Question detail
If the rate constant k for a reaction is 0.5 s^-1, what are the units of k for a second-order reaction?
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Rate equations (A-level only)
Question
- A. mol dm^-3 s^-1
- B. dm^3 mol^-1 s^-1
- C. s^-1
- D. mol^2 dm^-6 s^-1
Answer
dm^3 mol^-1 s^-1
Explanation
For a second-order reaction, the units of the rate constant k are derived from the rate equation, which is Rate = k[A]^2. The rate has units of mol dm^-3 s^-1, and since [A] is in mol dm^-3, k must have units of dm^3 mol^-1 s^-1 to ensure the equation is dimensionally consistent.
Common mistake
Incorrect Rate Equation Formation
Students often confuse the order of reaction with the coefficients in the balanced equation when writing rate equations.
To write the correct rate equation, identify the order of each reactant based on experimental data, not the coefficients. For example, if the rate law is rate = k[A]^2[B]^1, this indicates that the reaction is second order with respect to A and first order with respect to B. Ensure to use the correct orders derived from initial rate data.
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