Biological systems are characterized by change over time. Metabolic reactions convert sugars into energy over time. Cells divide and profilerate over time. Food is digested by your intestines and broken down into nutrients over time. The speed at which these processes happen depends on many variables including temperature, pressure, and substances in the environemnt. In order to describe these processes, we need a language to describe the various biochemical changes that occur. Chemical kinetics is one such language.
Chemical reactions can be divided in to two types: homogeneous and heterogeneous. Homogeneous reactions occur in a single phase, ususally gas or liquid. Heterogeneous reactions occur in a system with two phases and the reactions usually occur at the interface between them.
Chemical reactions can also be divided into reversible and irreversible. Reversible reactions can proceed either from reactants to products or from products to reactants until the system reaches a balance or equilibrium. Irreversible reactions proceed from reactants to products until one of the reactants is completely consumed. Theoretically, all reactions are reversible, but for some reactions the equilibrium is so far in favor of the products that we can treat it as if they were truly irreversible.
Consider the following general reaction.
aA + bB ⇄ cC + dD
Here the letters a, b, c, and d are called stoichiometric coefficients and they represent the number of molecules A, B, C, and D that participate in this reaction.
Here, the name of the molecule surrounded by square brackets indicates the concentration of that molecule. Note that the reactants have negative signs associated with them and the products have positive signs associated with them. This is because reactants are being consumed and products are being produced. r has units of concentration per time.
Intuitively, we would expect that the reaction rate will depend on the concentrations of the reactants since higher concentrations lead to higher numbers of encounters between reactants. We might also expect that the reaction rate will depend on temperature since molecules have higher energy and move faster at higher speeds. This intuition can written mathematically as a rate equation. Commonly, but not always, the temperature dependence and the concentration dependence are separated in a rate equation.
$$ r=k(T)f([A],[B]) $$Here k is called a rate constant and is dependent on temperature, T, and f() is a function than in general must be determined experimentally. One of the most common functions used for f() is the power law.
$$ r=k(T)[A]^α[B]^β $$ Here, α is the order of the reaction with respect to A, and β is the order of the reaction with respect to B. The overall order of the reaction is given as n=α+β.In general α and β can positive, negative, integer, or non-integer. In general, these reaction orders must be determined experimentally.A reaction may be an elementary reaction, in which case the reaction proceeds by the direct formation of the products from the interaction of the reactants or it may be a complex reaction where there are intermediate steps and intermediate species not represented in the stoichiometric equation. Elementary reactions are characterized by their molecularty, which is the number of molecules that participate in the reaction. A unimolecular reaction involves just one molecule, a bimolecular reaction involves two molecules, and a termolecular reaction involves three reactions. For elementary reactions the reaction orders are equal to the stoichiometric coefficients.
Updated on 230105