• Chemical kinetics can only?suggest?a reaction mechanism, they cannot prove it
  • However, they can be used to disprove a proposed mechanism

   

• Elementary steps are the steps involved in a reaction mechanism
  • For example, in the following general reaction:

   

A + B → C + D

• • The elementary steps could involve the formation of an intermediate:

Elementary step 1: A → R + D

Elementary step 2: R + B → C

• It is important that the elementary steps for a proposed mechanism agree with the overall stoichiometric equation
  • For example, combining the 2 elementary steps above gives the overall stoichiometric equation

   

A + R?+ B → R + C + D

A + B → C + D

Answers

Answer 1:

• • A one step reaction mechanism is simply the overall stoichiometric equation
  • Therefore, the correct answer is 2SO2?+ O2?→ 2SO3

   

Answer 2:

• • One of the two elementary steps for this two step mechanism can be taken from the question:

Elementary step 1: 2NO + O2?→ 2NO2

• • The second elementary step must involve the reaction of the nitrogen dioxide formed with sulfur dioxide:

Elementary step 2: NO2?+ SO2?→ NO + SO3?(or 2NO2?+ 2SO2?→ 2NO + 2SO3)

Predicting the reaction mechanism

• The overall reaction equation and rate equation can be used to predict a possible reaction mechanism of a reaction
  • This shows the individual reaction steps which are taking place

   

• For example, nitrogen dioxide (NO2) and carbon monoxide (CO) react to form nitrogen monoxide (NO) and carbon dioxide (CO2)
  • The overall reaction equation is:

   

NO2?(g) + CO (g) → NO (g) + CO2?(g)

• • The rate equation is:

Rate =?k?[NO2]2

• From the rate equation, it can be concluded that the reaction is zero-order with respect to CO (g) and second-order with respect to NO2?(g)
• This means that there are two molecules of NO2?(g) involved in the rate-determining step and zero molecules of CO (g)
  • This means that in terms of [popover id="GCXR2mfBwg279t3O" label=molecularity], the rate determining step is bimolecular

   

• A possible reaction mechanism could therefore be:

Step 1:

2NO2?(g) → NO (g) + NO3?(g)? ? ? ? ? ? ? ? ? ?slow?(rate-determining step)

Step 2:

NO3?(g) + CO (g) → NO2?(g) + CO2?(g)? ? ?fast

Overall:

2NO2?(g) +?NO3?(g)?+ CO (g) → NO (g) +?NO3?(g)?+?NO2?(g)?+ CO2?(g)

=? ? ?NO2?(g) + CO (g) → NO (g) + CO2?(g)

Predicting the reaction order & deducing the rate equation

• The?order?of a reactant and thus the rate equation can be deduced from a reaction mechanism if the rate-determining step is known
• For example, the reaction of nitrogen oxide (NO) with hydrogen (H2) to form nitrogen (N2) and water

2NO (g) + 2H2?(g) → N2?(g) + 2H2O (l)

• The reaction mechanism for this reaction is:

Step 1:

NO (g) + NO (g) → N2O2?(g)? ? ? ? ? ? ? ? ? ? ??fast

Step 2:

N2O2?(g) + H2?(g) → H2O (l) + N2O (g)??? ?slow?(rate-determining step)

Step 3:

N2O (g) + H2?(g) → N2?(g) + H2O (l)? ? ? ? ? ?fast

• The second step in this reaction mechanism is the?rate-determining step
• The rate-determining step consists of:
  • N2O2?which is formed from the reaction of?two NO molecules
  • One H2?molecule

   

• The reaction is, therefore,?second order?with respect to NO and?first order?with respect to H2
• So, the?rate?equation?becomes:

Rate =?k?[NO]2?[H2]

• The reaction is, therefore,?third order overall

• Nucleophilic substitution reactions can occur in two different ways (known as?SN2?and?SN1?reactions) depending on the structure of the halogenoalkane involved

SN1 reactions

• In?tertiary?halogenoalkanes, the carbon that is attached to the halogen is also bonded to three alkyl groups
• These halogenoalkanes undergo nucleophilic substitution by an?SN1?mechanism
  • ‘S’ stands for ‘substitution’
  • ‘N’ stands for ‘nucleophilic’
  • ‘1’ means that the rate of the reaction (which is determined by the slowest step of the reaction) depends on the concentration of only one reagent, the halogenoalkane

   

• The SN1 mechanism is a?two-step?reaction
• In the first step, the C-X bond breaks heterolytically and the halogen leaves the halogenoalkane as an X-?ion?(this is the?slow?and?rate-determining step)
  • As the rate-determining step only depends on the concentration of the halogenoalkane, the rate equation for an SN1 reaction is?rate =?k[halogenoalkane]
  • An SN1 reaction is described as unimolecular because there is only molecule involved in the rate-determining step
  • This forms a tertiary carbocation?(which is a tertiary carbon atom with a positive charge)
  • In the second step, the tertiary carbocation is attacked by the nucleophile

   

• For example, the nucleophilic substitution of 2-bromo-2-methylpropane by hydroxide ions to form 2-methyl-2-propanol

The mechanism of nucleophilic substitution in 2-bromo-2-methylpropane which is a tertiary halogenoalkane

SN2 reactions

• In?primary?halogenoalkanes,?the carbon that is attached to the halogen is bonded to one alkyl group
• These halogenoalkanes undergo nucleophilic substitution by an?SN2?mechanism
  • ‘S’ stands for ‘substitution’
  • ‘N’ stands for ‘nucleophilic’
  • ‘2’ means that the rate of the reaction (which is determined by the slowest step of the reaction) depends on the concentration of both the halogenoalkane and the nucleophile ions

   

• The SN2 mechanism is a?one-step?reaction
  • The nucleophile donates a pair of electrons to the δ+ carbon atom of the halogenoalkane to form a new bond
    • As this is a one-step reaction, the rate-determining step depends on the concentrations of the halogenoalkane and nucleophile, the rate equation for an SN2 reaction is?rate =?k[halogenoalkane][nucleophile]
    • An SN2 reaction is bimolecular as there are two molecules involved in the rate-determining step

     

  • At the same time, the C-X bond is breaking and the halogen (X) takes both electrons in the bond
  • The halogen leaves the halogenoalkane as an X-?ion

   

• For example, the nucleophilic substitution of bromoethane by hydroxide ions to form ethanol

The?SN2?mechanism of bromoethane with hydroxide causing an inversion of configuration