• 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) → N₂O_2?(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