Chemical kinetics is a branch of physical chemistry that is concerned with the investigation the rate of chemical reaction and how different experimental conditions can influence the reaction mechanism in order to facilitate the disappearance of reactants and conversion to products. At the macroscopic level of study, interest is made in the amount of reactants involved in the reaction, and the rate of formation of products while in molecular level, considerations is made is made in the reaction mechanism. For the product to be formed there must be breaking and forming of bonds as a result of collision within the molecules. The following are some principles that initiate chemical reaction in a system:
- The molecules must possess energy enough to proceed the reaction.
- The arrangement of the molecules in the space will be considered.
This is called reaction kinetics and with this study. Mathematical models can be developed in order describe the nature and behavior of the chemical reaction. The applicable principles if chemical kinetics is found in physical processes, it also relates to the rearrangement of atoms and formation of intermediates. One of the significance of chemical kinetics is that it provides mechanisms in which chemical reaction occurred.
In a chemical reaction, the breaking and rearrangement of reactant molecules and atoms (chemical bonds) are involved, these rearrangements give rise to product formation and new chemical position of atoms and molecules in the formed product. Therefore the reaction mechanism deals with the movement of atoms and electrons and the direction in which the reaction occurred is known as reaction path.
Chemical kinetics study has shown that some reactions follow a single path known as elementary reactions while others proceed in more than one path known as non elementary reaction.
The development of chemical kinetics was pioneered by peter waage and cato Guldberg in 1864. They developed the work through the law of mass action which state that the speed of a chemical reaction is proportional to the quantity of the reactants.
In this study did van’t Hoff in 1884 studied and published a work on chemical dynamics, he was awarded in 1901 for his discovery of the law of chemical dynamics. Hoff’s history. Also, his work presented experimental determination of rate law and constant, in the first order reaction for which the reaction rate are independent of concentration, first and second order reaction can be derived.
In consecutive reaction, the rate of reaction of each step in the mechanism of a reaction determines the kinetics of a reaction; first order reaction is a steady state approximation can simplify the rate law. There are many factors that influence the rate of chemical rate which include: temperature which is determined by Arrhenius equation, concentration, nature of reactant and catalyst involved in the reaction.
Chemical reaction rate is the rate of change of concentration of a substance with time. The factors that influence reaction rate are thermodynamic properties and kinetics. Thermodynamic properties in this context relates to the gain and loss of energy in chemical reaction.
Factors affecting reaction rate:
- Physical state of the reactant: the physical state of reactant affects the rate of reaction enormously,. Reactants which are separate by an interface, having the same phase are brought to contact by a thermal energy existing with the reaction vessel. Some reaction may be difficult to come into completion until the reaction content is stirred vigorously to have a homogeneous contact between the reactants. Example is a finely divided solid reactant involved in a reaction which has a greater surface per unit volume and this fastens the rate of reaction.
- Concentration of reactants: the concentration of reactants is a major determinant in procession of a reaction. This increases the frequency of the molecules and atoms which leads to a higher collision of the molecules or ions. The rate law may be differential or integrated rate law. Also the effective collision of the particles depends on their kinetic energy. The higher the kinetic energy the more the number of collision, the higher the concentration and the higher the rate of reaction. Notice that the concentration is proportional to the reaction rate.
- : the nature of a reactant is of importance when considering how fast a reaction can proceed; a reactant with increase in surface area reacts more than the reactants with low surface area. The increase surface area exposes the reactant and more contact is made between the aqueous solution and the reacting substances. Grounded substances have higher reaction rate than lump substances. Therefore the surface area of a reactant increases the reaction rate.
- this is more pronounced in the gaseous state reaction. Pressure increases the reaction rate, this is because the activity of a gas is proportional to the partial pressure of the gas. The kinetics of a reaction can be studied under high pressure approach.
- : the effect of temperature in a reaction is important, some reactions are endothermic while others are exothermic reactions. The temperature effect is concerned in the aspect of activation energy. The increase or decrease of reaction is dependent on the type of reaction involved ( endothermic or exothermic reaction)
- Presence of a catalyst: A catalyst is a substance that fastens or reduces the rate of chemical reaction by altering or lowering the activation energy of the reaction and at the end not consumed in the reaction. In biochemical reaction, protein acts as a catalyst called enzymes. The catalyst does not affect the rate constant of a reaction. the profile below shows a reaction involving a catalyst:
The order of a reaction has the following stages: initiation, propagation and termination stage.
Here we will be concerned about the formula of the order of the reaction and the procedures follows:
First order reaction: this has the relation as
When you make a plot of (In[A]) ( natural logarithm) against time a straight line graph is obtained with slope being negative (k). A and A0 are concentration at a particular time (t) and the initial concentration.
Second order reaction: This relation is when reactant concentrations are equal.
This gives a positive straight line with intercept as 1/[A0] when 1/[A] is plotted against time.
Third order equation:
Plot 1/2[A2] against time to obtain a straight line that has an intercept of 1/2[A02] and a slope of K. From the intercept the initial concentration can be determined.
Integrated Rate Law Summary
References and further reading:
Espenson H. J. chemical kinetics and reaction mechanism, 2nd edition. McGraw 2002. Pp5-8
Laidler K.J. chemical kinetics (3rd edition, Harper and Row 1987)
Fogler .S.H. Elements of chemical reaction engineering. 5th edition. Englewood cliffs, NJ:
Hill .C.G jr. an introduction to chemical engineering kinetics and reactor design. New York: john Wiley & Sons, inc 1977.