Chemical kinetics

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In physical chemistry, chemical kinetics or reaction kinetics study reaction rates in a chemical reaction. Analysing the influence of different reaction conditions on the reaction rate gives information about the reaction mechanism and the transition state of a chemical reaction.

Peter Waage developed the law of mass action in 1864 that stated for the first time that the speed of a chemical reaction is proportional to the quantity of the reacting substances.

1 Rate of reaction
2 Factors that influence the rate of a reaction
3 Equilibria
4 Enthalpy
5 See also
6 References
7 External links

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Rate of reaction

Kinetics deal with the experimental determination of reaction rates from which a rate law and reaction rate constant are derived. Essential rate laws exist for zero order reactions (for which reaction rates are independent of initial concentration), first order reactions and second order reactions, and can be derived for others through calculus. In consecutive reactions the rate-determining step often determines the kinetics. In consecutive first order reactions, a steady state approximation can simplify the rate law. The activation energy for a reaction is experimentally determinated through the Arrhenius equation and the Eyring equation. Factors that influence the reaction rate include the type of catalyst (homogeneous or heterogeneous), temperature, and the presence of catalysts.

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Factors that influence the rate of a reaction

Several factors can be controlled that have an effect on the rate of a certain reaction, these factors are the concentration of reactants, the physical state of reactants, the temperature, and the use of a catalyst.

Image:Collisions.jpg

Concentration plays a very important role in reactions. This is due to the fact that molecules must collide in order to react together. for example, consider the reaction where hypothetical components "A" and "B" react together in order to give a certain product "C": A_(g) + B_(g) → C_(g). Now imagine what the reaction would look like at the molecular level if this reaction was done in closed 50ml bottle. The atoms of A and B would be flying in all the directions, crashing into each other and into the bottle's walls. This reaction cannot occur if the molecules do not collide. And also the more molecules present in the bottle, the more they have chances to collide, therefore if we change the concentration of these molecules (by changing the volume of the bottle, or by adding or removing reactants) we also change the speed (or rate) of the reaction. Thus, reaction rate is proportional to the concentration of reactants.

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, as in aqueous solution, thermal motion brings them into contact. However, when they are in different phases, for example one is liquid and the other is gas, contact can only occur at the interface, in other words at the surface of the liquid. Therefore, vigorous shaking and stirring may be needed. So basically, 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 you start a fire, first you put wood chips and small branches, you dont start with big logs right away.

Temperature usually has a major effect on the speed of a reaction. Since a molecule has more energy when it is heated, then the more energy it has, the more chances it has to collide with other reactants. Thus, at a higher temperature, more collisions occur. More importantly however, is the fact that heating a molecules affects its kinetic energy, and therefore the "energy" of the collision. Take the example of two tomatoes traveling towards each other, if they are going considerably slow, they might just hit and recoil, however, if they are comming at each other very fast (with a lot of energy) they will certainly explode and stick to each other. So to restate, raising the temperature increases the reaction rate by increasing the number, and especially, the energy of the collisions. 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 catalyst is a substance that accelerates the rate of a chemical reaction. In autocatalysis a reaction product is itself a catalyst for that reaction possibly leading to a chain reaction. In biochemistry enzymes accelerate reactions. Michaelis-Menten kinetics describe the rate of enzyme mediated reactions.

In certain organic molecules specific substituents can have an influence on reaction rate in neighbouring group participation.

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Equilibria

In a reversible reaction, chemical equilibrium is reached when the reaction rate of the forward reaction is equal to the rate of the reverse reaction and the concentrations of the reactants and products no longer change. This is demonstrated in the classical example of the Haber-Bosch process. Le Chatelier's principle can then be used to predict the effect of change in concentration, temperature or pressure on the position of that chemical equilibrium. Chemical clock reactions such as the Belousov-Zhabotinsky reaction demonstrate in a spectacular way that component concentrations can oscillate for a long time before finally reaching equilibrium.

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Enthalpy

In general terms, the standard enthalpy change of reaction determines if a chemical reaction will take place, the kinetics will then tell how fast the reaction is. A reaction can be very exothermic but will not happen in practice if the reaction is too slow. If a reactant can react to form two different products, the thermodynamically most stable product 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 relationships.

The kinetic isotope effect is a difference in the rate of a chemical reaction when an atom in one of the reactants is replaced by one of its isotopes.

Chemical kinetics provide information on residence time and heat transfer in a chemical reactor in chemical engineering and the molar mass distribution in polymer chemistry.

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