Question

The order of reaction for which the unit of the velocity (rate) constant is \(mol\,L^{-1}\,s^{-1}\) is:

• A. First order
• B. Second order
• C. Zero order
• D. None of these

Hint:

Use \(k\) unit \(= mol^{(1-n)}L^{(n-1)}s^{-1}\). When rate \(=k\), the order is zero.

Answer 1

The Correct Option is C

Step 1: General unit of rate constant. For a reaction of order \(n\), rate \(= k[A]^n\). Since rate always has units \(mol\,L^{-1}\,s^{-1}\), the unit of \(k\) is \[k = mol^{(1-n)}\,L^{(n-1)}\,s^{-1}.\]
Step 2: Put \(n=0\). \(k = mol^{(1-0)}\,L^{(0-1)}\,s^{-1} = mol\,L^{-1}\,s^{-1}\). This matches the given unit.
Step 3: Check the others. First order (\(n=1\)): \(k = s^{-1}\). Second order (\(n=2\)): \(k = mol^{-1}\,L\,s^{-1}\). Neither equals \(mol\,L^{-1}\,s^{-1}\).
Step 4: Only a zero-order rate constant carries the units of rate itself, because for zero order rate \(= k\).
\[\boxed{\text{Zero order (option iii)}}\]