Unit 5: Topic 7 - Introduction to Reaction Mechanisms Flashcards
What is a reaction mechanism?
A reaction mechanism is the process by which a reaction occurs.
A chemical reaction does not always occur in one step. In fact, most reactions are the sum of many individual elementary reactions that make up the final reaction.
For example, consider the reaction: 2A + 2B ⟶ C + 2D
A reaction mechanism that could explain this overall reaction is:
2A ⟶ E
E + B ⟶ F + D
F + B ⟶ C + D
These individual reactions are elementary reactions that occur distinctly to produce the final reaction. You can propose many reaction mechanisms for a given reaction but you need experimental data to determine which is the true mechanism.
What are intermediates?
Intermediates are species that are made in a certain step and then consumed in a later step. They do not appear in the overall chemical equation of a reaction.
For example, consider the reaction A + D ⟶ E
A reaction mechanism that could explain this overall reaction is:
1) A + B ⟶ C
2) C + D ⟶ B + E
In this reaction mechanism, C is an intermediate because it is formed in elementary step 1 and used in elementary step 2.
What is a catalyst?
Catalysts are species that are used up in a certain step and then regenerated in a later step. They also increase the reaction rate by lowering the activation energy of certain steps.
Acids can be a catalyst (for this example let’s generalize acids as HA and their conjugate base as A-) :
For example, consider the hypothetical reaction A + D ⟶ E
A reaction mechanism that could explain this overall reaction is:
1) A + B ⟶ C
2) C + D ⟶ B + E
In this reaction mechanism, B is a catalyst because it is used in elementary step 1 and reformed in elementary step 2
How do you find the reaction equation when given the steps of the reaction?
We can find the reaction equation by simply adding up all the elementary steps.
Let’s use :
1) NO + NO ⟶ N₂O₂
2) N₂O₂ + H₂ ⟶ N₂O + H₂O
3) N₂O + H₂ ⟶ N₂ + H₂O
When we add up the left side and right side we get :
2NO + N₂O₂ + N₂O + 2H₂ ⟶ N₂O₂ + N₂O + N₂ + 2 N₂O
We can cancel out species that are on both sides of the equation. 1 N₂O₂ and 1 N₂O are canceled from both sides leaving us with:
2NO + 2H₂ ⟶ N₂ + 2N₂O
Why are reaction intermediates commonly found in low concentrations during a reaction?
After reaction intermediates are made, they’re used in a subsequent step. Therefore, their concentrations in a reaction mixture are usually low.
If there are multiple possible reaction mechanisms, how do researchers know which is correct?
Intermediates are very useful in solving this question.
Different reaction mechanisms will have varying intermediates and/or different stoichiometric ratios of these intermediates.
By detecting the different intermediates found in a reaction mixture, it’s possible to distinguish between reaction mechanisms.
