# Factors That Affect Reaction Rate – Chemical Kinetics

Several factors affect reaction rate. A chemical reaction only occurs if reactant particles collide with each other successfully. Anything which increases the likelihood of successful particle collisions increases reaction rate.

Using these factors to control the rate of a chemical reaction is important for many chemical processes. For example, slowing a highly exothermic reaction may prevent an explosion. Speeding the rate of a glow stick reaction makes its light glow brighter. Here is a list of factors that affect reaction rate, an explanation of why they work, and a look at the limitations of increasing rate.

### A Closer Look at the Factors

#### Temperature

Temperature is often the factor that has the greatest effect on reaction rate. Increasing temperature gives particles kinetic energy so they bounce around more quickly and are more likely to combine. More importantly, the added energy is more likely to meet the activation energy requirement for the reaction. In contrast, lowering the temperature makes molecules slower and less likely to react.

The rate of many chemicals reactions doubles for each 10 °C increase in temperature. The “rule” applies to most, but not all reactions. For example, many biochemical reaction rates double with much smaller temperature increases. Also, there is an upper temperature limit above which a reaction will slow or stop.

#### Pressure

Increasing pressure forces reactant particles closer together, increasing their interaction and the rate of reaction. As you might expect, pressure affects gases significantly more than liquids or solids.

#### Concentration

Increasing the concentration of liquid and gaseous reactants increases the number of collisions between particles and thus increases the reaction rate.

#### Use of a Catalyst

Catalysts or enzymes lower the activation energy of a chemical reaction. Since it’s easier for the reaction to occur, it’s faster.

Catalysts increase the frequency of collisions between reactants, alter molecular orientation, reduce intermolecular bonding within reactants, or donate electron density to reactants. The presence of a catalyst doesn’t change a chemical reaction, but it does help it reach equilibrium more quickly.

In contrast, some substances decrease the rate of a chemical reaction. These inhibitors may compete for a reactant, change reactant orientation, or alter electron density of chemical bond formation.

#### Particle Size – Surface Area

Smaller particle sizes and increased surface area maximize opportunities for reactants to collide. Crushing solids into powders increases surface area. For example, a chunk of magnesium metal oxidizes in air, but powdered magnesium oxidizes so quickly it can spontaneously ignite.

#### Physical State of Reactants

The physical state of the reactants (solid, liquid, gas) affects the rate of reaction. Liquid and gaseous reactant in the same phase tend to react quickly because thermal motion brings them together. The speed of a reaction is limited by the surface area of the interface when reactants are in different phases. Here, shaking and mixing can speed the rate of the reaction by bringing reactants together.

#### Absorption of Light

Light provides the activation energy needed for some reactions. For these reactions, increasing the amount of light increases the rate of reaction. Photosynthesis is a good example of a reaction affected by light.

#### Nature of the Reactants

The types of chemical bonds in the reactants influence how quickly reactions occur. For example, acid-base and ion-exchange reactions tend to be fast reactions. Reactions involving large molecules tend to be slower. Sometimes it’s possible to increase the rate of a reaction by choosing different compounds to yield the desired product. For example, in a substitution reaction, you’ll get a faster reaction using a soluble salt than an insoluble one because the soluble salt will dissolve into smaller particles.

### Limitations to Speeding the Rate of a Reaction

There is a limit regarding how much a factor can increase the rate of a chemical reaction. For example, increasing temperature speeds up a reaction, but above a certain temperature the reactants may denature. Adding a catalyst speeds a reaction, but adding more of it won’t cause a further rate increase.

### References

• Atkins P.; de Paula J. (2006). Physical Chemistry (8th ed.) W.H. Freeman. ISBN 0-7167-8759-8.
• Laidler, K. J. (1987). Chemical Kinetics (3rd ed.). Harper and Row. ISBN 0-06-043862-2.
• Steinfeld, J. I.; Francisco, J. S.; Hase, W. L. (1999). Chemical Kinetics and Dynamics (2nd ed.). Prentice-Hall. ISBN 0-13-737123-3.