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Encyclopedia > Reaction rate
Iron rusting - a chemical reaction with a slow reaction rate.
Iron rusting - a chemical reaction with a slow reaction rate.
Wood burning - a chemical reaction with a fast reaction rate.
Wood burning - a chemical reaction with a fast reaction rate.

The reaction rate or rate of reaction for a reactant or product in a particular reaction is intuitively defined as how fast a reaction takes place. For example the oxidation of iron under the atmosphere is a slow reaction which can take years, the combustion of butane in a fire is a reaction that takes place in fractions of a second. Image File history File linksMetadata Download high resolution version (3456x2304, 2455 KB) Summary Photographed by and copyright of (c) David Corby (User:Miskatonic, uploader) 2006 Licensing File links The following pages link to this file: Rust Redox Iron oxide User:Miskatonic Metadata This file contains additional information, probably added from... Image File history File linksMetadata Download high resolution version (3456x2304, 2455 KB) Summary Photographed by and copyright of (c) David Corby (User:Miskatonic, uploader) 2006 Licensing File links The following pages link to this file: Rust Redox Iron oxide User:Miskatonic Metadata This file contains additional information, probably added from... Vapours of hydrogen chloride in a beaker and ammonia in a test tube meet to form a cloud of a new substance, ammonium chloride A chemical reaction is a process that results in the interconversion of chemical substances. ... Large Bonfire File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Large Bonfire File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Vapours of hydrogen chloride in a beaker and ammonia in a test tube meet to form a cloud of a new substance, ammonium chloride A chemical reaction is a process that results in the interconversion of chemical substances. ... A reactant or reagent is any substance initially present in a chemical reaction. ... Vapours of hydrogen chloride in a beaker and ammonia in a test tube meet to form a cloud of a new substance, ammonium chloride A chemical reaction is a process that results in the interconversion of chemical substances. ... The most fundamental reactions in chemistry are the redox processes. ...


Chemical kinetics is the part of physical chemistry that studies reaction rates. The concepts of chemical kinetics are applied in many disciplines, such as chemical engineering, enzymology or environmental engineering. In physical chemistry, chemical kinetics or reaction kinetics study reaction rates in a chemical reaction. ... Physical chemistry is the application of physics to macroscopic, microscopic, atomic and particulate phenomena in chemical systems[1]within the field of chemistry traditionally using the principles, practices and concepts of thermodynamics, quantum chemistry, statistical mechanics and kinetics. ... Chemical engineering is the branch of engineering that deals with the application of physical science (e. ... Ribbon diagram of the catalytically perfect enzyme TIM. An enzyme is a protein that catalyzes, or speeds up, a chemical reaction. ... Environmental engineering[1][2] is the application of science and engineering principles to improve the environment (air, water, and/or land resources), to provide healthy water, air, and land for human habitation and for other organisms, and to remediate polluted sites. ...

Contents

Formal definition of reaction rate

According to IUPAC's Gold Book definition[1] the reaction rate v (also r or R) for the general chemical reaction aA + bB → pP + qQ, occurring in a closed system under constant-volume conditions, without a build-up of reaction intermediates, is defined as:ccv The International Union of Pure and Applied Chemistry (IUPAC) is an international non-governmental organization devoted to the advancement of chemistry. ... The Gold Book or Compendium of Chemical Terminology (ISBN 0865426848) is a book published by IUPAC containing internationally accepted definitions for terms in chemistry. ... In thermodynamics, a closed system, as contrasted with an isolated system, can exchange heat and work, but not matter, with its surroundings. ... The IUPAC Gold Book[1] defines a reaction intermediate or an intermediate a molecular entity (atom, ion, molecule. ...

v = - frac{1}{a} frac{d[A]}{dt} = - frac{1}{b} frac{d[B]}{dt} = frac{1}{p} frac{d[P]}{dt} = frac{1}{q} frac{d[Q]}{dt}

The IUPAC[1] recommends that the unit of time should always be the second. In such a case the rate of reaction differs from the rate of increase of concentration of a product P by a constant factor (the reciprocal of its stoichiometric number) and for a reactant A by minus the reciprocal of the stoichiometric number. Reaction rate usually has the units of mol dm-3 s-1. It is important to bear in mind that the previous definition is only valid for a single reaction, in a closed system of constant volume. This most usually implicit assumption must be stated explicitly, otherwise the definition is incorrect: If water is added to a pot containing salty water, the concentration of salt decreases, although there is no chemical reaction. In thermodynamics, a closed system, as contrasted with an isolated system, can exchange heat and work, but not matter, with its surroundings. ...


For any system in general the full mass balance must be taken into account: IN - OUT + GENERATION = ACCUMULATION A mass balance (also called a material balance) is an accounting of material entering and leaving a system. ...

F_{A0} - F_A + int_{0}^{V} r, dV = frac{dN_A}{dt}

When applied to the simple case stated previously this equation reduces to: v= frac{d[A]}{dt}


For a single reaction in a closed system of varying volume the so called rate of conversion can be is used, in order to avoid handling concentrations. It is defined as the derivative of the extent of reaction with respect to time. In a chemical reaction system the stoichiometric coefficient of the i–th component is defined as or where Ni is the number of molecules of i, and ξ is the progress variable or extent of reaction (Prigogine & Defay, p. ...


dot{&# 0; =frac{d&# 0;{dt} = frac{1}{nu_i} frac{dn_i}{dt} = frac{1}{nu_i} left(Vfrac{dC_i}{dt} + C_i frac{dV}{dt} right)


scriptstyle nu_i is the stoichiometric coefficient for substance i, scriptstyle V is the volume of reaction and scriptstyle C_i is the concentration of substance i.


When side products or reaction intermediates are formed, the IUPAC[1] recommends the use of the terms rate of appearance and rate of disappearance for products and reactants, respectively.


Reaction rates may also be defined on a basis that is not the volume of the reactor. When a catalyst is used the reaction rate may be stated on a catalyst weight (mol g-1 s-1) or surface area (mol m-2 s-1) basis. If the basis is a specific catalyst site that may be rigorously counted by a specified method, the rate is given in units of s-1 and is called a turnover frequency. It has been suggested that this article or section be merged into Catalysis. ...


Factors influencing rate of reaction

There are several factors that affect the rate of reaction:

  • Concentration: Reaction rate increases with concentration, as described by the rate law and explained by collision theory. As reactant concentration increases, the frequency of collision increases.
  • The nature of the reaction: Some reactions are naturally faster than others. The number of reacting species, their physical state (the particles that form solids move much more slowly than those of gases or those in solution), the complexity of the reaction and other factors can influence greatly the rate of a reaction.
  • Temperature: Usually conducting a reaction at a higher temperature delivers more energy into the system and increases the reaction rate by causing more collisions between particles, as explained by collision theory. However, the main reason why it increases the rate of reaction is that more of the colliding particles will have the necessary activation energy resulting in more successful collisions (when bonds are formed between reactants). The influence of temperature is described by the Arrhenius equation. As a rule of thumb, reaction rates for many reactions double or triple for every 10 degrees Celsius increase in temperature,[2] though the effect of temperature may be very much larger or smaller than this (to the extent that reaction rates can be independent of temperature or decrease with increasing temperature!)

For example, coal burns in a fireplace in the presence of oxygen but it doesn't when it is stored at room temperature. The reaction is spontaneous at low and high temperatures but at room temperature its rate is so slow that it is negligible. The increase in temperature, as created by a match, allows the reaction to start and then it heats itself, because it is exothermic. That is valid for many other fuels, such as methane, butane, hydrogen... In chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. ... A rate law is an equation that relates concentrations of reactants to the reaction rate. ... Collision theory is a theory, proposed by Max Trautz and William Lewis in 1916 that qualitatively explains how chemical reactions occur and why reaction rates differ for different reactions. ... FreQuency is a music video game developed by Harmonix and published by SCEI. It was released in November 2001. ... For other uses, see Collision (disambiguation). ... In the physical sciences, a phase is a set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties (i. ... Making a saline water solution by dissolving table salt (NaCl) in water This article is about chemical solutions. ... This article includes a list of works cited or a list of external links, but its sources remain unclear because it lacks in-text citations. ... The sparks generated by striking steel against a flint provide the activation energy to initiate combustion in this Bunsen burner. ... The Arrhenius equation is a simple, but remarkably accurate, formula for the temperature dependence of a chemical reaction rate, more correctly, of a rate coefficient, as this coefficient includes all magnitudes that affect reaction rate except for concentration. ... A rule of thumb is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination. ... Celsius is, or relates to, the Celsius temperature scale (previously known as the centigrade scale). ... Room temperature describes a certain temperature within enclosed space that is uses for various purposes by human beings. ... In thermodynamics, the word exothermic describes a process or reaction that releases energy in the form of heat. ... Methane is a chemical compound with the molecular formula CH4. ... Butane, also called n-butane, is the unbranched alkane with four carbon atoms, CH3CH2CH2CH3. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ...

  • Solvent: Many reactions take place in solution and the properties of the solvent affect the reaction rate. The ionic strength as well has an effect on reaction rate.
  • Pressure: The rate of gaseous reactions increases with pressure, which is, in fact, equivalent to an increase in concentration of the gas.
  • Electromagnetic Radiation: Electromagnetic radiation is a form of energy so it may speed up the rate or even make a reaction spontaneous, as it provides the particles of the reactants with more energy. This energy is in one way or another stored in the reacting particles (it may break bonds, promote molecules to electronically or vibrationally excited states...) creating intermediate species that react easily.

For example when methane reacts with chlorine in the dark, the reaction rate is very slow. It can be sped up when the mixture is put under diffused light. In bright sunlight, the reaction is explosive. A solvent is a liquid that dissolves a solid, liquid, or gaseous solute, resulting in a solution. ... The ionic strength of a solution is a function of the concentration of all ions present in a solution. ... The use of water pressure - the Captain Cook Memorial Jet in Lake Burley Griffin in Canberra, Australia. ... Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. ... Methane is a chemical compound with the molecular formula CH4. ... General Name, Symbol, Number chlorine, Cl, 17 Chemical series halogens Group, Period, Block 17, 3, p Appearance yellowish green Standard atomic weight 35. ... His name is Kushal Bhardwaj who has recently dicovered compounds, elements and mixtures. ...

  • A catalyst: The presence of a catalyst increases the reaction rate (in both the forward and reverse reactions) by providing an alternative pathway with a lower activation energy.

For example, platinum catalyzes the combustion of hydrogen with oxygen at room temperature. It has been suggested that this article or section be merged into Catalysis. ... The sparks generated by striking steel against a flint provide the activation energy to initiate combustion in this Bunsen burner. ... General Name, Symbol, Number platinum, Pt, 78 Chemical series transition metals Group, Period, Block 10, 6, d Appearance grayish white Standard atomic weight 195. ...

  • Isotopes: The kinetic isotope effect consists in a different reaction rate for the same molecule if it has different isotopes, usually hydrogen isotopes, because of the mass difference between hydrogen and deuterium.
  • Surface Area: In reactions on surfaces, which take place for example during heterogeneous catalysis, the rate of reaction increases as the surface area does. That is due to the fact that more particles of the solid are exposed and can be hit by reactant molecules.
  • Order: The order of the reaction controls how the reactant concentration affects reaction rate.

All the factors that affect a reaction rate are taken into account in the rate equation of the reaction. Isotopes are atoms of a chemical element whose nuclei have the same atomic number, Z, but different atomic weights, A. The word isotope, meaning at the same place, comes from the fact that isotopes are located at the same place on the periodic table. ... The kinetic isotope effect (KIE) is a variation in the reaction rate of a chemical reaction when an atom in one of the reactants is replaced by one of its isotopes. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... By reactions on surfaces it is understood reactions in which at least one of the steps of the reaction mechanism is the adsorption of one or more reactants. ... Heterogeneous catalysis is a chemistry term which describes catalysis where the catalyst is in a different phase (ie. ... Order in the context of a chemical reaction is a concept of reaction kinetics, a subdiscipline of physical chemistry. ...


Rate Equation

Main article: rate equation

For a chemical reaction n A + m B → C + D, the rate equation or rate law is a mathematical expression used in chemical kinetics to link the rate of a reaction to the concentration of each reactant. It is of the kind: For a chemical reaction, the rate law or rate equation is an equation which links the reaction rate with concentrations or pressures of reactants. ... Vapours of hydrogen chloride in a beaker and ammonia in a test tube meet to form a cloud of a new substance, ammonium chloride A chemical reaction is a process that results in the interconversion of chemical substances. ... A mathematical expression is a string of symbols which describes (or expresses) a (potential or actual) computation using operators and operands. ... In chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. ...

,r = k(T)[A]^{n'}[B]^{m'}

In this equation k(T) is the reaction rate coefficient or rate constant, although it is not really a constant, because it includes all the parameters that affect reaction rate, except for concentration, which is explicitly taken into account. Of all the parameters described before, temperature is normally the most important one.


The exponents n' and m' are called reaction orders and depend on the reaction mechanism. They sometimes are the same as the stoichiometric coefficients of A and B, but not necessarily. Order in the context of a chemical reaction is a concept of reaction kinetics, a subdiscipline of physical chemistry. ... In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs. ... In a chemical reaction system the stoichiometric coefficient of the i-th component is defined as or where Ni is the number of molecules of i and ξ is the progress variable or extent of reaction. ...


Stoichiometry, molecularity (the actual number of molecules colliding) and reaction order only coincide necessarily in elementary reactions, that is, those reactions that take place in just one step. The reaction equation for elementary reactions coincides with the process taking place at the atomic level, i.e. n molecules of type A are colliding with m molecules of type B (n plus m is the molecularity). It has been suggested that Stoichiometric coefficient and Gas stoichiometry be merged into this article or section. ... Molecularity in chemistry is the number of colliding molecules that are involved in a single reaction step. ... The Order of reaction [1] with respect to a certain reactant is defined, in chemical kinetics, as the power to which its concentration term in the rate equation is raised. ...


For gases the rate law can also be expressed in pressure units using e.g. the ideal gas law. Isotherms of an ideal gas The ideal gas law is the equation of state of a hypothetical ideal gas, first stated by Benoît Paul Émile Clapeyron in 1834. ...


By combining the rate law with a mass balance for the system in which the reaction occurs, an expression for the rate of change in concentration can be derived. For a closed system with constant volume such an expression can look like A mass balance (also called a material balance) is an accounting of material entering and leaving a system. ...

frac{d[C]}{dt} = k(T)[A]^{n'}[B]^{m'}

Temperature dependence

Main article: Arrhenius equation

Each reaction rate coefficient k has a temperature dependency, which is usually given by the Arrhenius equation: The Arrhenius equation is a simple, but remarkably accurate, formula for the temperature dependence of a chemical reaction rate, more correctly, of a rate coefficient, as this coefficient includes all magnitudes that affect reaction rate except for concentration. ... The Arrhenius equation is a simple, but remarkably accurate, formula for the temperature dependence of a chemical reaction rate, more correctly, of a rate coefficient, as this coefficient includes all magnitudes that affect reaction rate except for concentration. ...

k = A e^{ - frac{E_a}{RT} }

Ea is the activation energy and R is the gas constant. Since at temperature T the molecules have energies given by a Boltzmann distribution, one can expect the number of collisions with energy greater than Ea to be proportional to e^{frac{-E_a}{RT}}. A is the pre-exponential factor or frequency factor. The sparks generated by striking steel against a flint provide the activation energy to initiate combustion in this Bunsen burner. ... The gas constant (also known as the universal or ideal gas constant, usually denoted by symbol R) is a physical constant used in equations of state to relate various groups of state functions to one another. ... This article includes a list of works cited or a list of external links, but its sources remain unclear because it lacks in-text citations. ... In physics, the Boltzmann distribution predicts the distribution function for the fractional number of particles Ni / N occupying a set of states i which each has energy Ei: where is the Boltzmann constant, T is temperature (assumed to be a sharply well-defined quantity), is the degeneracy, or number of... In chemical kinetics, the frequency factor or A factor is the pre-exponential constant in the Arrhenius equation, which indicates how many collisions between reactants have the correct orientation to lead to the products. ...


The values for A and Ea are dependent on the reaction. There are also more complex equations possible, which describe temperature dependence of other rate constants which do not follow this pattern.


Example

For the reaction

2H_2 (g) + 2 NO(g) rarr N_2 (g) + 2 H_2O (g)

The rate equation is:

r = k [H_2]^1[NO]^2 ,

The rate equation does not simply reflect the reactants stoichiometric coefficients in the overall reaction: it is first order in H2, although the stoichiometric coefficient is 2 and it is second order in NO.


In chemical kinetics the overall reaction is usually proposed to occur through a number of elementary steps. Not all of these steps affect the rate of reaction; normally it is only the slowest elementary step that affect the reation rate. For example, in:

  1. 2 NO overrightarrowlongleftarrow N_2O_2 (fast equilibrium)
  2. N_2O_2 + H_2 rarr N_2O + H_2O (slow)
  3. N_2O + H_2 rarr N_2 + H_2O (fast)

Reactions 1 and 3 are very rapid compared to the second, so it is the slowest reaction that is reflected in the rate equation. The slow step is considered the rate determining step. The orders of the rate equation are those from the rate determining step.


See also

In chemistry, a steady state is a situation in which all state variables are constant in spite of ongoing processes that strive to change them. ... Collision theory is a theory, proposed by Max Trautz and William Lewis in 1916 that qualitatively explains how chemical reactions occur and why reaction rates differ for different reactions. ... The transition state of a chemical reaction is a particular configuration along the reaction coordinate. ...

Notes

  1. ^ a b c IUPAC definition of rate of reaction
  2. ^ Kenneth Connors, Chemical Kinetics, 1990, VCH Publishers, pg. 14

External links


  Results from FactBites:
 
Reaction rate - Wikipedia, the free encyclopedia (1167 words)
Pressure: The rate of gaseous reactions increases with pressure, which is, in fact, equivalent to an increase in concentration of the gas.
For a chemical reaction n A + m B → C + D, the rate equation or rate law is a mathematical expression used in chemical kinetics to link the rate of a reaction to the concentration of each reactant.
Reactions 1 and 3 are very rapid compared to the second, so it is the slowest reaction that is reflected in the rate equation.
  More results at FactBites »


 

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