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Methods of Determining Reaction Order - Practice Questions & MCQ

Edited By admin | Updated on Sep 18, 2023 18:35 AM | #JEE Main

Quick Facts

  • 9 Questions around this concept.

Solve by difficulty

Which among the following graphs pertain to a zero order reaction?

 

For a reaction A + B → products, the rate of reaction was doubled when concentration of A was doubled. When concentration of A and B both was doubled, the rate was again doubled, order of reaction with respect to A and B are respectively -

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 k1 and k2.
      \mathrm{k\: =\: \frac{2.303}{t}\, log_{10}\left [ \frac{A_{o}}{A} \right ]}
    2. If these two values k1 and 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 k1 and k2.
      \mathrm{k\: =\: \frac{A_{o}-A}{t}}
    2. Again, if these two values k1 and 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 k1 and k2.
      \mathrm{k\: =\: \frac{1}{t}\left [ \frac{1}{A}-\frac{1}{A_{o}} \right ]}
    2. Further, if these two values k1 and k2 are same, then this given reaction is of second-order. But if k1≠ k2, then check for third-order and so on.

Study it with Videos

How to Determine Order of Reaction: Half Life Method
How to Determine Order of Reaction: Graphical Method
How to Determine Order of Reaction - Integrated Rate Law Method

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