section_path: Kinetics
section_name: Kinetics
paragraph_text: Kinetics
  
  The speed at which reactions take place is studied by reaction kinetics. The rate depends on various parameters, such as:
  
  Several theories allow calculating the reaction rates at the molecular level. This field is referred to as reaction dynamics. The rate v of a first-order reaction, which could be the disintegration of a substance A, is given by:
  
  Its integration yields:
  
  Here k is the first-order rate constant, having dimension 1/time, (t) is the concentration at a time t and is the initial concentration. The rate of a first-order reaction depends only on the concentration and the properties of the involved substance, and the reaction itself can be described with a characteristic half-life. More than one time constant is needed when describing reactions of higher order. The temperature dependence of the rate constant usually follows the Arrhenius equation:
  
  where E is the activation energy and k is the Boltzmann constant. One of the simplest models of reaction rate is the collision theory. More realistic models are tailored to a specific problem and include the transition state theory, the calculation of the potential energy surface, the Marcus theory and the Rice–Ramsperger–Kassel–Marcus (RRKM) theory.