Question

Write the unit of (i) second order reaction and (ii) zero order reaction.

Hint:

Master formula for $k$ units: $M^{1-n} \cdot Time^{-1}$ where M is Molarity ($mol~L^{-1}$) and $n$ is the order.

Answer 1

Step 1: General unit formula.
From the rate equation $\text{Rate} = k[\text{conc}]^n$, the unit of the rate constant is $(mol~L^{-1})^{1-n}~s^{-1}$.
Step 2: Second order reaction ($n = 2$).
Substituting $n = 2$: $(mol~L^{-1})^{1-2}~s^{-1} = (mol~L^{-1})^{-1}~s^{-1} = L~mol^{-1}~s^{-1}$.
Step 3: Zero order reaction ($n = 0$).
Substituting $n = 0$: $(mol~L^{-1})^{1-0}~s^{-1} = mol~L^{-1}~s^{-1}$.
Step 4: Summary.
(i) Unit for second order reaction: $L~mol^{-1}~s^{-1}$. (ii) Unit for zero order reaction: $mol~L^{-1}~s^{-1}$.