Edexcel A Level Chemistry:复习笔记5.5.8 Rates & Reaction Mechanisms

• 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