<Dl> <Dd> k = k 0 e − E a / k B T (\ displaystyle k = k_ (0) e ^ ((- E_ (a)) / (k_ (B) T))) </Dd> </Dl> <Dd> k = k 0 e − E a / k B T (\ displaystyle k = k_ (0) e ^ ((- E_ (a)) / (k_ (B) T))) </Dd> <P> 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 . </P> <P> In a synthesis reaction, two or more simple substances combine to form a more complex substance . These reactions are in the general form: </P>

The substances that most directly control the rate of these reactions are known as