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COMEDK 2025 Afternoon Solved Paper

Question : 81 of 180

Marks: +1, -0

Which one of the following is incorrect?

Rate constant equals the rate of reaction when concentration of reactants equals unity.
Larger the value of rate constant $(\mathrm{k})$ value faster is the chemical reaction.
The unit for rate constant for $n^{\text {th }}$ order reaction is $\left(\mathrm{mol} \mathrm{L}^{-1}\right)^{n-1} \mathrm{~s}^{-1}$
The value of rate constant $(\mathrm{k})$ is independent of concentration of reactants.

Solution:

Concept:

Rate constant units depend on reaction order; option C gives the wrong exponent.

Explanation:

For an

n^{\text{th}}

-order reaction, rate

= k[A]^n

Rate has units

\text{mol L}^{-1} \text{s}^{-1}

So the units of

k

are:

[k] = \frac{\text{mol L}^{-1} \text{s}^{-1}}{(\text{mol L}^{-1})^n} = \text{mol}^{1-n} \text{L}^{n-1} \text{s}^{-1}

Option C claims units are

(\text{mol L}^{-1})^{n-1} \text{s}^{-1} = \text{mol}^{n-1} \text{L}^{-(n-1)} \text{s}^{-1}

This is the reciprocal of the correct expression, making option C incorrect.

All other options are correct:

A: When

[A] = 1

, rate

= k

B: Larger

k

means faster reaction.

k

does not depend on concentrations of reactants.

Answer:

Option C.

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