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Q3.1(ii):
From the rate expression for the following reactions, determine their order of reaction and the dimensions of the rate constants. (ii) \(H_{2}O_{2} (aq) + 3I^{-} (aq) + 2H^{+} \rightarrow 2H_{2}O (l) + I_{3}^{-}\) Rate = \(k[H_{2}O_{2}][I^{-}]\)

Solution :

Step 1: Determining the Order of Reaction

The rate law for the given reaction is expressed as:

$Rate = k[H_{2}O_{2}]^{1}[I^{-}]^{1}$

The order of reaction with respect to an individual reactant is the exponent to which its concentration term is raised in the rate law [Per the Rate Law definition].

  • Order with respect to $[H_{2}O_{2}] = 1$
  • Order with respect to $[I^{-}] = 1$

The overall order of the reaction is the sum of the individual orders of all reactants involved in the rate expression [Per the Law of Mass Action principles for kinetics]:

$n = 1 + 1 = 2$

Therefore, the reaction is a second-order reaction.

Step 2: Deriving the Dimensions of the Rate Constant ($k$)

The general expression for the rate constant $k$ of a reaction of order $n$ is given by the formula:

$k = \frac{Rate}{[Concentration]^{n}}$

Given that the rate of reaction is expressed in terms of change in concentration over time, its units are $mol \cdot L^{-1} \cdot s^{-1}$. The units of concentration are $mol \cdot L^{-1}$ [Per the SI unit convention for Molarity].

Substituting $n = 2$ into the dimensional formula:

$Units of k = \frac{mol \cdot L^{-1} \cdot s^{-1}}{(mol \cdot L^{-1})^{2}}$

$Units of k = \frac{mol \cdot L^{-1} \cdot s^{-1}}{mol^{2} \cdot L^{-2}}$

By canceling the common terms:

$Units of k = mol^{-1} \cdot L \cdot s^{-1}$

Step 3: Visual Representation of Second-Order Kinetics

Time (s) Rate Rate = k[A][B]

Summary of Analytical Results

Parameter Value/Unit
Order of Reaction $2$
Units of Rate Constant $L \cdot mol^{-1} \cdot s^{-1}$

Final Solution: The reaction is of the second order, and the dimensions of the rate constant $k$ are $L \cdot mol^{-1} \cdot s^{-1}$ (or $M^{-1} \cdot s^{-1}$).


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