Species that occur in a reaction pathway but not in the overall reaction are known as

Step 1: Analyze the question The question asks for the name of species that appear in the steps of a reaction mechanism (the reaction pathway) but do not appear in the overall balanced equation.

Step 2: Explain the concept Many reactions happen in several steps, called elementary steps. In these steps:

• Reactants are the starting materials. They appear on the left of the overall equation.
• Products are the final substances formed. They appear on the right of the overall equation.
• Intermediates are formed in one step and used up in a later step. They never show up in the overall equation because they cancel out.
• Inhibitors are substances that slow down or stop a reaction. They are not defined by the pathway-versus-overall idea.

Step 3: Example Consider a two-step mechanism:

• Step 1: A + B → C
• Step 2: C + D → E

Adding the steps gives A + B + C + D → C + E. Cancelling C from both sides gives the overall reaction A + B + D → E.

Species C is produced in step 1 and consumed in step 2. It is present in the pathway but absent from the overall reaction, so C is an intermediate.

Step 4: Check the options

• A. Reactants: these appear in the overall reaction. ✗
• B. Inhibitors: these slow reactions and have nothing to do with the cancellation of species. ✗
• C. Products: these appear in the overall reaction. ✗
• D. Intermediates: formed and then consumed, so absent from the overall equation. ✓

Answer: D. Intermediates

Exam tip: Think "in-between". An intermediate exists only between steps, so it is made and then used up. If you can cancel a species out when adding the steps, it is an intermediate. Don't confuse it with a catalyst, which is used first and then regenerated. Both vanish from the overall equation, but an intermediate is made first and then consumed, while a catalyst is consumed first and then remade.