Elementary Reactions (College Board AP® Chemistry)

Elementary Reactions

• Balanced chemical equations provide the overall details of what happens in a chemical reaction

  • For example, the equation for the reaction between carbon monoxide and nitrogen dioxide is given as:

CO (g) + NO₂ (g) → CO₂ (g) + NO (g)

• This equation shows that one mole of carbon monoxide reacts with one mole of nitrogen dioxide to form one mole of carbon dioxide and one mole of nitrogen monoxide

• It does not provide any details of how the reactants are turned into products

• On a molecular level, what happens in a chemical reaction may involve more than what is represented by a single chemical equation

  • Most reactions do not occur in one step but in a series of simple steps

• When reactions take place in more than one step, then the equation for each step is referred to as an elementary reaction

• For example, at 500 K, the reaction between carbon monoxide and nitrogen dioxide happens in two steps:

NO₂ (g) + NO₂ (g) → NO₃ (g) + NO (g)   (elementary reaction)

NO₃ (g) + CO (g) → CO₂ (g) + NO₂ (g)   (elementary reaction)

• An elementary reaction is a single molecular event that involves the collision of small particles

  • A set of elementary reactions that combine to give the overall chemical equation is called the reaction mechanism

  • From the elementary reactions above:

Step 1:           NO₂ (g) + NO₂ (g) → NO₃ (g) + NO (g)

Step 2:           NO₃ (g) + CO (g) → CO₂ (g) + NO₂ (g)

Overall equation:

NO₂(g) + NO₂(g) + NO₃(g) + CO(g) → NO₃(g) + NO(g) + CO₂(g) + NO_2(g)

• Chemical species such as NO₃ are called intermediates because they appear in the elementary steps but not in the overall balanced equation

Molecularity

• Elementary reactions are classified according to their molecularity

• The molecularity of a reaction is the number of molecules on the reactant side of an elementary reaction

• An elementary reaction may be

  • Unimolecular reactions involve only one reactant molecule

  • Bimolecular reactions involve two reactant molecules

  • Termolecular reactions involve three reactant molecules

• Unimolecular and bimolecular reactions are very common while reactions involving simultaneous collision between three molecules—termolecular reactions— are rare

  • For example, consider the overall reaction below:

CH₃Br + OH^– → CH₃OH + Br^–

• The reaction has been shown to involve two elementary steps:

CH₃Br + OH^– → CH₃OHBr^–                       (step 1)

CH₃OHBr^–  → CH₃OH + Br^–                      (step 2)

• The first step of the reaction involves two reactants and is bimolecular

• The second step involves one reactant and is unimolecular

Rate Laws and Elementary Reactions

• Knowing the elementary steps of a reaction enables us to deduce the rate law

• Consider the following elementary reaction:

A → products

• This reaction is unimolecular

• This means that the larger the number of A molecules present, the faster the rate of product formation

• Hence, the rate of this unimolecular reaction is directly proportional to the concentration of A or is first order in A

Rate = k[A]

• For a reaction involving two molecules, A and B

A + B → products

• The reaction is bimolecular

• The rate at which products are formed depends on how frequently the molecules of A and B collide

  • This in turn depends on the concentrations of A and B

• Hence, the rate law may be written as:
                          Rate = k[A][B]

• If the bimolecular reaction is of the type:
                          A + A → products or 2A → products

  • Then:
                  Rate = k[A]²

• In general, the reaction order for each reactant in an elementary reaction is equal to its stoichiometric coefficient in the chemical equation for that step