Reaction Mechanisms (HL) (DP IB Chemistry)

Reaction Mechanisms

What is a reaction mechanism?

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

• Each step is called an elementary step and involves a small number of particles

• Some of the products of an elementary step exist as intermediates and react in subsequent steps

• The sequence of elementary steps is called the reaction mechanism

• The sum of the elementary steps must equal the overall reaction equation

  • Intermediates that are produced in one elementary step and react in another step cancel out

• 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

What is the rate-determining step?

• A chemical reaction can only go as fast as the slowest part of the reaction

  • So, the rate-determining step is the slowest step in the reaction

What does the rate equation tell us about the rate-determining step?

• If a reactant appears in the rate-determining step, then the concentration of that reactant will also appear in the rate equation

• The order with respect to a reactant describes the number of particles of that reactant that take part in the rate-determining step

Predicting the reaction mechanism

• The overall reaction equation and rate equation can be used to predict a possible reaction mechanism of a reaction

• 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)

• 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)

   Simplify (remove species on both sides of the equation):

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:

Step 2:
 

Step 3:

• 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

Energy Profiles in Multistep Reactions

Single-step reactions

• When any reacting molecules collide with bond breaking and bond formation occurring, the interacting molecules will be in an unstable, high-energy state temporarily

  • This transition state will be of a higher energy than either the reactants or products and corresponds to the activation energy

• The exothermic reaction of hydrogen and iodine to form hydrogen iodide will be used to discuss how energy level diagrams relate to the rate-determining step

Energy profile of an exothermic reaction, showing the transition state

The energy profile for the exothermic reaction of hydrogen and iodine

• As the reaction proceeds, covalent bonds start to form between the hydrogen and iodine atoms from the hydrogen and iodine molecules

• At the same time, the covalent bonds within the hydrogen and iodine molecules grow longer and become weaker

• This results in the transition state complex shown

Transition state

The transition state complex for the reaction of hydrogen and iodine

• From this transition state, the bonds between the hydrogen and iodine atoms can continue to grow shorter and stronger resulting in the formation of hydrogen iodide

• Alternatively, the bonds within the hydrogen and iodine molecules can grow shorter and stronger which would result in the formation of the reactants

• Hydrogen iodide will only form if the hydrogen and iodine molecules collide with kinetic energy greater than or equal to the activation energy

  • The molecules will also need to collide in the correct orientations

• The reaction, or elementary, step with the greatest activation energy will be the rate-determining step and can be used to determine the rate equation

Multi-step reactions

• The exothermic reaction of nitrogen dioxide and fluorine to form nitryl fluoride (NO₂F) will be used to relate rate equations and rate-determining steps to the energy level diagram of a multi-step reaction:

2NO₂ (g) + F₂ (g) → 2NO₂F (g)

• This reaction is unlikely to occur in a single step as that would require three molecules to collide in the correct orientation and with sufficient kinetic energy

  • This is even less likely to occur as all three molecules are gaseous

• Experimental data shows that the rate equation for this reaction is:

Rate = k[NO₂][F₂]

• One proposed reaction mechanism for this reaction involves the following elementary steps:

  • Step 1: NO₂ + F₂ → NO₂F + F

  • Step 2: NO₂ + F → NO₂F

• Step 1 must be the rate-determining step as it is the only step that has reactants matching the rate equation

  • Therefore, on an energy profile, the activation energy for step 1 will be greater than the activation energy for step 2

• The species present after the first step must be:

NO₂ + NO₂F + F

• This can be deduced using the equation for elementary step 1 and the overall equation

  • The overall equation states that two NO₂ react with one F₂

  • Elementary step 1 uses one NO₂ to form the intermediates, NO₂F + F

  • This leaves one NO₂ along with NO₂F + F

• This leads to the following energy profile:

Energy profile of a multistep reaction

Energy profile for the formation of nitryl fluoride

• Key points from the energy profile are:

  • The overall reaction is exothermic, as stated

  • The rate-determining step is the step that has the greatest activation energy

  • There is a labelled energy level for the species present after the first step which include the intermediates and unreacted reactant