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)
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- 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) + NO2(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
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- 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
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- 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
- Then:
- 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
Exam Tip
- Rate laws are only written from balanced chemical equations where such equations represent elementary reactions
- Rate laws are never written from overall balanced chemical equations