Preferred Label : order of reaction;
Detailed label : order of reaction, n;
IUPAC definition : If the macroscopic (observed, empirical or phenomenological) rate of reaction ( em
v /em ) for any reaction can be expressed by an empirical differential rate equation
(or rate law) which contains a factor of the form k[A] α.[B] β ... (expressing in
full the dependence of the rate of reaction on the concentrations A, B ...) where
α, β are constant exponents (independent of concentration and time) and k is independent
of A and B etc. (rate constant, rate coefficient), then the reaction is said to be
of order α with respect to A, of order β with respect to B, ... , and of (total or
overall) order n α β ... The exponents α, β, ... can be positive or negative
integral or rational nonintegral numbers. They are the reaction orders with respect
to A, B, ... and are sometimes called 'partial orders of reaction'. Orders of reaction
deduced from the dependence of initial rates of reaction on concentration are called
'orders of reaction with respect to concentration'; orders of reaction deduced from
the dependence of the rate of reaction on time of reaction are called 'orders of reaction
with respect to time'. The concept of order of reaction is also applicable to chemical
rate processes occurring in systems for which concentration changes (and hence the
rate of reaction) are not themselves measurable, provided it is possible to measure
a chemical flux. For example, if there is a dynamic equilibrium according to the equation:
\[a{A}\rightleftharpoons p{P}\] and if a chemical flux is experimentally found, (e.g.
by NMR line-shape analysis) to be related to concentrations by the equation: \[\frac{Φ
_{-{A}}}{\alpha } k\ \left[{A}\right] {\alpha }\ \left[{L}\right] {\lambda }\] then
the corresponding reaction is of order α with respect to A ... and of total (or overall)
order n ( α λ ... ). The proportionality factor k above is called the (nth order)
'rate coefficient'. Rate coefficients referring to (or believed to refer to) elementary
reactions are called 'rate constants' or, more appropriately 'microscopic' (hypothetical,
mechanistic) rate constants. 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: \[v k\ \left[{A}\right] {\alpha }\ \left[{B}\right] {\beta }\]
but [B] stays constant, then the order of the reaction (with respect to time), as
observed from the concentration change of A with time, will be α, and the rate of
disappearance of A can be expressed in the form: \[v_{{A}} k_{{obs}}\ \left[{A}\right]
{\alpha }\] The proportionality factor kobs deduced from such an experiment is called
the 'observed rate coefficient' and it is related to the (α β)th order rate coefficient
k by the equation: \[k_{{obs}} k\ \left[{B}\right] {\beta }\] For the common case
when α 1, kobs is often referred to as a 'pseudo-first order rate coefficient' (kψ).
For a simple (elementary) reactions a partial order of reaction is the same as the
stoichiometric number of the reactant concerned and must therefore be a positive integer
(see rate of reaction). The overall order is then the same as the molecularity. For
stepwise reactions there is no general connection between stoichiometric numbers and
partial orders. Such reactions may have more complex rate laws, so that an apparent
order of reaction may vary with the concentrations of the chemical species involved
and with the progress of the reaction: in such cases it is not useful to speak of
orders of reaction, although apparent orders of reaction may be deducible from initial
rates. In a stepwise reactions, orders of reaction may in principle always be assigned
to the elementary steps.;
Origin ID : O04322;
Automatic exact mappings (from CISMeF team)
- See also
If the macroscopic (observed, empirical or phenomenological) rate of reaction ( em
v /em ) for any reaction can be expressed by an empirical differential rate equation
(or rate law) which contains a factor of the form k[A] α.[B] β ... (expressing in
full the dependence of the rate of reaction on the concentrations A, B ...) where
α, β are constant exponents (independent of concentration and time) and k is independent
of A and B etc. (rate constant, rate coefficient), then the reaction is said to be
of order α with respect to A, of order β with respect to B, ... , and of (total or
overall) order n α β ... The exponents α, β, ... can be positive or negative
integral or rational nonintegral numbers. They are the reaction orders with respect
to A, B, ... and are sometimes called 'partial orders of reaction'. Orders of reaction
deduced from the dependence of initial rates of reaction on concentration are called
'orders of reaction with respect to concentration'; orders of reaction deduced from
the dependence of the rate of reaction on time of reaction are called 'orders of reaction
with respect to time'. The concept of order of reaction is also applicable to chemical
rate processes occurring in systems for which concentration changes (and hence the
rate of reaction) are not themselves measurable, provided it is possible to measure
a chemical flux. For example, if there is a dynamic equilibrium according to the equation:
\[a{A}\rightleftharpoons p{P}\] and if a chemical flux is experimentally found, (e.g.
by NMR line-shape analysis) to be related to concentrations by the equation: \[\frac{Φ
_{-{A}}}{\alpha } k\ \left[{A}\right] {\alpha }\ \left[{L}\right] {\lambda }\] then
the corresponding reaction is of order α with respect to A ... and of total (or overall)
order n ( α λ ... ). The proportionality factor k above is called the (nth order)
'rate coefficient'. Rate coefficients referring to (or believed to refer to) elementary
reactions are called 'rate constants' or, more appropriately 'microscopic' (hypothetical,
mechanistic) rate constants. 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: \[v k\ \left[{A}\right] {\alpha }\ \left[{B}\right] {\beta }\]
but [B] stays constant, then the order of the reaction (with respect to time), as
observed from the concentration change of A with time, will be α, and the rate of
disappearance of A can be expressed in the form: \[v_{{A}} k_{{obs}}\ \left[{A}\right]
{\alpha }\] The proportionality factor kobs deduced from such an experiment is called
the 'observed rate coefficient' and it is related to the (α β)th order rate coefficient
k by the equation: \[k_{{obs}} k\ \left[{B}\right] {\beta }\] For the common case
when α 1, kobs is often referred to as a 'pseudo-first order rate coefficient' (kψ).
For a simple (elementary) reactions a partial order of reaction is the same as the
stoichiometric number of the reactant concerned and must therefore be a positive integer
(see rate of reaction). The overall order is then the same as the molecularity. For
stepwise reactions there is no general connection between stoichiometric numbers and
partial orders. Such reactions may have more complex rate laws, so that an apparent
order of reaction may vary with the concentrations of the chemical species involved
and with the progress of the reaction: in such cases it is not useful to speak of
orders of reaction, although apparent orders of reaction may be deducible from initial
rates. In a stepwise reactions, orders of reaction may in principle always be assigned
to the elementary steps.
