Reaction Mechanisms

Question 1

What is an elementary process?

Answer 1

• It expresses how molecules or ions react with each other at the molecular level, not the overall reaction.
• A series of elementary processes makes up a reaction mechanism
• Based on numbers of molecules involved in the elementary step, there are three kinds of elementary steps: unimolecular process, bimolecular process, and trimolecular process.

Question 2

How do you write the rate law for an elementary process?

Answer 2

• The exponent for the concentration is the same as the coefficient in the chemical equation for that elementary process
  
  • Ex: 2NO₂ –> NO₃ + NO
  • rate = k[NO₂]²
• (And if we don’t have the chemical equation for the elementary process, we need to get it experimentally.)

Question 3

What is a rate-determining step?

(aka rate-limiting step)

Answer 3

• It’s the slow step in a mechanism. The reaction can’t go any faster.
• The order for the rate-determining step is the same as the order of the overall reaction
  
  • So the rate law for the rate-determining step is directly related to the rate law for the overall reaction

Question 4

What is a reaction mechanism?

Answer 4

• A series of elementary processes
• We know that there are multiple steps if the experimentally determined rate law does not correspond to the coefficients in the chemical equation
  
  • The overall rate law corresponds with the rate of the slowest step

Question 5

Molecularity

What are unimolecular, bimolecular, and trimolecular processes?

Answer 5

Molecularity = how many molecules are involved on the reactant side.

3 types of elementary processes:

• Unimolecular - when a molecule or ion decomposes by itself
  
  • 1st order
  • Ex: O₃ –> O₂ + O Rate = k [O₃]
• Bimolecular - two reacting molecules or ions. Most common.
  
  • 2nd order
  • A + A = B + C Rate = k [A]²
    A + B = X + Y Rate = k [A] [B]
• Termolecular - 3 molecules. Statistically very unlikely.
  
  • 3rd order
  • A + A + A = products Rate = k [A]³
    A + A + B = products Rate = k [A]² [B]
    A + B + C = products Rate = k [A] [B] [C]

Question 6

What is a catalyst?

Answer 6

• A substance that changes the rate of a chemical reaction without being used up (not part of rxn)
• Provides a path to the products that has a rate-detmining step with a lower activation energy, allowing more of the reactant molecules to have the minimum energy needed

Extra from book:

• Positive catalysts: speed up reactions
• Negative catalysts (inhibitors): slow down
• Homogenous catalysts: same phase as reactants
• Heterogenous catalysts: different phase from reactants
  
  • Usually a solid. Promotes rxn through adsorption.

Question 7

What are enzymes?

Answer 7

• Biological catalysts, facilitate almost every reaction
• They have active sites

Question 8

The orders in a rate law have to match what?

Answer 8

• The order of the rate determining step
• If the rate determining step is the first step, then it’s easy! The exponents for the rate law will be the stoich coefficients of that step.
  
  • Ex: 2NO₂ → NO₃ + NO (slow)
  • NO₃ + CO → NO₂ + CO₂ (fast)
    Rate=k[NO₂]²
• If the rate determining step is later, have to use SSA or prequilibrium assumption to substitute out the intermediate

Question 9

How do you make sure a rate law is correct?

Answer 9

1. The order of the rate law should match the order / molecularity of the rate determining step.
2. No intermediates in the rate law.
   
   • (This is because we don’t know their concentrations, and we want to write in terms of reactants.)

Question 10

What are reaction intermediates?

Answer 10

• Product of the first step, reactant of the second step
• Notation: they are written in the mechanism, but cancelled for the BCE
• Graph: The local minimum of the new action pathway is where you would find the intermediates.
• Difference from transition state: a transition state is a very unstable combination of two species. Whereas n intermediate is isolatable, something you could potentially actually find in the beaker.

Question 11

Interpret graph of reaction mechanism.

What do the peaks and valleys represent?

Answer 11

• Number of peaks is the number of steps.
• The highest peak is the step with the highest activation barrier and will have the lowest rate.
• The valleys are the intermediates.

Question 12

What is steady state approximation?

Answer 12

• When there is not much of the intermediate in the beaker because as soon as it’s produced, it is consumed immediately. (See graph.)
  
  • Assumption: Δ [intermediate] / Δ time = 0
    
    • Δ = production - consumption
• Different from preequilibrium assumption because we aren’t assuming equilibrium
• Steps:
  
  1. Write the rate law from the rate determining step.
  2. Recognize the intermediate.
  3. Set up the assumption: Δ [intermediate] / Δ time = 0.
  4. Solve for [intermediate].
  5. Plug into the rate law.

Question 13

What is the preequilibrium assumption?

Answer 13

• When the equilibrium is the fast step (before the rate determining step)
• Assumes that the reactants and intermediates have reached equilibrium
• Set the rates of the forward and reverse reactions equal to each other, solve for the intermediate
• Plug into the rate law

Question 14

The rate constant is ____ if the step is faster.

The rate constant is ________ if the step is slower.

Answer 14

bigger

smaller

Question 15

How do you make sure a reaction mechanism is valid?

Answer 15

1. The individual elementary steps should add up to the overall reaction. (After cancelling out intermediates.)
2. The rate determining step should match the order of the experimental rate law.

Question 16

What is equal at equilibrium?

Answer 16

• At equilibrium the rates are equal. (NOT the rate constants.)
• k_forward[A]^a[B]^b = k_reverse[C]^c[D]^d
• k_forward / k_reverse = K_eq

Question 17

How do you identify the catalyst and reaction intermediate in a reaction mechanism?

Answer 17

• The catalyst is unchanged, present in the first step reactant and final step products
• The reaction intermediate is the product of the first step and reactant of the next step