Elementary reaction

An elementary reaction is a <a href="/facts/Chemical_reaction/GpqWlKrz">chemical reaction</a> in which one or more <a href="/facts/Chemical_species/A8BxM3oW">chemical species</a> react directly to form <a href="/facts/Product_(chemistry)/8uCoDUdv">products</a> in a single <a href="/facts/Reaction_step/TIsVN2hN">reaction step</a> and with a single <a href="/facts/Transition_state/34PsdqRd">transition state</a>. In practice, a reaction is assumed to be elementary if no <a href="/facts/Reaction_intermediates/BjD9HAHt">reaction intermediates</a> have been detected or need to be postulated to describe the reaction on a molecular scale. An apparently elementary reaction may be in fact a <a href="/facts/Stepwise_reaction/rtjq3YA5">stepwise reaction</a>, i.e. a complicated sequence of chemical reactions, with reaction intermediates of variable lifetimes.
In a <a href="/facts/Unimolecular/KSUIR0Kp">unimolecular</a> elementary reaction, a <a href="/facts/Molecule/N9ljSfFq">molecule</a> A <a href="/facts/Dissociation_(chemistry)/gDRGdeSN">dissociates</a> or <a href="/facts/Isomerisation/sxv23uKl">isomerises</a> to form the products(s)

A
          
        
        →
        
          
            products.
          
        
      
    
    {\displaystyle {\mbox{A}}\rightarrow {\mbox{products.}}}

At constant temperature, the <a href="/facts/Reaction_rate/Prwvxq0y">rate</a> of such a reaction is proportional to the concentration of the species A

d
              [
              
                
                  A
                
              
              ]
            
            
              d
              t
            
          
        
        =
        −
        k
        [
        
          
            A
          
        
        ]
        .
      
    
    {\displaystyle {\frac {d[{\mbox{A}}]}{dt}}=-k[{\mbox{A}}].}

In a <a href="/facts/Bimolecular/KSUIR0Kp">bimolecular</a> elementary reaction, two <a href="/facts/Atom/FoQ4kGN5">atoms</a>, molecules, <a href="/facts/Ion/3bePpDna">ions</a> or <a href="/facts/Radical_(chemistry)/qFymuVoT">radicals</a>, A and B, react together to form the product(s)

A + B
          
        
        →
        
          
            products.
          
        
      
    
    {\displaystyle {\mbox{A + B}}\rightarrow {\mbox{products.}}}

The rate of such a reaction, at constant temperature, is proportional to the product of the concentrations of the species A and B

d
              [
              
                
                  A
                
              
              ]
            
            
              d
              t
            
          
        
        =
        
          
            
              d
              [
              
                
                  B
                
              
              ]
            
            
              d
              t
            
          
        
        =
        −
        k
        [
        
          
            A
          
        
        ]
        [
        
          
            B
          
        
        ]
        .
      
    
    {\displaystyle {\frac {d[{\mbox{A}}]}{dt}}={\frac {d[{\mbox{B}}]}{dt}}=-k[{\mbox{A}}][{\mbox{B}}].}

The rate expression for an elementary bimolecular reaction is sometimes referred to as the <a href="/facts/Law_of_mass_action/kw0j6Re9">law of mass action</a> as it was first proposed by <a href="/facts/Cato_Maximilian_Guldberg/nBmwG5IY">Guldberg</a> and <a href="/facts/Peter_Waage/3Kcf3RtX">Waage</a> in 1864. An example of this type of reaction is a <a href="/facts/Cycloaddition/Gme6TDFP">cycloaddition</a> reaction.
This rate expression can be derived from first principles by using <a href="/facts/Collision_theory/65DaYrTP">collision theory</a> for <a href="/facts/Ideal_gases/80EfYGFj">ideal gases</a>. For the case of dilute fluids equivalent results have been obtained from simple probabilistic arguments.
According to collision theory the probability of three chemical species reacting simultaneously with each other in a <a href="/facts/Molecularity/KSUIR0Kp">termolecular</a> elementary reaction is negligible. Hence such termolecular reactions are commonly referred as non-elementary reactions and can be broken down into a more fundamental set of bimolecular reactions, in agreement with the law of mass action. It is not always possible to derive overall reaction schemes, but solutions based on <a href="/facts/Rate_equation/VK18zUhN">rate equations</a> are often possible in terms of <a href="/facts/Steady_state_(chemistry)/w7mgo1pD">steady-state</a> or <a href="/facts/Michaelis-Menten_kinetics/lYFykW3p">Michaelis-Menten</a> approximations.

Elementary reaction open-in-new

Elementary reaction