examples of slow chemical reactions

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From Wikipedia

Reaction mechanism

In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.

Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in a reaction mechanism. Recently, electrospray ionization mass spectrometry has been used to corroborate the mechanism of several organic reaction proposals.

Description

A chemical mechanism describes in detail exactly what takes place at each stage of an overall chemical reaction (transformation). It also describes each reaction intermediate, activated complex, and transition state, and which bonds are broken (and in what order), and which bonds are formed (and in what order). A complete mechanism must also account for all reactants used, the function of a catalyst, stereochemistry, all products formed and the amount of each, and what the relative rates of the steps are. Reaction intermediates are chemical species, often unstable and short-lived, which are not reactants or products of the overall chemical reaction, but are temporary products and reactants in the mechanism's reaction steps. Reaction intermediates are often free radicals or ions. Transition states can be unstable intermediate molecular states even in the elementary reactions. Transition states are commonly molecular entities involving an unstable number of bonds and/or unstable geometry which may be at chemical potential maxima.

The electron or arrow pushing method is often used in illustrating a reaction mechanism; for example, see the illustration of the mechanism for benzoin condensation in the following examples section.

A reaction mechanism must also account for the order in which molecules react. Often what appears to be a single step conversion is in fact a multistep reaction.

Examples

Consider the following reaction:

CO + NO2→ CO2 + NO

In this case, it has been experimentally determined that this reaction takes place according to the rate law R = k[NO_2]^2. Therefore, a possible mechanism by which this reaction takes place is:

2 NO2→ NO3 + NO (slow)
NO3 + CO → NO2 + CO2 (fast)

Each step is called an elementary step, and each has its own rate law and molecularity. The elementary steps should add up to the original reaction.

When determining the overall rate law for a reaction, the slowest step is the step that determines the reaction rate. Because the first step (in the above reaction) is the slowest step, it is the rate-determining step. Because it involves the collision of two NO2 molecules, it is a bimolecular reaction with a rate law of R = k[NO_2]^2. If we were to cancel out all the molecules that appear on both sides of the reaction, we would be left with the original reaction.

In organic chemistry, one of the first reaction mechanisms proposed was that for the benzoin condensation, put forward in 1903 by A. J. Lapworth.

Modelling

A correct reaction mechanism is an important part of accurate predictive modelling. For many combustion and plasma systems, detailed mechanisms are not available or require development.

Even when information is available, identifying and assembling the relevant data from a variety of sources, reconciling discrepant values and extrapolating to different conditions can be a difficult process without expert help. Rate constants or thermochemical data are often not available in the literature, so computational chemistry techniques or group-additivity methods must be used to obtain the required parameters.

At the different stages of a reaction mechanism's elaboration, appropriate methods must be used.

Molecularity

Molecularity in chemistry is the number of colliding molecular entities that are involved in a single reaction step.

  • A reaction involving one molecular entity is called unimolecular.
  • A reaction involving two molecular entities is called bimolecular.
  • A reaction involving three molecular entities is called termolecular.


From Yahoo Answers

Question:If someone wouldn't mind, could you explain to me 3 examples of chemical reactions that would quicken or slow down due to change of temperature. Eg. yeast rises faster with heat.

Answers:All reactions require a little bit of activation energy to get it going. Generally, however, Endothermic reactions speed up due to rise in temperature ex. reaction inside an ice pack (therefore ice pack is not cold for long in hotter weather) Exothermic reactions slow down due to rise in temperature ex. dissolving of oxygen into water

Question:

Answers:i guess you mean speed. fast: industrial reactions, using any fuel ie in a car, cooking slow: stop microbial growth in fridge, slow rust of metal objects, anti aging in us.

Question:can you please give me examples of these chemical reactions 1. Synthesis 2. Decomposition 3. Single Displacement 4. Double Displacement

Answers:1. 2 H2(g) + O2(g)--->2 H2O,(l) synthesis 2 .2 H2O-(l)--> O2)g)+2 H2 (g) , decomposition 3. 2NaCl(aq)+ F2(g)--->2 NaF(aq)+ Cl2(g), single displacement 4. AgNO3(aq)+ NaCl(aq)---> AgCl(s) + NaNO3(aq), double displacement

Question:pls. somene help me... what are the 4 types of chemical reaction? pls. give me at 2 examples of each so i can understand it well... thanks...

Answers:Isomerisation, in which a chemical compound undergoes a structural rearrangement without any change in its net atomic composition Direct combination or synthesis, in which two or more chemical elements or compounds unite to form a more complex product: N2 + 3 H2 2 NH3 Chemical decomposition or analysis, in which a compound is decomposed into smaller compounds or elements: 2 H2O 2 H2 + O2 Single displacement or substitution, characterized by an element being displaced out of a compound by a more reactive element: 2 Na(s) + 2 HCl(aq) 2 NaCl(aq) + H2(g) Metathesis or Double displacement reaction, in which two compounds exchange ions or bonds to form different compounds: NaCl(aq) + AgNO3(aq) NaNO3(aq) + AgCl(s) Precipitation (chemistry) Reactions where species in solution combine to form a solid product (precipitate). A typical example would be the reaction of methatesis described above. Acid-base reactions, broadly characterized as reactions between an acid and a base, can have different definitions depending on the acid-base concept employed. Some of the most common are: Arrhenius definition: Acids dissociate in water releasing H3O+ ions; bases dissociate in water releasing OH- ions. Br nsted-Lowry definition: Acids are proton (H+) donors; bases are proton acceptors. Includes the Arrhenius definition. Lewis definiton: Acids are electron-pair acceptors; bases are electron-pair donors. Includes the Br nsted-Lowry definition. Redox reactions, in which changes in oxidation numbers of atoms in involved species occur. Those reactions can often be interpreted as transferences of electrons between different molecular sites or species. A typical example of redox rection is: 2 S2O32 (aq) + I2(aq) S4O62 (aq) + 2 I (aq) In which I2 is reduced to I- and S2O32- (thiosulfate anion) is oxidized to S4O62-. Combustion, a kind of redox reaction in which any combustible substance combines with an oxidizing element, usually oxygen, to generate heat and form oxidized products. The term combustion is used usually only large-scale oxidation of whole molecules, i.e. a controlled oxidation of a single functional group is not combustion. C10H8+ 12 O2 10 CO2 + 4 H2O CH2S + 6 F2 CF4 + 2 HF + SF6 Organic reactions encompass a wide assortment of reactions involving compounds which have carbon as the main element in their molecular structure. The reactions an organic compound may take part are largely defined by its functional groups. Defined in opposition to inorganic reactions.

From Youtube

Chemical kinetics. Reaction mechanisms (1) :Chemistry: Chemical kinetics. Reaction mechanisms. Elementary steps; molecularity (unimolecular, bimolecular, termolecular). Rate laws for the overall reaction and for elementary steps. Rate-determining step. Mechanisms with slow first steps. Mechanisms with fast forward and reverse first steps This is a recording of a tutoring session, posted with the students' permission. These videos are offered on a "pay-what-you-like" basis. You can pay for the use of the videos at my website: www.freelance-teacher.com For a list of all the available video series, arranged in suggested viewing order, go to my website. For a playlist containing all the videos in this series, click here: www.youtube.com (1) Reaction mechanisms (2) Elementary steps. Molecularity (unimolecular, bimolecular, termolecular). Rate expression for the overall reaction (3) Rate laws for elementary steps. Intermediates (4) An example (5) Elementary reaction. Rate-determining step. Mechanisms with slow first steps: an example (6) Continued. Another example (7) Mechanisms with fast forward and reverse first steps (8) Continued (9) Continued

Chemical kinetics. Reaction mechanisms (2) :Chemistry: Chemical kinetics. Reaction mechanisms. Elementary steps; molecularity (unimolecular, bimolecular, termolecular). Rate laws for the overall reaction and for elementary steps. Rate-determining step. Mechanisms with slow first steps. Mechanisms with fast forward and reverse first steps This is a recording of a tutoring session, posted with the students' permission. These videos are offered on a "pay-what-you-like" basis. You can pay for the use of the videos at my website: www.freelance-teacher.com For a list of all the available video series, arranged in suggested viewing order, go to my website. For a playlist containing all the videos in this series, click here: www.youtube.com (1) Reaction mechanisms (2) Elementary steps. Molecularity (unimolecular, bimolecular, termolecular). Rate expression for the overall reaction (3) Rate laws for elementary steps. Intermediates (4) An example (5) Elementary reaction. Rate-determining step. Mechanisms with slow first steps: an example (6) Continued. Another example (7) Mechanisms with fast forward and reverse first steps (8) Continued (9) Continued