Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. 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 Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition states, as well as the construction of mathematical models that can describe the characteristics of a chemical reaction. Chemistry, a reaction mechanism is the step by step Sequence of Elementary reactions by which overall Chemical change occurs. The transition state of a Chemical reaction is a particular configuration along the Reaction coordinate. 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. Peter Waage ( June 29, 1833 &ndash January 13, 1900) was a significant Norwegian Chemist and professor at the Cato Maximilian Guldberg ( August 11 1836 in Christiania (today Oslo) – January 14 1902, Oslo) was a Norwegian In chemistry Law of Mass Action has two aspects 1 the equilibrium aspect concerning the composition of a reaction mixture at equilibrium and 2 the kinetic
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Rate of reaction
Chemical kinetics deals with the experimental determination of reaction rates from which rate laws and rate constants are derived. 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 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 The rate law or rate equation for a Chemical reaction is an equation which links the Reaction rate with concentrations or pressures of reactants and constant In Chemical kinetics a reaction rate constant k or \lambda quantifies the speed of a Chemical reaction. Relatively simple rate laws exist for zero order reactions (for which reaction rates are independent of concentration), first order reactions, and second order reactions, and can be derived for others. The rate law or rate equation for a Chemical reaction is an equation which links the Reaction rate with concentrations or pressures of reactants and constant The rate law or rate equation for a Chemical reaction is an equation which links the Reaction rate with concentrations or pressures of reactants and constant 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 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 In consecutive reactions the rate-determining step often determines the kinetics. The rate-determining step (RDS is a Chemistry term for the slowest step in a Chemical reaction. In consecutive first order reactions, a steady state approximation can simplify the rate law. For other uses of the term steady state see Steady state (disambiguation In Chemistry, a steady state is a situation in which all The rate law or rate equation for a Chemical reaction is an equation which links the Reaction rate with concentrations or pressures of reactants and constant The activation energy for a reaction is experimentally determined through the Arrhenius equation and the Eyring equation. In Chemistry, activation energy, also called midnight energy, is a term introduced in 1889 by the Swedish scientist Svante Arrhenius, that is defined The Arrhenius equation is a simple but remarkably accurate formula for the temperature dependence of the Rate constant, and therefore rate of a chemical reaction The Eyring equation also known as Eyring–Polanyi equation in Chemical kinetics relates the Reaction rate to Temperature. 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. 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 A state of matter (or physical state, or form of matter) has physical properties which are qualitatively different from other states of matter In Chemistry, concentration is the measure of how much of a given substance there is mixed with another substance Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Catalysis is the process in which the rate of a Chemical reaction is increased by means of a Chemical substance known as a catalyst
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. A salt, in Chemistry, is defined as the product formed from the neutralisation reaction of Acids and bases. Ion exchange is an exchange of Ions between two Electrolytes or between an electrolyte Solution and a complex. 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. A state of matter (or physical state, or form of matter) has physical properties which are qualitatively different from other states of matter A solid' object is in the States of matter characterized by resistance to Deformation and changes of Volume. Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of This page is about the physical properties of gas as a state of matter When reactants are in the same phase, as in aqueous solution, thermal motion brings them into contact. 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 In Chemistry, a solution is a Homogeneous Mixture composed of two or more substances 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. Surface area is the measure of how much exposed Area an object has The volume of any solid plasma vacuum or theoretical object is how much three- Dimensional space it occupies often quantified numerically 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 reactions are the exception to the rule that homogeneous reactions take place faster than heterogeneous reactions. On water reactions are a group of Organic reactions that take place as an Emulsion in water and that exhibit an unusual Reaction rate acceleration compared
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. In Chemistry, concentration is the measure of how much of a given substance there is mixed with another substance The Collision theory, proposed by Max Trautz and William Lewis in 1916 and 1918 qualitatively explains how Chemical reactions occur and why Reaction 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. Frequency is a measure of the number of occurrences of a repeating event per unit 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. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Molecules at a higher temperature have more thermal energy. Thermal energy is the sum of the sensible energy and latent 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. A refrigerator (often called a " fridge " for short is a cooling appliance comprising a thermally insulated compartment and a Heat pump - 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. An oven is an enclosed compartment for Heating, Baking or Drying.
A reaction's kinetics can also be studied with a temperature jump approach. A temperature jump is a piece of apparatus useful in the study of Chemical kinetics. This involves using a sharp rise in temperature and observing the relaxation rate of an equilibrium process.
Catalysts
A catalyst is a substance that accelerates the rate of a chemical reaction but remains chemically unchanged afterwards. Catalysis is the process in which the rate of a Chemical reaction is increased by means of a Chemical substance known as a catalyst Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties The catalyst increases rate reaction by providing a different reaction mechanism to occur with a lower activation energy. Chemistry, a reaction mechanism is the step by step Sequence of Elementary reactions by which overall Chemical change occurs. In Chemistry, activation energy, also called midnight energy, is a term introduced in 1889 by the Swedish scientist Svante Arrhenius, that is defined In autocatalysis a reaction product is itself a catalyst for that reaction leading to positive feedback. A single Chemical reaction is said to have undergone autocatalysis, or be autocatalytic, if the reaction product is itself the Catalyst for that reaction Positive feedback, sometimes referred to as "cumulative causation" is a Feedback loop system in which the system responds to perturbation in the same direction Proteins that act as catalysts in biochemical reactions are called enzymes. Enzymes are Biomolecules that catalyze ( ie increase the rates of Chemical reactions Almost all enzymes are Proteins Michaelis-Menten kinetics describe the rate of enzyme mediated reactions. Enzyme kinetics is the study of the Chemical reactions that are catalysed by Enzymes, with a focus on their Reaction rates The study of
In certain organic molecules specific substituents can have an influence on reaction rate in neighbouring group participation. Neighbouring group participation or NGP in Organic chemistry has been defined by IUPAC as the interaction of a reaction centre with a Lone pair
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 Physics, thermodynamics (from the Greek θερμη therme meaning " Heat " and δυναμις dynamis meaning " In a reversible reaction, chemical equilibrium is reached when the rates of the forward and reverse reactions are equal and the concentrations of the reactants and products no longer change. A reversible reaction is a Chemical reaction that results in an equilibrium mixture of Reactants and products. A reagent or reactant is a substance or compound consumed during a Chemical reaction. A product is a substance that forms as a result of a Biological - or Chemical reaction. This is demonstrated by, for example, the Haber-Bosch process for combining nitrogen and hydrogen to produce ammonia. The Haber process, also called the Haber–Bosch process, is the Nitrogen fixation reaction of Nitrogen and Hydrogen, over an iron substrate Chemical clock reactions such as the Belousov-Zhabotinsky reaction demonstrate that component concentrations can oscillate for a long time before finally reaching equilibrium. A chemical clock is a complex mixture of reacting chemical compounds in which the Concentration of one component shows an abrupt change accompanied by a A Belousov-Zhabotinsky reaction, or BZ reaction, is one of a class of reactions that serve as a classical example of Non-equilibrium thermodynamics, resulting in the
Free energy
In general terms, the free energy change (ΔG) of a reaction determines if a chemical change will take place, but kinetics describes how fast the reaction is. In Thermodynamics, the term thermodynamic free energy refers to the amount of work that can be extracted from a System, and is helpful in Engineering 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. In Thermodynamics, the word exothermic "outside heating" describes a process or reaction that releases Energy usually in the form of Heat, but In Thermodynamics (a branch of Physics) entropy, symbolized by S, is a measure of the unavailability of a system ’s Energy 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. Thermodynamic reaction control or kinetic reaction control in a Chemical reaction can decide the composition in a reaction product when competing reactions lead to 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 relationships. In Physical organic chemistry, a free-energy relationship or linear Gibbs energy relation relates the Logarithm of a Reaction rate constant
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 isotopes. 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 Isotopes (Greek isos = "equal" tópos = "site place" are any of the different types of atoms ( Nuclides
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. Residence time is a broadly useful concept that expresses how fast something moves through a system in equilibrium In thermal physics, heat transfer is the passage of Thermal energy from a hot to a colder body In Chemical engineering, chemical reactors are vessels designed to contain Chemical reactions The design of a chemical reactor deals with multiple aspects of Chemical engineering is the branch of Engineering that deals with the application of Physical science (e The Molar mass distribution (also known as the molecular weight distribution in a Polymer describes the relationship between a polymer fraction and the Molar mass Polymer chemistry or macromolecular chemistry is a multidisciplinary Science that deals with the Chemical synthesis and chemical properties of
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. Fluid catalytic cracking (FCC is the most important conversion process used in petroleum refineries.
See also
- Collision theory
- Arrhenius equation
- Beer's law
- Chemical reaction
- Autocatalytic reactions and order creation
References
- Preparing for the Chemistry AP Exam. The Collision theory, proposed by Max Trautz and William Lewis in 1916 and 1918 qualitatively explains how Chemical reactions occur and why Reaction The Arrhenius equation is a simple but remarkably accurate formula for the temperature dependence of the Rate constant, and therefore rate of a chemical reaction In A chemical reaction is a process that always results in the interconversion of Chemical substances The substance or substances initially involved in a chemical reaction are called Autocatalytic reactions are Chemical reactions in which at least one of the products is also a Reactant. Upper Saddle River, New Jersey: Pearson Education, 2004. 131-134. ISBN 0-536-73157-8
External links
- Chemical Kinetics
- Chemistry applets
- University of Waterloo
- Washington state university
- Chemical Kinetics Lecture
- Chemical Kinetics of Gas Phase Reactions
Dictionary
chemical kinetics
-noun
- (chemistry) the study of the reaction rates of chemical reactions, especially as a means of elucidating reaction pathways or mechanisms
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