Order of a chemical reaction

The overall order of a chemical reaction is equal to the sum of the exponents of all the concentration terms in the differential rate expression for a reaction considered in one direction only. For example, if the empirical rate law for a particular chemical reaction was... 

We can now introduce a general treatment of the concept of the order of a chemical reaction. A chemical reaction is said to he of the mh order if its rate is directly proportional to the product of n concentrations. Therefore die decomposition of A. if described by the equation... 

Under experimental conditions, it maybe found that the rate of formation of product is proportional to the first, second, or (rarely) third power of the concentration of the reagent then the reaction is classified as first-, second-, or third-order with respect to that reagent. Thus, the order of a chemical reaction is strictly an empirical finding. 

For each set of constant input and output concentration constraints a system of linear chemical reactions has a unique steady state. For a network of nonlinear biochemical reactions, however, there could be several steady states compatible with a given set of constraints. The number and character of these steady states are determined by the structure of the network including the extent of nonlinearity, the number and connectivity of the individual chemical reactions and the values of the reaction rate constants and the concentrations of the reactants. The higher the order of a chemical reaction, the more steady states may be compatible with a given set of chemical constraints. The simple trimolecular reaction system of Schlogl [13] illustrates how a third-order chemical reaction can have two stable steady states compatible with a single set of chemical constraints ... 

The graphing calculator can run a program that can tell you the order of a chemical reaction, provided you indicate the reactant concentrations and reaction rates for two experiments involving the same reaction. 

The order of a chemical reaction is the mathematical relationship between the concentration of the reactants and the time course of the reaction. Most chemical reactions fall into three categories zero, first, or second order. 

Other reaction orders The overall reaction order of a chemical reaction is the sum of the orders for the individual reactants in the rate law. Many chemical reactions, particularly those having more than one reactant, are not first order. Consider the general form for a chemical reaction with two reactants. In this chemical equation, a and b are coefficients. 

To further complicate matters, the order of a chemical reaction cannot be predicted from the chemical equation, even if it has been balanced. The order of a reaction is determined experimentally from accurate measurements of the rate under different conditions. It is possible for reactions to be third order, zero order (often found in solid-state reactions such as the release of drug from pharmaceutical suspensions) or even of a fractional order. 

The order of a chemical reaction cannot be determined by inspection, it must be determined experimentally. In practice, this means measuring the decomposition of the compound under controlled conditions and applying each of the rate equations in turn to see which type of equation fits the data and gives the best straight line. This is what scientists term an empirical method, and what the man in the street calls trial and error ... 

In one method of fixing the order of a chemical reaction it is necessary to find the time which is required for the transformation of equal fractional parts of a given substance in two separate systems. Let x denote the concentration of the- reacting sub-Itance at the time t a, the initial concentration of the reacting substance and suppose that the following numbers were obtained —... 

Apply the method of initial rates to determine the order of a chemical reaction with respect to Reactant X. Create a set of hypothetical experimental data that would lead you to conclude that the reaction is second order in X. 

It is often mistakenly assumed that the order of a chemical reaction can be predicted directly fiom its stoichiometry. However, it must be understood that the exponents of concentration in a differential rate law expression are not necessarily equal to the corresponding molar coefficient molar coefficients in the balanced equation. The order of a reaction can be determined by experunentadon there is no precise way of predicting it dhectly fiom the balanced chemical equation. 

Letort M. Definition and Determination of the Two Orders of a Chemical Reaction. Jourrud de Chimie Physique 1937 34 206 quoted from Benson S. The foundations of chemical kinetics. New York, NY McGraw-Hill 1960. p. 379, 380, 381, 383. 

Although the order of a chemical reaction cannot be predicted from the overall reaction, the order of an elementary process is predictable. For example, for the general unimolecular process... 

Show that if the overall order of a chemical reaction is zero, the concentration [A]t of a reactant at time t is given by... 

This method can be used to determine the order of a chemical reaction (Chapter 16). 

Knowledge of the value of the rate of the reaction at different reactant concentrations would allow for determination of the rate and order of a chemical reaction. For the reaction A B, for example, reactant or product concentration-time curves are determined at different initial reactant concentrations. The absolute value of slope of the curve at t = 0, d[A]/dt)o or rf[B]/rft)o, corresponds to the initial rate or initial velocity of the reaction (Fig. 1.8). 

We can, however, use kinetic data to make inferences about the order of a chemical reaction. Consider the following process in which tert- mty bromide reacts with water (in a solvent such as tetrahydrofuran) to give t rr-butyl alcohol ... 

In chemical reaction kinetics, one of the methods for determination of the order of a chemical reaction is the method of initial rates. In this method, the reaction starts with different initial concentrations of the reactant A, and changes in the eoncentration of A with time are measured. For each initial concentration, the initial reaction rate can be calculated from differentiation of concentration with respect to time at the beginning of the reaction ... 

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