Question

If the rate constant is measured in per second, which reaction order does this indicate?

• A. Zero order
• B. First order
• C. Either zero or first order
• D. None of these

Hint:

For reactions where the rate constant is measured in \(\text{s}^{-1}\), the reaction is typically first order.

Answer 1

The Correct Option is B

Step 1: Units of the rate constant.
The units of the rate constant \( k \) depend on the order of the reaction. If the rate constant is measured in \(\text{s}^{-1}\), this indicates the units are consistent with the first-order reaction.
Step 2: Explanation of the options.
For a first-order reaction, the rate law is \( r = k[A] \), where the rate \( r \) has units of mol/L·s, and \([A]\) has units of mol/L. Therefore, the unit of \( k \) for a first-order reaction is \(\text{s}^{-1}\).
Step 3: Conclusion.
Thus, if the rate constant is measured in \(\text{s}^{-1}\), the reaction is first order, which corresponds to option (B).

Answer 2

Step 1: For a zero-order reaction, the rate constant's unit is mol L^-1 s^-1.

Step 2: For a first-order reaction, the concentration term cancels out, so the unit of k is just s^-1 ('per second').

Step 3: Since the given unit here is 'per second', it matches a first-order reaction — that's option (B).

Answer 3

Elimination approach: Every reaction order has its own signature unit for the rate constant, so no two different orders ever share a unit. That rules out "either zero or first order" and "none of these" straight away. Checking a zero-order rate law, \( r = k \), \( k \) has to carry the same unit as rate itself, mol per litre per second, which does not match "per second". A unit of \( \text{s}^{-1} \) belongs only to a first-order reaction, so option (B) is correct.