Chemical kinetics
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'''Chemical kinetics''', also known as reaction kinetics, is the study of [[reaction rate|rates]] of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the [[reaction mechanism|reaction's mechanism]] and [[transition state]]s, as well as the construction of mathematical models that can describe the characteristics of a chemical reaction. In 1864, [[Peter Waage]] and [[Cato Guldberg]] pioneered the development of chemical kinetics by formulating the [[law of mass action]], which states that the speed of a chemical reaction is proportional to the quantity of the reacting substances.
[[Image:Molecular-collisions.jpg|thumb|right|400px|[[Reaction rate]] tends to increase with [[concentration]] - a phenomenon explained by [[collision theory]].]]
== Rate of reaction ==
{{main|reaction rate}}
Chemical kinetics deals with the experimental determination of [[reaction rate]]s from which [[rate law]]s and [[reaction rate constant|rate constants]] are derived. Relatively simple [[rate law]]s exist for [[Rate law#Zero-order reactions|zero order reactions]] (for which reaction rates are independent of concentration), [[first order reaction]]s, and [[second order reaction]]s, and can be derived for others. In consecutive reactions the [[rate-determining step]] often determines the kinetics. In consecutive first order reactions, a [[steady state (chemistry)|steady state]] approximation can simplify the [[rate law]]. The [[activation energy]] for a reaction is experimentally determined through the [[Arrhenius equation]] and the [[Eyring equation]]. The main factors that influence the [[reaction rate]] include: the [[physical state]] of the reactants, the [[concentrations]] of the reactants, the [[temperature]] at which the reaction occurs, and whether or not any [[catalysts]] are present in the reaction.
== Factors affecting reaction rate ==
===Nature of the Reactants===
Depending upon what substances are reacting, the time varies. Acid reactions, the formation of [[salts]], and [[ion exchange]] are fast reactions. When covalent bond formation takes place between the molecules and when large molecules are formed, the reactions tend to be very slow.
===Physical State===
The [[physical state]] ([[solid]], [[liquid]], or [[gas]]) of a reactant is also an important factor of the rate of change. When reactants are in the same [[phase (matter)|phase]], as in [[aqueous]] [[solution]], thermal motion brings them into contact. However, when they are in different phases, the reaction is limited to the interface between the reactants. Reaction can only occur at their area of contact, in the case of a liquid and a gas, at the surface of the liquid. Vigorous shaking and stirring may be needed to bring the reaction to completion. This means that the more finely divided a solid or liquid reactant, the greater its [[surface area]] per unit [[volume]], and the more contact it makes with the other reactant, thus the faster the reaction. To make an analogy, for example, when one starts a fire, one uses wood chips and small branches—one doesn't start with large logs right away. In organic chemistry [[On water reaction]]s are the exception to the rule that homogeneous reactions take place faster than heterogeneous reactions.
===Concentration===
[[Concentration]] plays an important role in reactions according to the [[collision theory]] of chemical reactions, this is because molecules must collide in order to react together. As the concentration of the reactants increases, the [[frequency]] of the molecules colliding increases, striking each other faster by being in closer contact at any given point in time. Imagine two reactants being in a closed container. All the molecules contained within are colliding constantly. By increasing the amount of one or more of the reactants you cause these collisions to happen more often, increasing the reaction rate (Figure 1.1).
===Temperature===
[[Temperature]] usually has a major effect on the speed of a reaction. Molecules at a higher temperature have more [[thermal energy]]. When reactants (reactant + reactant → product) in a chemical reaction are heated, the more energetic atoms or molecules have a greater probability to collide with one another. Thus, more collisions occur at a higher temperature, making a product in a chemical reaction. More importantly however, is the fact that at higher temperatures molecules have more vibrational energy, that is, atoms are vibrating much more violently, so raising the temperature not only increases the number of collisions but also collisions that can result in rearrangement of atoms within the reactant molecules. For example, a [[refrigerator]] slows down the speed of the rate of reaction since it cools the molecules. On the other hand, an [[oven]] gives heat (energy) to the molecules which in turn speeds up the rate of reaction, cooking the food faster.
A reaction's kinetics can also be studied with a [[temperature jump]] approach. This involves using a sharp rise in temperature and observing the relaxation rate of an equilibrium process.
===Catalysts===
[[Image:Activation energy.svg|thumb|right|292px|Generic potential energy diagram showing the effect of a catalyst in an hypothetical exothermic chemical reaction. The presence of the catalyst opens a different reaction pathway (shown in red) with a lower activation energy. The final result and the overall thermodynamics are the same.]]
A [[catalyst]] is a substance that accelerates the rate of a chemical reaction but remains [[chemically]] unchanged afterwards. The catalyst increases rate reaction by providing a different [[reaction mechanism]] to occur with a lower [[activation energy]]. In [[autocatalysis]] a reaction product is itself a catalyst for that reaction leading to [[positive feedback]]. Proteins that act as catalysts in biochemical reactions are called [[enzyme]]s. [[Michaelis-Menten kinetics]] describe the [[enzyme kinetics|rate of enzyme mediated reactions]].
In certain organic molecules specific substituents can have an influence on reaction rate in [[neighbouring group participation]].
Agitating or mixing a solution will also accelerate the rate of a chemical reaction, as this gives the particles greater kinetic energy, increasing the number of collisions between reactants and therefore the possibility of successful collisions.
Increasing the pressure in a gaseous reaction will increase the number of collisions between reactants, increasing the rate of reaction. This is because the [[activity]] of a gas is directly proportional to the partial pressure of the gas. This is similar to the effect of increasing the concentration of a solution.
A catalyst does not affect the position of the equilibria, as the catalyst speeds up the backward and forward reactions equally.
== Equilibrium ==
While chemical kinetics is concerned with the rate of a chemical reaction, [[thermodynamics]] determines the extent to which reactions occur. In a [[reversible reaction]], chemical equilibrium is reached when the rates of the forward and reverse reactions are equal and the concentrations of the [[reactant]]s and [[Product (chemistry)|products]] no longer change. This is demonstrated by, for example, the [[Haber-Bosch process]] for combining nitrogen and hydrogen to produce ammonia. [[Chemical clock]] reactions such as the [[Belousov-Zhabotinsky reaction]] demonstrate that component concentrations can oscillate for a long time before finally reaching equilibrium.
== Free energy ==
In general terms, the [[Thermodynamic free energy|free energy change (ΔG)]] of a reaction determines if a chemical change will take place, but kinetics describes how fast the reaction is. A reaction can be very [[exothermic]] and have a very positive [[entropy]] change but will not happen in practice if the reaction is too slow. If a reactant can produce two different products, the thermodynamically most stable one will generally form except in special circumstances when the reaction is said to be under [[kinetic reaction control]]. The [[Curtin-Hammett principle]] applies when determining the product ratio for two reactants interconverting rapidly, each going to a different product. It is possible to make predictions about reaction rate constants for a reaction from [[Free-energy relationship]]s.
The [[kinetic isotope effect]] is the difference in the rate of a chemical reaction when an atom in one of the reactants is replaced by one of its [[isotope]]s.
Chemical kinetics provides information on [[residence time]] and [[heat transfer]] in a [[chemical reactor]] in [[chemical engineering]] and the [[molar mass distribution]] in [[polymer chemistry]].
==Applications==
The mathematical models that describe chemical reaction kinetics provide chemists and chemical engineers with tools to better understand and describe chemical processes such as food decomposition, microorganism growth, stratospheric ozone decomposition, and the complex chemistry of biological systems. These models can also be used in the design or modification of chemical reactors to optimize product yield, more efficiently separate products, and eliminate environmentally harmful by-products. When performing [[catalytic cracking]] of heavy hydrocarbons into gasoline and light gas, for example, kinetic models can be used to find the temperature and pressure at which the highest yield of heavy hydrocarbons into gasoline will occur.
== See also ==
* [[Collision theory]]
* [[Arrhenius equation]]
* [[Beer's law]]
* [[Chemical reaction]]
* [[Autocatalytic reactions and order creation]]
== References ==
* ''Preparing for the Chemistry AP Exam''. Upper Saddle River, New Jersey: Pearson Education, 2004. 131-134. ISBN 0-536-73157-8
== External links ==
* [http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch22/rateframe.html Chemical Kinetics]
* [http://www.chm.davidson.edu/ChemistryApplets/kinetics/ Chemistry applets]
* [http://www.science.uwaterloo.ca/~cchieh/cact/c123/chmkntcs.html University of Waterloo]
* [http://www.sci.wsu.edu/idea/ChemKinetics/ Washington state university]
* [http://cinarz.zdo.com/moodle/mod/resource/view.php?id=10 Chemical Kinetics Lecture]
* [http://mark.jelezniak.de/Chemked/index.htm Chemical Kinetics of Gas Phase Reactions]
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[[Category:Chemical kinetics| ]]
[[bs:Hemijska kinetika]]
[[ca:Cinètica química]]
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[[eo:Kemia kinetiko]]
[[fa:سینتیک شیمیایی]]
[[fr:Cinétique chimique]]
[[id:Kinetika kimia]]
[[it:Cinetica chimica]]
[[he:קינטיקה כימית]]
[[hu:Reakciókinetika]]
[[ja:反応速度論]]
[[pl:Kinetyka chemiczna]]
[[pt:Cinética química]]
[[ru:Химическая кинетика]]
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