pressure has the same effect as an increase in concentration. Pure solids and liquids have a constant concentration, so changing their amount does not change the rate, though it may change the surface area.
The effect of a concentration change depends on the reaction order. If the rate is proportional to [A], doubling the concentration doubles the rate. If it is proportional to [A]2, it quadruples the rate. A burning splint glows in air, but in pure oxygen it bursts into flame because the concentration of oxygen is about five times higher.
How does temperature affect the rate?
Raising the temperature has two effects. Particles move faster, so they collide more often, but this is only a small effect (the collision frequency is proportional to the square root of T, which increases only by about 1.7 % when the temperature goes from 300 K to 310 K). The main effect is that a much larger fraction of the collisions have energy greater than the activation energy. The fraction of particles with enough energy to react rises roughly exponentially with temperature, as described by the Arrhenius equation.
Many reactions near room temperature roughly double or triple in rate for each rise of 10 °C. This is only a rule of thumb, since the size of the effect depends on the activation energy.
How does surface area affect the rate?
A reaction between a solid and a liquid or gas can only take place at the surface of the solid. Dividing the solid into smaller pieces exposes more surface. For a cube with a side of 1 cm, the surface area is 6 cm2. If it is cut into 1000 cubes of 1 mm side, each has a surface of 6 mm2, giving a total of 6000 mm2 = 60 cm2, which is ten times greater. Powdered magnesium reacts with acid far faster than a ribbon, and sugar dissolves faster as powder than as a lump.
Very fine powders and dusts, such as flour, coal dust or grain dust, can react explosively with oxygen, since the surface is so large that the reaction can spread through the whole cloud in an instant. Dust explosions have caused serious industrial accidents.
What other factors are important?
- Catalysts: they provide another route with a lower activation energy and speed up the reaction without being used up.
- Nature of reactants: reactions that need only the rearrangement of electrons, as in acid–base reactions of ions, are very fast, while reactions that require the breaking of many strong covalent bonds are slow.
- Light: photochemical reactions, such as photosynthesis and the reaction of hydrogen with chlorine, need photons to provide the energy.
- Mixing and diffusion: stirring helps to bring the reactants together.
- Solvent and ionic strength: these can change the rate of reactions in solution.
Summary table
| Factor | Change | Effect on particles | Everyday example |
|---|---|---|---|
| Concentration | Increase | More collisions per second | Splint burns faster in pure oxygen |
| Temperature | Increase | More particles with E ≥ Ea | Food spoils faster when warm |
| Surface area | Increase | More particles exposed | Powdered sugar dissolves quickly |
| Catalyst | Add | Lower activation energy | Enzymes in digestion |
| Pressure (gases) | Increase | Higher concentration | Industrial gas reactors |
Worked examples
Example 1: the effect of temperature on time
A reaction takes 160 minutes at 20 °C. If the rate doubles for every 10 °C rise, how long will it take at 60 °C? The temperature increase is 40 °C, which is four steps of 10 °C, so the rate rises by 24 = 16 times. The time falls to 160 / 16 = 10 minutes.
Example 2: the effect of concentration
The rate of a reaction is given by rate = k[A]2. By what factor does the rate change if [A] is tripled? The rate changes by 32 = 9. If [A] is halved, the rate changes by (½)2 = ¼.
Example 3: the effect of surface area
Cutting a cube into smaller cubes with a side length 10 times smaller increases the total area by a factor of 10, and with a side length 100 times smaller by a factor of 100. A reaction between a lump of marble and acid that takes 100 s to finish therefore takes about 10 s if the lump is ground into pieces with a tenth of the side length, assuming that the rate is proportional to the exposed area.
Who found the temperature rule?
The observation that reaction rates double or triple for every 10 K increase was formulated by Jacobus van ‘t Hoff in 1884. The exponential dependence on temperature was given a theoretical form by Svante Arrhenius in 1889, who proposed that molecules must acquire a minimum energy before they can react. Collision theory, developed by Max Trautz in 1916 and William Lewis in 1918, explained why the fraction of energetic collisions rises so steeply with temperature.
Which mistakes are common?
- Thinking that more collisions are the main effect of a higher temperature: it is the higher fraction of energetic collisions.
- Assuming that doubling every 10 °C is a law: it is only an approximation.
- Believing that a catalyst changes the equilibrium: it speeds up the forward and reverse reactions alike.
- Forgetting the order: doubling the concentration does not always double the rate.
- Mixing rate and extent: a faster reaction does not make more product at equilibrium.