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Introduction to Reaction Mechanisms

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A reaction mechanism is the step-by-step sequence of elementary steps by which a reaction actually happens. Each step is a single molecular event, and the steps sum to the overall balanced equation. Species produced in one step and consumed in a later one are intermediates, which cancel out of the overall equation.

UNIT 5 TOPIC 5.7 • INTRODUCTION TO REACTION MECHANISMS MECHANISM MAP A mechanism is a sequence of elementary steps that adds up to the overall reaction. Elementary steps each step is a single molecular event Step 1 A + B → C + I Step 2 I + D → E Step 3 E → F + G Intermediates I and E are highlighted. Overall reaction add all steps, then cancel intermediates Sum of all steps: A + B + I + D + E → C + I + E + F + G cancel I and E — each is on both sides A + B + D → C + F + G Intermediates I and E do not appear in the overall reaction. Both I and E are intermediates I formed in step 1, consumed in step 2 E formed in step 2, consumed in step 3 An intermediate is produced in one step and consumed in a later step; it does not appear in the overall reaction. Catalyst vs intermediate Intermediate: made first, then used up Catalyst: used up first, then regenerated CED anchor · Unit 5 Kinetics Summing the three steps and cancelling I and E gives A + B + D → C + F + G — the intermediates never appear. AP Chemistry · Unit 5 · Kinetics
A mechanism is the actual sequence of elementary steps by which a reaction occurs. Each step is a single molecular event; the steps sum to the overall balanced equation, with intermediates (formed then consumed) canceling out.
Mechanism Stepper · Open the sandbox →

The trap is summing the steps incorrectly — failing to cancel intermediates properly so the steps do not add up to the overall reaction. A valid mechanism's elementary steps must sum to the balanced overall equation.

The work

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Lesson
Reaction Mechanisms ›

A mechanism's elementary steps sum to the overall reaction, with intermediates canceling. The lesson adds steps and identifies intermediates, then closes with a ten-scenario check.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the Topic 5.7 misconceptions: mechanism steps summed incorrectly (intermediates not canceling), and catalyst or intermediate roles misread.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions
Diagnostic first