More Complex Reactions | Chemistry Optional Notes for UPSC PDF Download

Introduction

A major goal in chemical kinetics is to determine the sequence of elementary reactions, or the reaction mechanism, that comprise complex reactions. For example, Sherwood Rowland and Mario Molina won the Nobel Prize in Chemistry in 1995 for proposing the elementary reactions involving chlorine radicals that contribute to the overall reaction of O₃ → O₂ in the troposphere. In the following sections, we will derive rate laws for complex reaction mechanisms, including reversible, parallel and consecutive reactions.

Parallel Reactions

Consider the reaction in which chemical species  A undergoes one of two irreversible first order reactions to form either species  B or species  C:

The overall reaction rate for the consumption of A can be written as:

Integrating [A] with respect to t, we obtain the following equation:

Plugging this expression into the equation for

Integrating [B] with respect to t, we obtain:

At t = 0, [B] = 0. Therefore,

Likewise,

The ratio of [B] to  [C] is simply:

An important parallel reaction in industry occurs in the production of ethylene oxide, a reagent in many chemical processes and also a major component in explosives. Ethylene oxide is formed through the partial oxidation of ethylene:

However, ethylene can also undergo a combustion reaction:

To select for the first reaction, the oxidation of ethylene takes place in the presence of a silver catalyst, which significantly increases k₁ compared to k₂. Figure 9.4.1 displays the concentration profiles for species A, B, and C in a parallel reaction in which k₁ > k₂.

Consecutive Reactions

Consider the following series of first-order irreversible reactions, where species A reacts to form an intermediate species, I, which then reacts to form the product, P:

We can write the reaction rates of species A, I and P as follows:

As before, integrating  [A] with respect to t leads to:

The concentration of species I can be written as

Then, solving for  [P], we find that:

Figure  9.4.2 displays the concentration profiles for species  A,  I, and  P in a consecutive reaction in which  k₁ = k₂. As can be seen from the figure, the concentration of species  I reaches a maximum at some time,  tmax. Oftentimes, species  I is the desired product. Returning to the oxidation of ethylene into ethylene oxide, it is important to note another reaction in which ethylene oxide can decompose into carbon dioxide and water through the following reaction

Thus, to maximize the concentration of ethylene oxide, the oxidation of ethylene is only allowed proceed to partial completion before the reaction is stopped.
Finally, in the limiting case when  k₂ ≫ k₁, we can write the concentration of  P as

Thus, when k₂ ≫ k₁, the reaction can be approximated as A → P and the apparent rate law follows 1st order kinetics.

Consecutive Reactions With an Equilibrium

Consider the reactions

We can write the reaction rates as:

The exact solutions of these is straightforward, in principle, but rather involved, so we will just state the exact solutions, which are

where

Steady-State Approximations

Consider the following consecutive reaction in which the first step is reversible:

We can write the reaction rates as:

These equations can be solved explicitly in terms of [A], [I], and [P], but the math becomes very complicated quickly. If, however, k₂ + k₋₁ ≫ k₁ (in other words, the rate of consumption of I is much faster than the rate of production of I ), we can make the approximation that the concentration of the intermediate species, I, is small and constant with time:

Equation 21.22 can now be written as

where [I]_ss is a constant represents the steady state concentration of intermediate species, [I]. Solving for [I]_ss,

We can then write the rate equation for species A as

Integrating,

Equation 21.28 is the same equation we would obtain for apparent 1st order kinetics of the following reaction:

where

Figure 9.4.3 displays the concentration profiles for species, A, I, and P with the condition that k₂ + k₋₁ ≫ k₁. These types of reaction kinetics appear when the intermediate species, I, is highly reactive.

Lindemann Mechanism

Consider the isomerization of methylisonitrile gas,  CH₃NC, to acetonitrile gas,  CH₃CN:

If the isomerization is a unimolecular elementary reaction, we should expect to see 1st order rate kinetics. Experimentally, however, 1st order rate kinetics are only observed at high pressures. At low pressures, the reaction kinetics follow a 2nd order rate law:

To explain this observation, J.A. Christiansen and F.A. Lindemann proposed that gas molecules first need to be energized via intermolecular collisions before undergoing an isomerization reaction. The reaction mechanism can be expressed as the following two elementary reactions

where M can be a reactant molecule, a product molecule or another inert molecule present in the reactor. Assuming that the concentration of A∗ is small, or k₁ ≪ k₂ + k₋₁, we can use a steady-state approximation to solve for the concentration profile of species B with time:

Solving for [A^∗],

The reaction rates of species  A and  B  can be written as

where

At high pressures, we can expect collisions to occur frequently, such that k₋₁[M] ≫ k₂. Equation 21.33 then becomes

which follows  1st order rate kinetics.
At low pressures, we can expect collisions to occurs infrequently, such that  k₋₁[M] ≪ k₂. In this scenario, equation 21.33 becomes

which follows second order rate kinetics, consistent with experimental observations.

Equilibrium Approximations 

Consider again the following consecutive reaction in which the first step is reversible:

Now let us consider the situation in which k₂ ≪ k₁ and k₋₁. In other words, the conversion of I to P is slow and is the rate-limiting step. In this situation, we can assume that [A] and [I] are in equilibrium with each other. As we derived before for a reversible reaction in equilibrium,

or, in terms of [I],

These conditions also result from the exact solution when we set  k₂ ≈ 0. When this is done, we have the approximate expressions from the exact solution:

and the approximate solutions become

In the long-time limit, when equilibrium is reached and transient behavior has decayed away, we find

Plugging the above equation into the expression for  d[P]/dt,

The reaction can thus be approximated as a 1st order reaction

with

Figure  9.4.4 displays the concentration profiles for species,  A,  I, and  P with the condition that  k₂ ≪ k₁ = k₋₁. When  k₁ = k₋₁, we expect  [A] = [I]. As can be seen from the figure, after a short initial startup time, the concentrations of species  A and  I are approximately equal during the reaction.