Methods of Determining Reaction Order MCQ - Practice Questions & Answers

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11 Questions around this concept.

Concepts Covered - 3

How to Determine Order of Reaction: Half Life Method

It is used when the rate law involves only one concentration term.

$\mathrm{t_{1 / 2} \propto(a)^{1-n}}$
or
$\mathrm{t_{1 / 2} \propto 1 / a^{n-1}}$

For two different concentrations, we have:

$\mathrm{\frac{\left(t^{1 / 2}\right)_{1}}{\left(t^{1 / 2}\right)_{2}}=\left(\frac{a_{2}}{a_{1}}\right)^{n-1}}$

On taking logarithms on both sides, we get:

$\mathrm{\log _{10} \frac{\left(t_{1 / 2}\right)_{1}}{\left(t_{1 / 2}\right)_{2}}=(n-1) \log _{10}\left(a_{2} / a_{1}\right)}$

Hence,

$\mathrm{n}=1+\frac{\log \left(\mathrm{t}^{1 / 2}\right)_{1}-\log \left(\mathrm{t}^{1 / 2}\right)_{2}}{\log \mathrm{a}_{2}-\log \mathrm{a}_{1}}$
Here, n is the order of the reaction.

How to Determine Order of Reaction: Graphical Method

Here graphs are plotted between rate and concentration to find the order of the reaction.

$\left[\text {Rate }=\mathrm{k}(\text {concentration})^{\mathrm{n}}\right]$

Plots of Rate vs Concentration

How to Determine Order of Reaction - Integrated Rate Law Method

If the data for time(t) and [A] is given then this method is applicable. Thus follows the steps given below to find the order of reaction by using the integrated rate law method.

Check for First Order:
1. Use the formula given below to find out the two values of k as k₁and k₂.
  $\mathrm{k\: =\: \frac{2.303}{t}\, log_{10}\left [ \frac{A_{o}}{A} \right ]}$
2. If these two values k₁and k₂ are same, then this given reaction is of first-order. But if k₁≠ k₂, then check for zero-order.
Check for Zero-Order:
1. Use the formula given below to find out the two values of k as k₁and k₂.
  $\mathrm{k\: =\: \frac{A_{o}-A}{t}}$
2. Again, if these two values k₁and k₂ are same, then this given reaction is of zero-order. But if k₁≠ k₂, then check for second-order.
Check for Third-Order:
1. Use the formula given below to find out the two values of k as k₁and k₂.
  $\mathrm{k\: =\: \frac{1}{t}\left [ \frac{1}{A}-\frac{1}{A_{o}} \right ]}$
2. Further, if these two values k₁and k₂ are same, then this given reaction is of second-order. But if k₁≠ k₂, then check for third-order and so on.