Concentration Changes Over Time

AP Chemistry· difficulty 3/5

Reaction A + B -> P. [B] is held nearly constant (large excess). [A] decays exponentially.

[B] (excess) [A]

Why does [A] follow first-order behavior even though true rate law is rate = k[A][B]?

  • A

    Stoichiometry is 1:1

  • B

    First order with respect to time only

  • C

    B is a catalyst

  • D

    [B] ≈ constant absorbs into k_obs

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Explanation

With [B] ≈ [B]0 constant, rate = k[B]0[A] is effectively first order in A.

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