Factors affecting reaction rates

From Chemistry Resource
Jump to: navigation, search

Under any given set of conditions a reaction will proceed at a given rate. By changing the reaction conditions the rate may change.

Factors which affect the rate of reaction are:

  • Chemical nature of reactants
Even though fluorine and neon are right next to each other on the periodic table then have very different chemical reactivities. F2 is a very strong oxidizer and is more chemical reactive than any other element. Whereas Ne forms no know neutral compounds.
Consider the combustion of compounds with O2 in air. Some substances do not burn, e.g. H2O, CO2 and N2. Other substances such as H2, CH4 and gasoline will not burn until ignited with a spark. While others such as SiH4, P4, and AsH3 and spontaneously burn. The rate of combustion of these substances varies from 0 to some very high number. It is the chemical nature of these compounds that causes this variation in combustion rate.
Chemical properties that affect the rate of reaction include the strength and number of bonds that must be broken and formed. Generally the weaker and fewer bonds that need to be broken the faster the reaction will be.
  • The ability of the reactants to come in contact with each other
Wooden logs do not easily ignite but when the wood is split into small pieces it is much easier to light to start. When sawdust is dispersed in the air is can burn with explosive violence. These differences in reaction rates are due to the increased surface area as the particle size gets smaller. As the surface area increases, there are more molecules available to take part in the reaction. In general, when the surface area of a substance increases the substance reacts at a faster rate.
Another factor is the phase of the reactants. Fast reactions are favoured by situations when the reactants are free to move around and are near each other. Reactions between solids tend to be slow because the molecules are not free to move around. In gases, the molecules are free to move about but then are relatively far apart so reactions between gases tend to react at intermediate rates. Reactions between two liquids tend to also have an intermediate rate because the molecules are close together. Reactions between ions in solutions tend to be fast because of they are close to each other and they are free to move around. In general the rate of reaction goes as aqueous ions > gases or liquids > solids.
A homogenous reaction is a reaction in which the reactants are in the same phase, e.g. the reaction between hydrogen gas and oxygen gas to give water. A heterogeneous reaction is a reaction in which the reactants are different phases, e.g. the reaction between metallic iron and oxygen to give rust.
  • Temperature
Higher temperatures generally increase the rate of reaction. For example food is put into a refrigerator to slow down the decay of food. Heating the reactants causes the molecules to faster. This increases the number of collisions between reactants. But more importantly the reactant will collide with higher energy. So a higher percentage of collisions will have enough energy to overcome any barrier to the formation of products.
  • Concentration of the reactants
For a reaction to occur the reactant must collide together. The greater the number of reactant molecules in a given volume, the rate of collisions between them. When the collision frequency increase then the reaction rate increases
  • Pressure
For a gas the pressure is proportional to the gas’s concentration. As the pressure is increased the collision frequency between the molecules also increases. Therefore the reaction rate increases with increasing pressure.
  • Presence of catalysts
A catalyst is a chemical which can be added to a chemical reaction to increase the rate of the reaction. The catalyst is not consumed in the reaction. An example would the rusting of iron. Iron that is kept dry rust much slower than iron that is exposed to moisture. It this case water acts as a catalyst. The catalyst usually changes the reaction mechanism to one with lower activation energy.