A proposed reaction mechanism is given below. Give the overall (i.e. chemical) equation for this reaction.

A + A --- A2 (slow)

A2 + B --- C + D (fast)

C + B --- E + D (fast)

Question

A proposed reaction mechanism is given below. Give the overall (i.e. chemical) equation for this reaction.

A + A ---> A2                          (slow)

A2 + B ---> C + D                    (fast)

C + B ---> E + D                      (fast)

BlackTom AI · Accepted Answer

To determine the overall chemical equation, first identify the slow, rate-determining step and which species are intermediates. 
The given mechanism starts with A + A → A2 as the slow step. This means A2 is an intermediate formed from two A molecules and will not appear in the final net equation unless produced and consumed in subsequent steps. 
Next, A2 + B → C + D is fast, converting the intermediate A2 and B into products C and D. 
Then, C + B → E + D is also fast, consuming C with B to form E and D. 
Now we combine the steps, canceling intermediates that are produced and consumed across steps. The first step produces A2, which is consumed in the second step; thus A2 does not appear in the overall equation. 
Collect the reactants that appear on the left and the products that appear on the right across all steps: from step 1, 2 A atoms are used; from steps 2 and 3, we consume 2 B atoms total (one B in step 2 and one B in step 3). The products formed are E and D, with D appearing in both steps 2 and 3 (two D molecules in total) and E appearing from step 3. 
Therefore, the net chemical equation is 2 A + 2 B → E + 2 D. 
Now, evaluating the alternatives: 
- Option 1: A + A → E + D would imply direct conversion of A to E and D in a single step, which does not reflect the intermediate A2 formation and the subsequent steps; this ignores the actual intermediacy and the B reactants. 
- Option 2: A + A → A2 represents the slow step, not the overall reaction, so it cannot be the complete net equation. 
- Option 3: 2A + B → E + D would neglect the second B necessary in the mechanism (the B atom is consumed in two separate fast steps), so this is not balanced with respect to B and the mechanism. 
- Option 4: 2A + 2B → E + 2D matches the stoichiometry derived from combining all steps and canceling intermediates, aligning with the rate-determining step and subsequent fast steps.
Thus, the correct overall equation is 2A + 2B → E + 2D, and the other choices misrepresent either the intermediates, the number of B atoms involved, or the net stoichiometry.

A proposed reaction mechanism is given below. Give the overall (i.e. chemical) equation for this reaction. A + A ---> A2 (slow) A2 + B ---> C + D (fast) C + B ---> E + D (fast)单项选择题

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