Question detail
If the rate constant (k) for a reaction is 0.5 s^-1, what is the unit of the rate for a first-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
- B. mol/dm^3
- C. s^-1
- D. dm^3/mol/s
Answer
mol/dm^3/s
Explanation
The rate of a reaction is expressed in terms of concentration change over time. For a first-order reaction, the unit of rate is mol/dm^3/s, which corresponds to the change in concentration (mol/dm^3) per second (s).
Common mistake
Incorrect Rate Equation Prediction
Students often confuse the overall reaction equation with the rate-determining step when predicting the rate equation from a proposed mechanism.
To fix this, clearly identify the rate-determining step in the mechanism and use only the reactants involved in that step to write the rate equation. For example, if the rate-determining step is A + B → C, the rate equation would be rate = k[A][B], where k is the rate constant.
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