**ii.** The order of the reaction for which the units of rate constant are mol dm⁻³ s⁻¹ is a. 1 b. 3 c. 0 d. 2

Choose the most correct option · Q2

^{-3}

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✓ Free question

Step 1. For an nth order reaction, k = rate/[A]^n, so the units of k are (concentration/time) / (concentration)^n = (concentration)^(1-n) (time)^-1.

Step 2. For k to have units of mol dm-3 s-1 (pure concentration/time, i.e. concentration power of exactly +1), we need $1-n=1$ , so $n=0$ .

Step 3. This matches the zero order case directly: for a zero order reaction, k=(([A]0-[A]t))/t, which is literally concentration/time.

✓Final answer

c. 0

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2020Set ANNUAL1 mark
$Q. Write the mathematical relation between half life of zero order reaction and its rate constant.$
›Reveal solutionSolution

Half-life of a zero order reaction is directly proportional to the initial concentration.

For a zero order reaction, the integrated rate law is $[A] = [A]_0 - kt$ . At $t = t_{1/2}$ , $[A] = [A]_0/2$ :

$\frac{[A]_0}{2} = [A]_0 - kt_{1/2} \implies kt_{1/2} = \frac{[A]_0}{2} \implies t_{1/2} = \frac{[A]_0}{2k}$

Unlike a first order reaction, the half-life here is directly proportional to the initial concentration, not independent of it.

✓Final answer
$t_{1/2} = [A]_0/(2k)$
CBSE 2018Set ANNUAL1 markMCQ
$A \rightarrow$
(a) $\dfrac{[A]_0}{k}$
(b) $\dfrac{0.693}{k}$
(c) $\dfrac{[A]_0}{2k}$
(d) $\dfrac{2[A]_0}{k}$
›Reveal solutionSolution

Substituting $[A]=[A]_0/2$ at $t=t_{1/2}$ into the zero-order integrated rate law gives $t_{1/2}=[A]_0/2k$ .

The integrated rate law for a zero order reaction is $[A]_t = [A]_0 - kt$ . At the half-life, $t=t_{1/2}$ and $[A]_t = [A]_0/2$ :

$\dfrac{[A]_0}{2} = [A]_0 - k\,t_{1/2} \Rightarrow k\,t_{1/2} = [A]_0 - \dfrac{[A]_0}{2} = \dfrac{[A]_0}{2}$

$\Rightarrow t_{1/2} = \dfrac{[A]_0}{2k}$

(Unlike first order kinetics, the zero-order half-life DOES depend on the initial concentration.)

✓Final answer
$\dfrac{[A]_0}{2k}$

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Q.ii. The order of the reaction for which the units of rate constant are mol dm $^{-3}$ s $^{-1}$ is a. 1
b. 3
c. 0
d. 2

Concept understanding — Zero-Order Reactions

Q.ii. The order of the reaction for which the units of rate constant are mol dm−3^{-3}−3 s−1^{-1}−1 is a. 1 b. 3 c. 0 d. 2

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Q.ii. The order of the reaction for which the units of rate constant are mol dm $^{-3}$ s $^{-1}$ is a. 1
b. 3
c. 0
d. 2

Unlock everything free for 14 days