Question

The reaction $L\to M$ is started with 10 g /L. After 30 and 90 minutes, 5 g and 1.25 g /L left respectively. The order of the reaction is:
(a)- 0
(b)- 2
(c)- 1
(d)- 3

Answer 1

Hint: When the reduction of the concentration of the compound is the same then we can easily predict the order of the reaction. At 30 minutes the concentration of the compound becomes half, if this continues then the order can be predicted easily.

Complete step by step solution: When the reaction occurs, the reactants convert into products. As time increases, the concentration of the reactants decreases, and the concentration of the products increases.
With the help of the reaction between the concentration and time of the reaction, we can calculate the order of the reaction.
Order of the reaction means the actual sum of the concentration of the terms on which the rate of the reaction depends.
So, in the given data of the question, the initial concentration is 10 g /L, after 30 minutes the concentration of the compound left is 5 g /L and 5 g /L has been converted into a product. This means that the concentration reduces to half.
Now, at 90 minutes, the concentration left is 1.25 g /L. So, we can see that when the time increases by 30minutes, then the concentration becomes half. This is shown below:
$10\text{ g /L}\xrightarrow{30\text{ minutes}}5\text{ g /L}\xrightarrow{30\text{ minutes}}2.5\text{ g /L}\xrightarrow{30\text{ minutes}}1.25\text{ g /L}$
So, at an equal interval of time, the concentration becomes half, therefore, the order of the reaction will be 1.

A correct answer is an option (c).

Note: First-order reaction means the concentration of one term is responsible for the rate of the reaction. If the reaction is a zero-order reaction, then the rate of the reaction does not depend on the concentration of the reactant.