How to calculate rate constant or rate equation value?, Chemistry

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Various methods are employed to calculate the rate constant. The method to be adopted and the proper selection depend on the experimental condition and nature of reactants and products. Some methods employed are discussed below:

Volumetric method

Used for reactions where reactants or products are acid or base or species undergoes a redox reaction. The idea is illustrated by the example below:

(i) Decomposition of H2O2
                                                      
H2O 451_index.png  H2O + ½ O2

If [A0] = initial conc. At t = 0 and [A] is conc. At time t then the conc. Of H2O2 at different intervals can be calculated by titration of the reactant using KMnO4 in acidic medium. At any time volume of KMnO4[H2O2]4 at time t.

Thus, V0 = volume of KMnO4 at start = [A0]

Vt = volume of KMnO4 at time t = [A]

Hence for first order reaction, k = 2.303/t log V0/Vt.

(ii) Decomposition of nitrogen pentaoxide

N2O5 decomposes in the gaseous state as well as in the form of its solution in an inert solvent. The decomposition is shown below.

976_index.png 

When the reaction is dome in its solution, N2O4 and NO2 remain in solution and the volume of oxygen gas collected is noted at different intervals of time. Thus,

Volume of O2 gas collected at any time (Vt) ∝ Amt. of N2O5 decomposed (x)
                                                              
i.e. x ∝ Vt

Also vol. of O2 gas collected at infinite time (V∞) Amt. of N2O5 taken initially (a)
                                                               
i.e. a ∝ V∞

Using the first order equation
                                            
k = 2.303/t log V∞/(V∞ - Vt)

 

 


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