Chemical kinetics: Encyclopedia II - Chemical kinetics - Factors that influence the rate of a reaction

Chemical kinetics - Factors that influence the rate of a reaction

We can control four factors that have an effect on the rate of a certain reaction, these factors are: the concentration of reactants, the physical state of reactants, the temperature, and the use of a catalyst.

Chemical kinetics - Concentration

Concentration plays a very important role in reactions. This is due to the fact that molecules must collide in order to react together. for example, consider the reaction where hypothetical components "A" and "B" react together in order to give a certain product "C": A_(g) + B_(g) → C_(g). Now imagine what the reaction would look like at the molecular level if this reaction was done in closed 50ml bottle. The atoms of A and B would be flying in all the directions, crashing into each other and into the bottle's walls. This reaction cannot occur if the molecules do not collide. And also the more molecules present in the bottle, the more they have chances to collide, therefore if we change the concentration of these molecules (by changing the volume of the bottle, or by adding or removing reactants) we also change the speed (or rate) of the reaction. Thus, reaction rate is proportional to the concentration of reactants.

Chemical kinetics - Physical state

The physical state (solid, liquid or gas) of a reactant is also an important factor of the rate of change. When reactants are in the same phase, as in aquaeous solution, thermal motion brings them into contact. However, when they are in different phases, for example one is liquid and the other is gas, contact can only occur at the interface, in other words at the surface of the liquid. Therefore, vigorous shaking and stirring may be needed. So basically, this means that the more finely divided a solid or liquid reactant, the greater its surface area per unit volume, and the more contact it makes with the other reactant, thus the faster the reaction. To make an analogy, for example when you start a fire, first you put wood chips and small branches, you dont start with big logs right away.

Chemical kinetics - Temperature

Temperature usually has a major effect on the speed of a reaction. Since a molecule has more energy when it is heated, then the more energy it has, the more chances it has to collide with other reactants. Thus, at a higher temperature, more collisions occur. More importantly however, is the fact that heating a molecules affects its kinetic energy, and therefore the "energy" of the collision. Take the example of two tomatoes traveling towards each other, if they are going considerably slow, they might just hit and recoil, however, if they are comming at each other very fast (with a lot of energy) they will certainly explode and stick to each other. So to restate, raising the temperature increases the reaction rate by increasing the number, and especially, the energy of the collisions.

A good example of this is something we use everyday. A refrigerator slows down the speed of the reaction since it cools the molecules, on the other hand, an oven gives heat (energy) to the molecules which in turn speeds up the rate of the reaction, cooking the food faster.

Chemical kinetics - Catalysts

A catalyst is a substance that accelerates the rate of a chemical reaction. In autocatalysis a reaction product is itself a catalyst for that reaction possibly leading to a chain reaction. In biochemistry enzymes accelerate reactions. Michaelis-Menten kinetics describe the rate of enzyme mediated reactions.

Adapted from the Wikipedia article "Factors that influence the rate of a reaction", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki