Overall order of a reaction

When the overall order of a reaction is greater than three, the mechanism probably has one or more equilibria and intermediates prior to the rate-determining step. 

A certain element of confusion is to be met with both in textbooks, and in the literature, over the use and meaning of the terms order (cf. p. 39) and molecularity as applied to reactions. The order is an experimentally determined quantity, the overall order of a reaction being the sum of the powers of the concentration terms that appear in the rate equation ... 

The units of k, the rate constant, depend on the overall order of a reaction. For what overall reaction order does the rate constant have the same units as the reaction rate ... 

Therefore, for this reaction, the rate is first order with respect to hydrogen and first order with respect to iodide. The overall order of a reaction is found by summing the individual orders. Therefore, the overall order for the above reaction is 2. 

Each exponent is termed the order of reaction for the species concerned, while the sum of the exponents of the concentration terms defines the overall order of a reaction. 

It is important to recognize the difference between the order of a reaction with respect to a specific reactant and the overall order of a reaction. The /order of a reaction with respect to a particular reactant is the power to which the concentration of that reactant must be raised to have direct proportionality between concentration and reaction rate. According to Equation 8-2 the rate of the chloromethane-hydroxide ion reaction is first order with respect to chloromethane and first order with respect to hydroxide ion. In Equation 8-1 the rate is first order with respect to chloromethane and zero order with respect to hydroxide ion because [OH0]0 = 1. The overall order of reaction is the sum of the orders of the respective reactants. Thus Equations 8-1 and 8-2 express the rates of overall first-order and second-order reactions, respectively. 

The order is not a result of the nature of the chemical equation it is dependent on data collected from experiments. Those data are then used to determine a mathematical equation that fits, the rate equation. The overall order of a reaction is the sum of the orders with respect to various substances (the sum of the exponents). Further, the order of a reaction is stated with respect to a named substance in the reaction (see listing above). 

The overall order of a reaction does not have to be an integer, nor does the order have to be an integer with respect to any indivtdual component. As an example, consider the gas-phase synthesis of phosgene ... 

Since the overall order of a reaction is a guide to the molecularity of individual steps, a determination of the order is important in establishing... 

The (overall) order of a reaction cannot be deduced from measurements of a rate of appearance or rate of disappearance at a single value of the concentration of a species whose concentration is constant (or effectively constant) during the course of the reaction. If the overall rate of reaction is, for example, given by... 

Order of a reaction The sum of the powers to which all concentrations are raised in the rate-law expression also called overall order of a reaction. 

The overall order of a reaction is a quantity that is determined experimentally and, depending on the form of the rate equation, can apply equally well to reactions that are elementary or composite. 

The exponents in the rate equation are usually small whole numbers, such as 1 or 2. (There are also some cases in which the exponent 0 occurs.) The values of the exponents are related to the number of molecules involved in the detailed steps that constitute the mechanism. The overall order of a reaction is the sum of all the exponents. If, for example, the rate of a reaction A P is given by the rate equation... 

In general, the overall order of a reaction is equal to the sum of the exponents a and h in the rate equation Rate = k A [B] If in some other reaction, for example, we found that Rate = [A] [B], then we would say that the reaction is second order with respect to [A], first order with respect to [B], and third order overall. 

The overall order of a reaction equals the sum of the orders of the reactant species in the rate law. In this example, the overall order is 2 that is, the reaction is second order overall. 

We have seen that the units of k depends on the overall order of a reaction. Derive a general expression for the units of k for a reaction of any overall order, based on the order of the reaction (o) and the units of concentration (M) and time (s). 

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