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MDCAT Chemistry QUESTION #4358
Question 1
For the reaction $2A + B \rightarrow 3C + D$, which of the following does NOT express the reaction rate?
  • $-\dfrac{d[B]}{dt}$
  • $\dfrac{d[D]}{dt}$
  • $-\dfrac{1}{2}\dfrac{d[A]}{dt}$
  • $-\dfrac{1}{3}\dfrac{d[C]}{dt}$✔️
Correct Answer Explanation
Rate $= -\dfrac{1}{2}\dfrac{d[A]}{dt} = -\dfrac{d[B]}{dt} = +\dfrac{1}{3}\dfrac{d[C]}{dt} = +\dfrac{d[D]}{dt}$. The expression $-\dfrac{1}{3}\dfrac{d[C]}{dt}$ is wrong because C is a product so its rate should be positive: $+\dfrac{1}{3}\dfrac{d[C]}{dt}$.