KAR - Factors Affecting Reaction Rates (Lesson)

Factors Affecting Reaction Rates

Chemical changes can occur at different rates. These rates can be thought of as expressed in terms of how fast reactants disappear or how fast products appear as the reaction occurs. By convention, reaction rates are always expressed in terms of the reactants.

Reactions can be fast, like an explosion, or slow, like the rusting of an iron nail. Chemical kinetics is the study of reaction rates and the factors that determine those rates. A basic understanding of chemical kinetics can be gained by looking at certain factors that affect reaction rates.  These factors can be evaluated using what is referred to as collision theory which seeks to understand reaction rates on a particle level. Listed below are certain factors that affect the number of collisions that occur during a chemical reaction and therefore affect the reaction rate.

Factors Affecting Rates

Chemical Nature of the Reactants

In order for reactions to occur, the atoms, ions, or molecules must come into contact with each other. If the reactants are in gas phase or dissolved in a solution, they intermingle and collide easily. In solid form, there is less chance for particles to interact. Even then, the ability of an individual type of element to react contributes to the rate. Both sodium and potassium metals react with water. But potassium loses electrons more easily than sodium, so its reaction is explosively fast.

Reactions are classified by the states of the reactants into two categories: homogeneous reactions and heterogeneous reactions. An acid and base neutralization reaction, where each substance is in aqueous solution, is an example of a homogeneous reaction. Another example would be when hydrogen gas and oxygen gas combined to form water vapor. 

The formation of rust from solid iron and oxygen found either in liquid water or gaseous water vapor is an example of a heterogeneous reaction. In these reactions, the reactants are only able to interact at the interface between the phases. For this reason, the area of contact between the phases affects reaction rate. The smaller the particles, the faster the reaction is. Crushing a solid will increase surface area and produce more collisions per second.  

Particle Size (Surface Area)

Dust explosions are an example of how increased surface area affects reaction rate. The explosions occur when tiny dust particles of a combustible material such as grain, sugar, or coal are ignited. These reactions are much faster than when trying to react a large chunk of the same material because of the increase in surface area.  An increase in surface area leads to more collisions and a faster reaction rate.

Concentration

The concentration of reactants at the beginning of a reaction affects rate. Wood burns quickly in air with 20% oxygen, but in pure oxygen, wood burns explosively fast. But rate increases with increasing concentration may be explained by what is known as collision theory. The logic here is that a higher concentration of reactant molecules leads to more collisions and, as a result, a faster reaction rate. This is similar to what is described above with regard to surface area.

Presence of a Catalyst

Catalysts are unique substances that will increase the reaction rate of the specific reaction that they catalyze. Catalysts are not a generic compound that can be used to speed up any reaction but rather are specifically tailored for certain reactions. While the factors above increase the number of collisions that occur, and thereby the reaction rate, catalyzes increase reactions rates for a different reason. These reasons will be explored in future lessons.

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