Chemical kinetics
From Pharmpedia
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.
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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 first order reactions and second order reactions. In consecutive reactions the rate-determining step often determine 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 of through the Arrhenius equation. Among the factors that influence a reaction rate are the homogeneous or heterogeneous environment.
Catalysts
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.
Equlibria
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 behave oscillatory a for a long time before finally reaching equillibrium.
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. 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.
References
- Preparing for the Chemistry AP Exam. Upper Saddle River, New Jersey: Pearson Education, 2004. 131-134. ISBN 0536731578
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